stap - Online in the Cloud

PROGRAM:

NAME

stap - systemtap script translator/driver

SYNOPSIS

stap [ OPTIONS ] FILENAME [ ARGUMENTS ]
stap [ OPTIONS ] - [ ARGUMENTS ]
stap [ OPTIONS ] -e SCRIPT [ ARGUMENTS ]
stap [ OPTIONS ] -l PROBE [ ARGUMENTS ]
stap [ OPTIONS ] -L PROBE [ ARGUMENTS ]
stap [ OPTIONS ] --dump-probe-types
stap [ OPTIONS ] --dump-probe-aliases
stap [ OPTIONS ] --dump-functions

DESCRIPTION

The stap program is the front-end to the Systemtap tool. It accepts probing instructions
written in a simple domain-specific language, translates those instructions into C code,
compiles this C code, and loads the resulting module into a running Linux kernel or a
DynInst user-space mutator, to perform the requested system trace/probe functions. You
can supply the script in a named file (FILENAME), from standard input (use - instead of
FILENAME), or from the command line (using -e SCRIPT). The program runs until it is
interrupted by the user, or if the script voluntarily invokes the exit() function, or by
sufficient number of soft errors.

The language, which is described the SCRIPT LANGUAGE section below, is strictly typed,
expressive, declaration free, procedural, prototyping-friendly, and inspired by awk and C.
It allows source code points or events in the system to be associated with handlers, which
are subroutines that are executed synchronously. It is somewhat similar conceptually to
"breakpoint command lists" in the gdb debugger.

OPTIONS

The systemtap translator supports the following options. Any other option prints a list
of supported options. Options may be given on the command line, as usual. If the file
$SYSTEMTAP_DIR/rc exist, options are also loaded from there and interpreted first.
($SYSTEMTAP_DIR defaults to $HOME/.systemtap if unset.)

- Use standard input instead of a given FILENAME as probe language input, unless -e
SCRIPT is given.

-h --help
Show help message.

-V --version
Show version message.

-p NUM Stop after pass NUM. The passes are numbered 1-5: parse, elaborate, translate,
compile, run. See the PROCESSING section for details.

-v Increase verbosity for all passes. Produce a larger volume of informative (?)
output each time option repeated.

--vp ABCDE
Increase verbosity on a per-pass basis. For example, "--vp 002" adds 2 units of
verbosity to pass 3 only. The combination "-v --vp 00004" adds 1 unit of verbosity
for all passes, and 4 more for pass 5.

-k Keep the temporary directory after all processing. This may be useful in order to
examine the generated C code, or to reuse the compiled kernel object.

-g Guru mode. Enable parsing of unsafe expert-level constructs like embedded C.

-P Prologue-searching mode. This is equivalent to --prologue-searching=always.
Activate heuristics to work around incorrect debugging information for function
parameter $context variables.

-u Unoptimized mode. Disable unused code elision and many other optimizations during
elaboration / translation.

-w Suppressed warnings mode. Disables all warning messages.

-W Treat all warnings as errors.

-b Use bulk mode (percpu files) for kernel-to-user data transfer. Use the stap-merge
program to multiplex them back together later.

-t Collect timing information on the number of times probe executes and average amount
of time spent in each probe-point. Also shows the derivation for each probe-point.

-sNUM Use NUM megabyte buffers for kernel-to-user data transfer. On a multiprocessor in
bulk mode, this is a per-processor amount.

-I DIR Add the given directory to the tapset search directory. See the description of
pass 2 for details.

-D NAME=VALUE
Add the given C preprocessor directive to the module Makefile. These can be used
to override limit parameters described below.

-B NAME=VALUE
Add the given make directive to the kernel module build's make invocation. These
can be used to add or override kconfig options.

-a ARCH
Use a cross-compilation mode for the given target architecture. This requires
access to the cross-compiler and the kernel build tree, and goes along with the -B
CROSS_COMPILE=arch-tool-prefix- and -r /build/tree options.

--modinfo NAME=VALUE
Add the name/value pair as a MODULE_INFO macro call to the generated module. This
may be useful to inform or override various module-related checks in the kernel.

-G NAME=VALUE
Sets the value of global variable NAME to VALUE when staprun is invoked. This
applies to scalar variables declared global in the script/tapset.

-R DIR Look for the systemtap runtime sources in the given directory.

-r /DIR
Build for kernel in given build tree. Can also be set with the SYSTEMTAP_RELEASE
environment variable.

-r RELEASE
Build for kernel in build tree /lib/modules/RELEASE/build. Can also be set with
the SYSTEMTAP_RELEASE environment variable.

-m MODULE
Use the given name for the generated kernel object module, instead of a unique
randomized name. The generated kernel object module is copied to the current
directory.

-d MODULE
Add symbol/unwind information for the given module into the kernel object module.
This may enable symbolic tracebacks from those modules/programs, even if they do
not have an explicit probe placed into them.

--ldd Add symbol/unwind information for all shared libraries suspected by ldd to be
necessary for user-space binaries being probe or listed with the -d option.
Caution: this can make the probe modules considerably larger.

--all-modules
Equivalent to specifying "-dkernel" and a "-d" for each kernel module that is
currently loaded. Caution: this can make the probe modules considerably larger.

-o FILE
Send standard output to named file. In bulk mode, percpu files will start with
FILE_ (FILE_cpu with -F) followed by the cpu number. This supports strftime(3)
formats for FILE.

-c CMD Start the probes, run CMD, and exit when CMD finishes. This also has the effect of
setting target() to the pid of the command ran.

-x PID Sets target() to PID. This allows scripts to be written that filter on a specific
process. Scripts run independent of the PID's lifespan.

-e SCRIPT
Run the given SCRIPT specified on the command line.

-E SCRIPT
Run the given SCRIPT specified. This SCRIPT is run in addition to the main script
specified, through -e, or as a script file. This option can be repeated to run
multiple scripts, and can be used in listing mode (-l/-L).

-l PROBE
Instead of running a probe script, just list all available probe points matching
the given single probe point. The pattern may include wildcards and aliases, but
not comma-separated multiple probe points. The process result code will indicate
failure if there are no matches.

-L PROBE
Similar to "-l", but list probe points and script-level local variables.

-F Without -o option, load module and start probes, then detach from the module
leaving the probes running. With -o option, run staprun in background as a daemon
and show its pid.

-S size[,N]
Sets the maximum size of output file and the maximum number of output files. If
the size of output file will exceed size , systemtap switches output file to the
next file. And if the number of output files exceed N , systemtap removes the
oldest output file. You can omit the second argument.

--skip-badvars
Ignore unresolvable or run-time-inaccessible context variables and substitute with
0, without errors.

--prologue-searching[=WHEN]
Prologue-searching mode. Activate heuristics to work around incorrect debugging
information for function parameter $context variables. WHEN can be either
"never", "always", or "auto" (i.e. enabled by heuristic). If WHEN is missing, then
"always" is assumed. If the option is missing, then "auto" is assumed.

--suppress-handler-errors
Wrap all probe handlers into something like this

try { ... } catch { next }

block, which causes any runtime errors to be quietly suppressed. Suppressed errors
do not count against MAXERRORS limits. In this mode, the MAXSKIPPED limits are
also suppressed, so that many errors and skipped probes may be accumulated during a
script's runtime. Any overall counts will still be reported at shutdown.

--compatible VERSION
Suppress recent script language or tapset changes which are incompatible with given
older version of systemtap. This may be useful if a much older systemtap script
fails to run. See the DEPRECATION section for more details.

--check-version
This option is used to check if the active script has any constructs that may be
systemtap version specific. See the DEPRECATION section for more details.

--clean-cache
This option prunes stale entries from the cache directory. This is normally done
automatically after successful runs, but this option will trigger the cleanup
manually and then exit. See the CACHING section for more details about cache
limits.

--color[=WHEN], --colour[=WHEN]
This option controls coloring of error messages. WHEN can be either "never",
"always", or "auto" (i.e. enable only if at a terminal). If WHEN is missing, then
"always" is assumed. If the option is missing, then "auto" is assumed.

Colors can be modified using the SYSTEMTAP_COLORS environment variable. The format
must be of the form key1=val1:key2=val2:key3=val3 ...etc. Valid keys are "error",
"warning", "source", "caret", and "token". Values constitute Select Graphic
Rendition (SGR) parameter(s). Consult the documentation of your terminal for the
SGRs it supports. As an example, the default colors would be expressed as
error=01;31:warning=00;33:source=00;34:caret=01:token=01. If SYSTEMTAP_COLORS is
absent, the default colors will be used. If it is empty or invalid, coloring is
turned off.

--disable-cache
This option disables all use of the cache directory. No files will be either read
from or written to the cache.

--poison-cache
This option treats files in the cache directory as invalid. No files will be read
from the cache, but resulting files from this run will still be written to the
cache. This is meant as a troubleshooting aid when stap's cached behavior seems to
be misbehaving.

--privilege[=stapusr | =stapsys | =stapdev]
This option instructs stap to examine the script looking for constructs which are
not allowed for the specified privilege level (see UNPRIVILEGED USERS).
Compilation fails if any such constructs are used. If stapusr or stapsys are
specified when using a compile server (see --use-server), the server will examine
the script and, if compilation succeeds, the server will cryptographically sign the
resulting kernel module, certifying that is it safe for use by users at the
specified privilege level.

If --privilege has not been specified, -pN has not been specified with N < 5, and
the invoking user is not root, and is not a member of the group stapdev, then stap
will automatically add the appropriate --privilege option to the options already
specified.

--unprivileged
This option is equivalent to --privilege=stapusr.

--use-server[=HOSTNAME[:PORT] | =IP_ADDRESS[:PORT] | =CERT_SERIAL]
Specify compile-server(s) to be used for compilation and/or in conjunction with
--list-servers and --trust-servers (see below). If no argument is supplied, then
the default in unprivileged mode (see --privilege) is to select compatible servers
which are trusted as SSL peers and as module signers and currently online.
Otherwise the default is to select compatible servers which are trusted as SSL
peers and currently online. --use-server may be specified more than once, in which
case a list of servers is accumulated in the order specified. Servers may be
specified by host name, ip address, or by certificate serial number (obtained using
--list-servers). The latter is most commonly used when adding or revoking trust in
a server (see --trust-servers below). If a server is specified by host name or ip
address, then an optional port number may be specified. This is useful for
accessing servers which are not on the local network or to specify a particular
server.

IP addresses may be IPv4 or IPv6 addresses.

If a particular IPv6 address is link local and exists on more than one interface,
the intended interface may be specified by appending the address with a percent
sign (%) followed by the intended interface name. For example,
"fe80::5eff:35ff:fe07:55ca%eth0".

In order to specify a port number with an IPv6 address, it is necessary to enclose
the IPv6 address in square brackets ([]) in order to separate the port number from
the rest of the address. For example, "[fe80::5eff:35ff:fe07:55ca]:5000" or
"[fe80::5eff:35ff:fe07:55ca%eth0]:5000".

If --use-server has not been specified, -pN has not been specified with N < 5, and
the invoking user not root, is not a member of the group stapdev, but is a member
of the group stapusr, then stap will automatically add --use-server to the options
already specified.

--use-server-on-error[=yes|=no]
Instructs stap to retry compilation of a script using a compile server if
compilation on the local host fails in a manner which suggests that it might
succeed using a server. If this option is not specified, the default is no. If no
argument is provided, then the default is yes. Compilation will be retried for
certain types of errors (e.g. insufficient data or resources) which may not occur
during re-compilation by a compile server. Compile servers will be selected
automatically for the re-compilation attempt as if --use-server was specified with
no arguments.

--list-servers[=SERVERS]
Display the status of the requested SERVERS, where SERVERS is a comma-separated
list of server attributes. The list of attributes is combined to filter the list of
servers displayed. Supported attributes are:

all specifies all known servers (trusted SSL peers, trusted module signers,
online servers).

specified
specifies servers specified using --use-server.

online filters the output by retaining information about servers which are
currently online.

trusted
filters the output by retaining information about servers which are trusted
as SSL peers.

signer filters the output by retaining information about servers which are trusted
as module signers (see --privilege).

compatible
filters the output by retaining information about servers which are
compatible with the current kernel release and architecture.

If no argument is provided, then the default is specified. If no servers were
specified using --use-server, then the default servers for --use-server are listed.

Note that --list-servers uses the avahi-daemon service to detect online servers. If
this service is not available, then --list-servers will fail to detect any online
servers. In order for --list-servers to detect servers listening on IPv6 addresses,
the avahi-daemon configuration file /etc/avahi/avahi-daemon.conf must contain an
active "use-ipv6=yes" line. The service must be restarted after adding this line in
order for IPv6 to be enabled.

--trust-servers[=TRUST_SPEC]
Grant or revoke trust in compile-servers, specified using --use-server as specified
by TRUST_SPEC, where TRUST_SPEC is a comma-separated list specifying the trust
which is to be granted or revoked. Supported elements are:

ssl trust the specified servers as SSL peers.

signer trust the specified servers as module signers (see --privilege). Only root
can specify signer.

all-users
grant trust as an ssl peer for all users on the local host. The default is
to grant trust as an ssl peer for the current user only. Trust as a module
signer is always granted for all users. Only root can specify all-users.

revoke revoke the specified trust. The default is to grant it.

no-prompt
do not prompt the user for confirmation before carrying out the requested
action. The default is to prompt the user for confirmation.

If no argument is provided, then the default is ssl. If no servers were specified
using --use-server, then no trust will be granted or revoked.

Unless no-prompt has been specified, the user will be prompted to confirm the trust
to be granted or revoked before the operation is performed.

--dump-probe-types
Dumps a list of supported probe types and exits. If --privilege=stapusr is also
specified, the list will be limited to probe types available to unprivileged users.

--dump-probe-aliases
Dumps a list of all probe aliases found in library files and exits.

--dump-functions
Dumps a list of all functions found in library files and exits. Also includes their
parameters and types. A function of type 'unknown' indicates a function that does
not return a value. Note that not all function/parameter types may be resolved
(these are also shown by 'unknown'). This features is very memory-intensive and
thus may not work properly with --use-server if the target server imposes an rlimit
on process memory (i.e. through the ~stap-server/.systemtap/rc configuration file,
see stap-server(8)).

--remote URL
Set the execution target to the given host. This option may be repeated to target
multiple execution targets. Passes 1-4 are completed locally as normal to build
the script, and then pass 5 will copy the module to the target and run it.
Acceptable URL forms include:

[USER@]HOSTNAME, ssh://[USER@]HOSTNAME
This mode uses ssh, optionally using a username not matching your own. If a
custom ssh_config file is in use, add SendEnv LANG to retain
internationalization functionality.

libvirt://DOMAIN, libvirt://DOMAIN/LIBVIRT_URI
This mode uses stapvirt to execute the script on a domain managed by
libvirt. Optionally, LIBVIRT_URI may be specified to connect to a specific
driver and/or a remote host. For example, to connect to the local privileged
QEMU driver, use:

--remote libvirt://MyDomain/qemu:///system

See the page at <http://libvirt.org/uri.html> for supported URIs. Also see
stapvirt(1) for more information on how to prepare the domain for stap
probing.

unix:PATH
This mode connects to a UNIX socket. This can be used with a QEMU virtio-
serial port for executing scripts inside a running virtual machine.

direct://
Special loopback mode to run on the local host.

--remote-prefix
Prefix each line of remote output with "N: ", where N is the index of the remote
execution target from which the given line originated.

--download-debuginfo[=OPTION]
Enable, disable or set a timeout for the automatic debuginfo downloading feature
offered by abrt as specified by OPTION, where OPTION is one of the following:

yes enable automatic downloading of debuginfo with no timeout. This is the same
as not providing an OPTION value to --download-debuginfo

no explicitly disable automatic downloading of debuginfo. This is the same as
not using the option at all.

ask show abrt output, and ask before continuing download. No timeout will be
set.

<timeout>
specify a timeout as a positive number to stop the download if it is taking
too long.

--rlimit-as=NUM
Specify the maximum size of the process's virtual memory (address space), in bytes.
If nothing is specified, no limits are imposed.

--rlimit-cpu=NUM
Specify the CPU time limit, in seconds. If nothing is specified, no limits are
imposed.

--rlimit-nproc=NUM
Specify the maximum number of processes that can be created. If nothing is
specified, no limits are imposed.

--rlimit-stack=NUM
Specify the maximum size of the process stack, in bytes. If nothing is specified,
no limits are imposed.

--rlimit-fsize=NUM
Specify the maximum size of files that the process may create, in bytes. If nothing
is specified, no limits are imposed.

--sysroot=DIR
Specify sysroot directory where target files (executables, libraries, etc.) are
located. With -r RELEASE, the sysroot will be searched for the appropriate kernel
build directory. With -r /DIR, however, the sysroot will not be used to find the
kernel build.

--sysenv=VAR=VALUE
Provide an alternate value for an environment variable where the value on a remote
system differs. Path variables (e.g. PATH, LD_LIBRARY_PATH) are assumed to be
relative to the directory provided by --sysroot, if provided.

--suppress-time-limits
Disable -DSTP_OVERLOAD related options as well as -DMAXACTION and -DMAXTRYLOCK.
This option requires guru mode.

--runtime=MODE
Set the pass-5 runtime mode. Valid options are kernel (default) and dyninst. See
ALTERNATE RUNTIMES below for more information.

--dyninst
Shorthand for --runtime=dyninst.

--save-uprobes
On machines that require SystemTap to build its own uprobes module (kernels prior
to version 3.5), this option instructs SystemTap to also save a copy of the module
in the current directory (creating a new "uprobes" directory first).

--target-namespaces=PID
Allow for a set of target namespaces to be set based on the namespaces the given
PID is in. This is for namespace-aware tapset functions. If the target namespaces
was not set, the target defaults to the stap process' namespaces.

ARGUMENTS

Any additional arguments on the command line are passed to the script parser for
substitution. See below.

SCRIPT LANGUAGE

The systemtap script language resembles awk and C. There are two main outermost
constructs: probes and functions. Within these, statements and expressions use C-like
operator syntax and precedence.

GENERAL SYNTAX
Whitespace is ignored. Three forms of comments are supported:
# ... shell style, to the end of line, except for $# and @#
// ... C++ style, to the end of line
/* ... C style ... */
Literals are either strings enclosed in double-quotes (passing through the usual C escape
codes with backslashes, and with adjacent string literals glued together, also as in C),
or integers (in decimal, hexadecimal, or octal, using the same notation as in C). All
strings are limited in length to some reasonable value (a few hundred bytes). Integers
are 64-bit signed quantities, although the parser also accepts (and wraps around) values
above positive 2**63.

In addition, script arguments given at the end of the command line may be inserted. Use
$1 ... $<NN> for insertion unquoted, @1 ... @<NN> for insertion as a string literal. The
number of arguments may be accessed through $# (as an unquoted number) or through @# (as a
quoted number). These may be used at any place a token may begin, including within the
preprocessing stage. Reference to an argument number beyond what was actually given is an
error.

PREPROCESSING
A simple conditional preprocessing stage is run as a part of parsing. The general form is
similar to the cond ? exp1 : exp2 ternary operator:

%( CONDITION %? TRUE-TOKENS %)
%( CONDITION %? TRUE-TOKENS %: FALSE-TOKENS %)

The CONDITION is either an expression whose format is determined by its first keyword, or
a string literals comparison or a numeric literals comparison. It can be also composed of
many alternatives and conjunctions of CONDITIONs (meant as in previous sentence) using ||
and && respectively. However, parentheses are not supported yet, so remembering that
conjunction takes precedence over alternative is important.

If the first part is the identifier kernel_vr or kernel_v to refer to the kernel version
number, with ("2.6.13-1.322FC3smp") or without ("2.6.13") the release code suffix, then
the second part is one of the six standard numeric comparison operators <, <=, ==, !=, >,
and >=, and the third part is a string literal that contains an RPM-style version-release
value. The condition is deemed satisfied if the version of the target kernel (as
optionally overridden by the -r option) compares to the given version string. The
comparison is performed by the glibc function strverscmp. As a special case, if the
operator is for simple equality (==), or inequality (!=), and the third part contains any
wildcard characters (* or ? or [), then the expression is treated as a wildcard (mis)match
as evaluated by fnmatch.

If, on the other hand, the first part is the identifier arch to refer to the processor
architecture (as named by the kernel build system ARCH/SUBARCH), then the second part is
one of the two string comparison operators == or !=, and the third part is a string
literal for matching it. This comparison is a wildcard (mis)match.

Similarly, if the first part is an identifier like CONFIG_something to refer to a kernel
configuration option, then the second part is == or !=, and the third part is a string
literal for matching the value (commonly "y" or "m"). Nonexistent or unset kernel
configuration options are represented by the empty string. This comparison is also a
wildcard (mis)match.

If the first part is the identifier systemtap_v, the test refers to the systemtap
compatibility version, which may be overridden for old scripts with the --compatible flag.
The comparison operator is as is for kernel_v and the right operand is a version string.
See also the DEPRECATION section below.

If the first part is the identifier systemtap_privilege, the test refers to the privilege
level that the systemtap script is compiled with. Here the second part is == or !=, and
the third part is a string literal, either "stapusr" or "stapsys" or "stapdev".

If the first part is the identifier guru_mode, the test refers to if the systemtap script
is compiled with guru_mode. Here the second part is == or !=, and the third part is a
number, either 1 or 0.

If the first part is the identifier runtime, the test refers to the systemtap runtime
mode. See ALTERNATE RUNTIMES below for more information on runtimes. The second part is
one of the two string comparison operators == or !=, and the third part is a string
literal for matching it. This comparison is a wildcard (mis)match.

Otherwise, the CONDITION is expected to be a comparison between two string literals or two
numeric literals. In this case, the arguments are the only variables usable.

The TRUE-TOKENS and FALSE-TOKENS are zero or more general parser tokens (possibly
including nested preprocessor conditionals), and are passed into the input stream if the
condition is true or false. For example, the following code induces a parse error unless
the target kernel version is newer than 2.6.5:

%( kernel_v <= "2.6.5" %? **ERROR** %) # invalid token sequence

The following code might adapt to hypothetical kernel version drift:

probe kernel.function (
%( kernel_v <= "2.6.12" %? "__mm_do_fault" %:
%( kernel_vr == "2.6.13*smp" %? "do_page_fault" %:
UNSUPPORTED %) %)
) { /* ... */ }

%( arch == "ia64" %?
probe syscall.vliw = kernel.function("vliw_widget") {}
%)

PREPROCESSOR MACROS
The preprocessor also supports a simple macro facility, run as a separate pass before
conditional preprocessing.

Macros are defined using the following construct:

@define NAME %( BODY %)
@define NAME(PARAM_1, PARAM_2, ...) %( BODY %)

Macros, and parameters inside a macro body, are both invoked by prefixing the macro name
with an @ symbol:

@define foo %( x %)
@define add(a,b) %( ((@a)+(@b)) %)

@foo = @add(2,2)

Macro expansion is currently performed in a separate pass before conditional compilation.
Therefore, both TRUE- and FALSE-tokens in conditional expressions will be macroexpanded
regardless of how the condition is evaluated. This can sometimes lead to errors:

// The following results in a conflict:
%( CONFIG_UTRACE == "y" %?
@define foo %( process.syscall %)
%:
@define foo %( **ERROR** %)
%)

// The following works properly as expected:
@define foo %(
%( CONFIG_UTRACE == "y" %? process.syscall %: **ERROR** %)
%)

The first example is incorrect because both @defines are evaluated in a pass prior to the
conditional being evaluated.

Normally, a macro definition is local to the file it occurs in. Thus, defining a macro in
a tapset does not make it available to the user of the tapset. Publically available
library macros can be defined by including .stpm files on the tapset search path. These
files may only contain @define constructs, which become visible across all tapsets and
user scripts.

VARIABLES
Identifiers for variables and functions are an alphanumeric sequence, and may include _
and $ characters. They may not start with a plain digit, as in C. Each variable is by
default local to the probe or function statement block within which it is mentioned, and
therefore its scope and lifetime is limited to a particular probe or function invocation.

Scalar variables are implicitly typed as either string or integer. Associative arrays
also have a string or integer value, and a tuple of strings and/or integers serving as a
key. Here are a few basic expressions.

var1 = 5
var2 = "bar"
array1 [pid()] = "name" # single numeric key
array2 ["foo",4,i++] += 5 # vector of string/num/num keys
if (["hello",5,4] in array2) println ("yes") # membership test

The translator performs type inference on all identifiers, including array indexes and
function parameters. Inconsistent type-related use of identifiers signals an error.

Variables may be declared global, so that they are shared amongst all probes and live as
long as the entire systemtap session. There is one namespace for all global variables,
regardless of which script file they are found within. Concurrent access to global
variables is automatically protected with locks, see the SAFETY AND SECURITY section for
more details. A global declaration may be written at the outermost level anywhere, not
within a block of code. Global variables which are written but never read will be
displayed automatically at session shutdown. The translator will infer for each its value
type, and if it is used as an array, its key types. Optionally, scalar globals may be
initialized with a string or number literal. The following declaration marks variables as
global.

global var1, var2, var3=4

Global variables can also be set as module options. One can do this by either using the -G
option, or the module must first be compiled using stap -p4. Global variables can then be
set on the command line when calling staprun on the module generated by stap -p4. See
staprun(8) for more information.

Arrays are limited in size by the MAXMAPENTRIES variable -- see the SAFETY AND SECURITY
section for details. Optionally, global arrays may be declared with a maximum size in
brackets, overriding MAXMAPENTRIES for that array only. Note that this doesn't indicate
the type of keys for the array, just the size.

global tiny_array[10], normal_array, big_array[50000]

Arrays may be configured for wrapping using the '%' suffix. This causes older elements to
be overwritten if more elements are inserted than the array can hold. This works for both
associative and statistics typed arrays.

global wrapped_array1%[10], wrapped_array2%

Many types of probe points provide context variables, which are run-time values, safely
extracted from the kernel or userspace program being probed. These are prefixed with the
$ character. The CONTEXT VARIABLES section in stapprobes(3stap) lists what is available
for each type of probe point. These context variables become normal string or numeric
scalars once they are stored in normal script variables. See the TYPECASTING section
below on how to to turn them back into typed pointers for further processing as context
variables.

STATEMENTS
Statements enable procedural control flow. They may occur within functions and probe
handlers. The total number of statements executed in response to any single probe event
is limited to some number defined by a macro in the translated C code, and is in the
neighbourhood of 1000.

EXP Execute the string- or integer-valued expression and throw away the value.

{ STMT1 STMT2 ... }
Execute each statement in sequence in this block. Note that separators or
terminators are generally not necessary between statements.

; Null statement, do nothing. It is useful as an optional separator between
statements to improve syntax-error detection and to handle certain grammar
ambiguities.

if (EXP) STMT1 [ else STMT2 ]
Compare integer-valued EXP to zero. Execute the first (non-zero) or second STMT
(zero).

while (EXP) STMT
While integer-valued EXP evaluates to non-zero, execute STMT.

for (EXP1; EXP2; EXP3) STMT
Execute EXP1 as initialization. While EXP2 is non-zero, execute STMT, then the
iteration expression EXP3.

foreach (VAR in ARRAY [ limit EXP ]) STMT
Loop over each element of the named global array, assigning current key to VAR.
The array may not be modified within the statement. By adding a single + or -
operator after the VAR or the ARRAY identifier, the iteration will proceed in a
sorted order, by ascending or descending index or value. If the array contains
statistics aggregates, adding the desired @operator between the ARRAY identifier
and the + or - will specify the sorting aggregate function. See the STATISTICS
section below for the ones available. Default is @count. Using the optional limit
keyword limits the number of loop iterations to EXP times. EXP is evaluated once
at the beginning of the loop.

foreach ([VAR1, VAR2, ...] in ARRAY [ limit EXP ]) STMT
Same as above, used when the array is indexed with a tuple of keys. A sorting
suffix may be used on at most one VAR or ARRAY identifier.

foreach ([VAR1, VAR2, ...] in ARRAY [INDEX1, INDEX2, ...] [ limit EXP ]) STMT
Same as above, where iterations are limited to elements in the array where the keys
match the index values specified. The symbol * can be used to specify an index and
will be treated as a wildcard.

foreach (VALUE = VAR in ARRAY [ limit EXP ]) STMT
This variant of foreach saves current value into VALUE on each iteration, so it is
the same as ARRAY[VAR]. This also works with a tuple of keys. Sorting suffixes on
VALUE have the same effect as on ARRAY.

foreach (VALUE = VAR in ARRAY [INDEX1, INDEX2, ...] [ limit EXP ]) STMT
Same as above, where iterations are limited to elements in the array where the keys
match the index values specified. The symbol * can be used to specify an index and
will be treated as a wildcard.

break, continue
Exit or iterate the innermost nesting loop (while or for or foreach) statement.

return EXP
Return EXP value from enclosing function. If the function's value is not taken
anywhere, then a return statement is not needed, and the function will have a
special "unknown" type with no return value.

next Return now from enclosing probe handler. This is especially useful in probe
aliases that apply event filtering predicates.

try { STMT1 } catch { STMT2 }
Run the statements in the first block. Upon any run-time errors, abort STMT1 and
start executing STMT2. Any errors in STMT2 will propagate to outer try/catch
blocks, if any.

try { STMT1 } catch(VAR) { STMT2 }
Same as above, plus assign the error message to the string scalar variable VAR.

delete ARRAY[INDEX1, INDEX2, ...]
Remove from ARRAY the element specified by the index tuple. If the index tuple
contains a * in place of an index, the * is treated as a wildcard and all elements
with keys that match the index tuple will be removed from ARRAY. The value will no
longer be available, and subsequent iterations will not report the element. It is
not an error to delete an element that does not exist.

delete ARRAY
Remove all elements from ARRAY.

delete SCALAR
Removes the value of SCALAR. Integers and strings are cleared to 0 and ""
respectively, while statistics are reset to the initial empty state.

EXPRESSIONS
Systemtap supports a number of operators that have the same general syntax, semantics, and
precedence as in C and awk. Arithmetic is performed as per typical C rules for signed
integers. Division by zero or overflow is detected and results in an error.

binary numeric operators
* / % + - >> << & ^ | && ||

binary string operators
. (string concatenation)

numeric assignment operators
= *= /= %= += -= >>= <<= &= ^= |=

string assignment operators
= .=

unary numeric operators
+ - ! ~ ++ --

binary numeric, string comparison or regex matching operators
< > <= >= == != =~ !~

ternary operator
cond ? exp1 : exp2

grouping operator
( exp )

function call
fn ([ arg1, arg2, ... ])

array membership check
exp in array
[exp1, exp2, ...] in array
[*, *, ... ]in array

REGULAR EXPRESSION MATCHING
The scripting language supports regular expression matching. The basic syntax is as
follows:

exp =~ regex
exp !~ regex

(The first operand must be an expression evaluating to a string; the second operand must
be a string literal containing a syntactically valid regular expression.)

The regular expression syntax supports most of the features of POSIX Extended Regular
Expressions, except for subexpression reuse ("\1") functionality. The ability to capture
and extract the contents of the matched string and subexpressions has not yet been
implemented.

PROBES
The main construct in the scripting language identifies probes. Probes associate abstract
events with a statement block ("probe handler") that is to be executed when any of those
events occur. The general syntax is as follows:

probe PROBEPOINT [, PROBEPOINT] { [STMT ...] }
probe PROBEPOINT [, PROBEPOINT] if (CONDITION) { [STMT ...] }

Events are specified in a special syntax called "probe points". There are several
varieties of probe points defined by the translator, and tapset scripts may define further
ones using aliases. Probe points may be wildcarded, grouped, or listed in preference
sequences, or declared optional. More details on probe point syntax and semantics are
listed on the stapprobes(3stap) manual page.

The probe handler is interpreted relative to the context of each event. For events
associated with kernel code, this context may include variables defined in the source code
at that spot. These "context variables" are presented to the script as variables whose
names are prefixed with "$". They may be accessed only if the kernel's compiler preserved
them despite optimization. This is the same constraint that a debugger user faces when
working with optimized code. In addition, the objects must exist in paged-in memory at
the moment of the systemtap probe handler's execution, because systemtap must not cause
(suppresses) any additional paging. Some probe types have very little context. See the
stapprobes(3stap) man pages to see the kinds of context variables available at each kind
of probe point.

Probes may be decorated with an arming condition, consisting of a simple boolean
expression on read-only global script variables. While disarmed (condition evaluates to
false), some probe types reduce or eliminate their run-time overheads. When an arming
condition evaluates to true, probes will be soon re-armed, and their probe handlers will
start getting called as the events fire. (Some events may be lost during the arming
interval. If this is unacceptable, do not use arming conditions for those probes.)

New probe points may be defined using "aliases". Probe point aliases look similar to
probe definitions, but instead of activating a probe at the given point, it just defines a
new probe point name as an alias to an existing one. There are two types of alias, i.e.
the prologue style and the epilogue style which are identified by "=" and "+="
respectively.

For prologue style alias, the statement block that follows an alias definition is
implicitly added as a prologue to any probe that refers to the alias. While for the
epilogue style alias, the statement block that follows an alias definition is implicitly
added as an epilogue to any probe that refers to the alias. For example:

probe syscall.read = kernel.function("sys_read") {
fildes = $fd
if (execname() == "init") next # skip rest of probe
}

defines a new probe point syscall.read, which expands to kernel.function("sys_read"), with
the given statement as a prologue, which is useful to predefine some variables for the
alias user and/or to skip probe processing entirely based on some conditions. And

probe syscall.read += kernel.function("sys_read") {
if (tracethis) println ($fd)
}

defines a new probe point with the given statement as an epilogue, which is useful to take
actions based upon variables set or left over by the the alias user. Please note that in
each case, the statements in the alias handler block are treated ordinarily, so that
variables assigned there constitute mere initialization, not a macro substitution.

An alias is used just like a built-in probe type.

probe syscall.read {
printf("reading fd=%d\n", fildes)
if (fildes > 10) tracethis = 1
}

FUNCTIONS
Systemtap scripts may define subroutines to factor out common work. Functions take any
number of scalar (integer or string) arguments, and must return a single scalar (integer
or string). An example function declaration looks like this:

function thisfn (arg1, arg2) {
return arg1 + arg2
}

Note the general absence of type declarations, which are instead inferred by the
translator. However, if desired, a function definition may include explicit type
declarations for its return value and/or its arguments. This is especially helpful for
embedded-C functions. In the following example, the type inference engine need only infer
type type of arg2 (a string).

function thatfn:string (arg1:long, arg2) {
return sprint(arg1) . arg2
}

Functions may call others or themselves recursively, up to a fixed nesting limit. This
limit is defined by a macro in the translated C code and is in the neighbourhood of 10.

PRINTING
There are a set of function names that are specially treated by the translator. They
format values for printing to the standard systemtap output stream in a more convenient
way. The sprint* variants return the formatted string instead of printing it.

print, sprint
Print one or more values of any type, concatenated directly together.

println, sprintln
Print values like print and sprint, but also append a newline.

printd, sprintd
Take a string delimiter and two or more values of any type, and print the values
with the delimiter interposed. The delimiter must be a literal string constant.

printdln, sprintdln
Print values with a delimiter like printd and sprintd, but also append a newline.

printf, sprintf
Take a formatting string and a number of values of corresponding types, and print
them all. The format must be a literal string constant.

The printf formatting directives similar to those of C, except that they are fully type-
checked by the translator:

%b Writes a binary blob of the value given, instead of ASCII text. The width
specifier determines the number of bytes to write; valid specifiers are %b
%1b %2b %4b %8b. Default (%b) is 8 bytes.

%c Character.

%d,%i Signed decimal.

%m Safely reads kernel memory at the given address, outputs its content. The
optional precision specifier (not field width) determines the number of
bytes to read - default is 1 byte. %10.4m prints 4 bytes of the memory in a
10-character-wide field.

%M Same as %m, but outputs in hexadecimal. The minimal size of output is
double the optional precision specifier - default is 1 byte (2 hex chars).
%10.4M prints 4 bytes of the memory as 8 hexadecimal characters in a
10-character-wide field.

%o Unsigned octal.

%p Unsigned pointer address.

%s String.

%u Unsigned decimal.

%x Unsigned hex value, in all lower-case.

%X Unsigned hex value, in all upper-case.

%% Writes a %.

The # flag selects the alternate forms. For octal, this prefixes a 0. For hex, this
prefixes 0x or 0X, depending on case. For characters, this escapes non-printing values
with either C-like escapes or raw octal.

Examples:

a = "alice", b = "bob", p = 0x1234abcd, i = 123, j = -1, id[a] = 1234, id[b] = 4567
print("hello")
Prints: hello
println(b)
Prints: bob\n
println(a . " is " . sprint(16))
Prints: alice is 16
foreach (name in id) printdln("|", strlen(name), name, id[name])
Prints: 5|alice|1234\n3|bob|4567
printf("%c is %s; %x or %X or %p; %d or %u\n",97,a,p,p,p,j,j)
Prints: a is alice; 1234abcd or 1234ABCD or 0x1234abcd; -1 or 18446744073709551615\n
printf("2 bytes of kernel buffer at address %p: %2m", p, p)
Prints: 2 byte of kernel buffer at address 0x1234abcd: <binary data>
printf("%4b", p)
Prints (these values as binary data): 0x1234abcd
printf("%#o %#x %#X\n", 1, 2, 3)
Prints: 01 0x2 0X3
printf("%#c %#c %#c\n", 0, 9, 42)
Prints: \000 \t *

STATISTICS
It is often desirable to collect statistics in a way that avoids the penalties of
repeatedly exclusive locking the global variables those numbers are being put into.
Systemtap provides a solution using a special operator to accumulate values, and several
pseudo-functions to extract the statistical aggregates.

The aggregation operator is <<<, and resembles an assignment, or a C++ output-streaming
operation. The left operand specifies a scalar or array-index lvalue, which must be
declared global. The right operand is a numeric expression. The meaning is intuitive:
add the given number to the pile of numbers to compute statistics of. (The specific list
of statistics to gather is given separately, by the extraction functions.)

foo <<< 1
stats[pid()] <<< memsize

The extraction functions are also special. For each appearance of a distinct extraction
function operating on a given identifier, the translator arranges to compute a set of
statistics that satisfy it. The statistics system is thereby "on-demand". Each execution
of an extraction function causes the aggregation to be computed for that moment across all
processors.

Here is the set of extractor functions. The first argument of each is the same style of
lvalue used on the left hand side of the accumulate operation. The @count(v), @sum(v),
@min(v), @max(v), @avg(v) extractor functions compute the
number/total/minimum/maximum/average of all accumulated values. The resulting values are
all simple integers. Arrays containing aggregates may be sorted and iterated. See the
foreach construct above.

Histograms are also available, but are more complicated because they have a vector rather
than scalar value. @hist_linear(v,start,stop,interval) represents a linear histogram from
"start" to "stop" by increments of "interval". The interval must be positive. Similarly,
@hist_log(v) represents a base-2 logarithmic histogram. Printing a histogram with the
print family of functions renders a histogram object as a tabular "ASCII art" bar chart.

probe timer.profile {
x[1] <<< pid()
x[2] <<< uid()
y <<< tid()
}
global x // an array containing aggregates
global y // a scalar
probe end {
foreach ([i] in x @count+) {
printf ("x[%d]: avg %d = sum %d / count %d\n",
i, @avg(x[i]), @sum(x[i]), @count(x[i]))
println (@hist_log(x[i]))
}
println ("y:")
println (@hist_log(y))
}

TYPECASTING
Once a pointer (see the CONTEXT VARIABLES section of stapprobes(3stap)) has been saved
into a script integer variable, the translator loses the type information necessary to
access members from that pointer. Using the @cast() operator tells the translator how to
interpret the number as a typed pointer.

@cast(p, "type_name"[, "module"])->member

This will interpret p as a pointer to a struct/union named type_name and dereference the
member value. Further ->subfield expressions may be appended to dereference more levels.
NOTE: the same dereferencing operator -> is used to refer to both direct containment or
pointer indirection. Systemtap automatically determines which. The optional module tells
the translator where to look for information about that type. Multiple modules may be
specified as a list with : separators. If the module is not specified, it will default
either to the probe module for dwarf probes, or to "kernel" for functions and all other
probes types.

The translator can create its own module with type information from a header surrounded by
angle brackets, in case normal debuginfo is not available. For kernel headers, prefix it
with "kernel" to use the appropriate build system. All other headers are build with
default GCC parameters into a user module. Multiple headers may be specified in sequence
to resolve a codependency.

@cast(tv, "timeval", "<sys/time.h>")->tv_sec
@cast(task, "task_struct", "kernel<linux/sched.h>")->tgid
@cast(task, "task_struct",
"kernel<linux/sched.h><linux/fs_struct.h>")->fs->umask

Values acquired by @cast may be pretty-printed by the $ and $$ suffix operators, the same
way as described in the CONTEXT VARIABLES section of the stapprobes(3stap) manual page.

When in guru mode, the translator will also allow scripts to assign new values to members
of typecasted pointers.

Typecasting is also useful in the case of void* members whose type may be determinable at
runtime.

probe foo {
if ($var->type == 1) {
value = @cast($var->data, "type1")->bar
} else {
value = @cast($var->data, "type2")->baz
}
print(value)
}

EMBEDDED C
When in guru mode, the translator accepts embedded code in the top level of the script.
Such code is enclosed between %{ and %} markers, and is transcribed verbatim, without
analysis, in some sequence, into the top level of the generated C code. At the outermost
level, this may be useful to add #include instructions, and any auxiliary definitions for
use by other embedded code.

Another place where embedded code is permitted is as a function body. In this case, the
script language body is replaced entirely by a piece of C code enclosed again between %{
and %} markers. This C code may do anything reasonable and safe. There are a number of
undocumented but complex safety constraints on atomicity, concurrency, resource
consumption, and run time limits, so this is an advanced technique.

The memory locations set aside for input and output values are made available to it using
macros STAP_ARG_* and STAP_RETVALUE. Errors may be signalled with STAP_ERROR. Output may
be written with STAP_PRINTF. The function may return early with STAP_RETURN. Here are
some examples:

function integer_ops (val) %{
STAP_PRINTF("%d\n", STAP_ARG_val);
STAP_RETVALUE = STAP_ARG_val + 1;
if (STAP_RETVALUE == 4)
STAP_ERROR("wrong guess: %d", (int) STAP_RETVALUE);
if (STAP_RETVALUE == 3)
STAP_RETURN(0);
STAP_RETVALUE ++;
%}
function string_ops (val) %{
strlcpy (STAP_RETVALUE, STAP_ARG_val, MAXSTRINGLEN);
strlcat (STAP_RETVALUE, "one", MAXSTRINGLEN);
if (strcmp (STAP_RETVALUE, "three-two-one"))
STAP_RETURN("parameter should be three-two-");
%}
function no_ops () %{
STAP_RETURN(); /* function inferred with no return value */
%}

The function argument and return value types have to be inferred by the translator from
the call sites in order for this to work. The user should examine C code generated for
ordinary script-language functions in order to write compatible embedded-C ones.

The last place where embedded code is permitted is as an expression rvalue. In this case,
the C code enclosed between %{ and %} markers is interpreted as an ordinary expression
value. It is assumed to be a normal 64-bit signed number, unless the marker /* string */
is included, in which case it's treated as a string.

function add_one (val) {
return val + %{ 1 %}
}
function add_string_two (val) {
return val . %{ /* string */ "two" %}
}

The embedded-C code may contain markers to assert optimization and safety properties.

/* pure */
means that the C code has no side effects and may be elided entirely if its value
is not used by script code.

/* stable */
means that the C code always has the same value (in any given probe handler
invocation), so repeated calls may be automatically replaced by memoized values.
Such functions must take no parameters, and also be pure.

/* unprivileged */
means that the C code is so safe that even unprivileged users are permitted to use
it.

/* myproc-unprivileged */
means that the C code is so safe that even unprivileged users are permitted to use
it, provided that the target of the current probe is within the user's own process.

/* guru */
means that the C code is so unsafe that a systemtap user must specify -g (guru
mode) to use this.

/* unmangled */
in an embedded-C function, means that the legacy (pre-1.8) argument access syntax
should be made available inside the function. Hence, in addition to STAP_ARG_foo
and STAP_RETVALUE one can use THIS->foo and THIS->__retvalue respectively inside
the function. This is useful for quickly migrating code written for SystemTap
version 1.7 and earlier.

/* unmodified-fnargs */
in an embedded-C function, means that the function arguments are not modified
inside the function body.

/* string */
in embedded-C expressions only, means that the expression has const char * type and
should be treated as a string value, instead of the default long numeric.

BUILT-INS
A set of builtin probe point aliases are provided by the scripts installed in the
directory specified in the stappaths(7) manual page. The functions are described in the
stapprobes(3stap) manual page.

PROCESSING

The translator begins pass 1 by parsing the given input script, and all scripts (files
named *.stp) found in a tapset directory. The directories listed with -I are processed in
sequence, each processed in "guru mode". For each directory, a number of subdirectories
are also searched. These subdirectories are derived from the selected kernel version (the
-R option), in order to allow more kernel-version-specific scripts to override less
specific ones. For example, for a kernel version 2.6.12-23.FC3 the following patterns
would be searched, in sequence: 2.6.12-23.FC3/*.stp, 2.6.12/*.stp, 2.6/*.stp, and finally
*.stp. Stopping the translator after pass 1 causes it to print the parse trees.

In pass 2, the translator analyzes the input script to resolve symbols and types.
References to variables, functions, and probe aliases that are unresolved internally are
satisfied by searching through the parsed tapset script files. If any tapset script file
is selected because it defines an unresolved symbol, then the entirety of that file is
added to the translator's resolution queue. This process iterates until all symbols are
resolved and a subset of tapset script files is selected.

Next, all probe point descriptions are validated against the wide variety supported by the
translator. Probe points that refer to code locations ("synchronous probe points")
require the appropriate kernel debugging information to be installed. In the associated
probe handlers, target-side variables (whose names begin with "$") are found and have
their run-time locations decoded.

Next, all probes and functions are analyzed for optimization opportunities, in order to
remove variables, expressions, and functions that have no useful value and no side-effect.
Embedded-C functions are assumed to have side-effects unless they include the magic string
/* pure */. Since this optimization can hide latent code errors such as type mismatches
or invalid $context variables, it sometimes may be useful to disable the optimizations
with the -u option.

Finally, all variable, function, parameter, array, and index types are inferred from
context (literals and operators). Stopping the translator after pass 2 causes it to list
all the probes, functions, and variables, along with all inferred types. Any inconsistent
or unresolved types cause an error.

In pass 3, the translator writes C code that represents the actions of all selected script
files, and creates a Makefile to build that into a kernel object. These files are placed
into a temporary directory. Stopping the translator at this point causes it to print the
contents of the C file.

In pass 4, the translator invokes the Linux kernel build system to create the actual
kernel object file. This involves running make in the temporary directory, and requires a
kernel module build system (headers, config and Makefiles) to be installed in the usual
spot /lib/modules/VERSION/build. Stopping the translator after pass 4 is the last chance
before running the kernel object. This may be useful if you want to archive the file.

In pass 5, the translator invokes the systemtap auxiliary program staprun program for the
given kernel object. This program arranges to load the module then communicates with it,
copying trace data from the kernel into temporary files, until the user sends an interrupt
signal. Any run-time error encountered by the probe handlers, such as running out of
memory, division by zero, exceeding nesting or runtime limits, results in a soft error
indication. Soft errors in excess of MAXERRORS block of all subsequent probes (except
error-handling probes), and terminate the session. Finally, staprun unloads the module,
and cleans up.

ABNORMAL TERMINATION
One should avoid killing the stap process forcibly, for example with SIGKILL, because the
stapio process (a child process of the stap process) and the loaded module may be left
running on the system. If this happens, send SIGTERM or SIGINT to any remaining stapio
processes, then use rmmod to unload the systemtap module.

EXAMPLES

See the stapex(3stap) manual page for a brief collection of samples, or a large set of
installed samples under the systemtap documentation/testsuite directories. See
stappaths(7stap) for the likely location of these on the system.

CACHING

The systemtap translator caches the pass 3 output (the generated C code) and the pass 4
output (the compiled kernel module) if pass 4 completes successfully. This cached output
is reused if the same script is translated again assuming the same conditions exist (same
kernel version, same systemtap version, etc.). Cached files are stored in the
$SYSTEMTAP_DIR/cache directory. The cache can be limited by having the file cache_mb_limit
placed in the cache directory (shown above) containing only an ASCII integer representing
how many MiB the cache should not exceed. In the absence of this file, a default will be
created with the limit set to 256MiB. This is a 'soft' limit in that the cache will be
cleaned after a new entry is added if the cache clean interval is exceeded, so the total
cache size may temporarily exceed this limit. This interval can be specified by having the
file cache_clean_interval_s placed in the cache directory (shown above) containing only an
ASCII integer representing the interval in seconds. In the absence of this file, a default
will be created with the interval set to 300 s.

SAFETY AND SECURITY

Systemtap is may be used as a powerful administrative tool. It can expose kernel internal
data structures and potentially private user information. (In dyninst runtime mode, this
is not the case, see the ALTERNATE RUNTIMES section below.)

The translator asserts many safety constraints during compilation and more during run-
time. It aims to ensure that no handler routine can run for very long, allocate boundless
memory, perform unsafe operations, or in unintentionally interfere with the system. Uses
of script global variables are automatically read/write locked as appropriate, to protect
against manipulation by concurrent probe handlers. (Deadlocks are detected with timeouts.
Use the -t flag to receive reports of excessive lock contention.) Experimenting with
scripts is therefore generally safe. The guru-mode -g option allows administrators to
bypass most safety measures, which permits invasive or state-changing operations,
embedded-C code, and increases the risk of upset. By default, overload prevention is
turned on for all modules. If you would like to disable overload processing, use the
--suppress-time-limits option.

Errors that are caught at run time normally result in a clean script shutdown and a pass-5
error message. The --suppress-handler-errors option lets scripts tolerate soft errors
without shutting down.

PERMISSIONS
For the normal linux-kernel-module runtime, to run the kernel objects systemtap builds, a
user must be one of the following:

· the root user;

· a member of the stapdev and stapusr groups;

· a member of the stapsys and stapusr groups; or

· a member of the stapusr group.

The root user or a user who is a member of both the stapdev and stapusr groups can build
and run any systemtap script.

A user who is a member of both the stapsys and stapusr groups can only use pre-built
modules under the following conditions:

· The module has been signed by a trusted signer. Trusted signers are normally systemtap
compile-servers which sign modules when the --privilege option is specified by the
client. See the stap-server(8) manual page for more information.

· The module was built using the --privilege=stapsys or the --privilege=stapusr options.

Members of only the stapusr group can only use pre-built modules under the following
conditions:

· The module is located in the /lib/modules/VERSION/systemtap directory. This directory
must be owned by root and not be world writable.

or

· The module has been signed by a trusted signer. Trusted signers are normally systemtap
compile-servers which sign modules when the --privilege option is specified by the
client. See the stap-server(8) manual page for more information.

· The module was built using the --privilege=stapusr option.

The kernel modules generated by stap program are run by the staprun program. The latter
is a part of the Systemtap package, dedicated to module loading and unloading (but only in
the white zone), and kernel-to-user data transfer. Since staprun does not perform any
additional security checks on the kernel objects it is given, it would be unwise for a
system administrator to add untrusted users to the stapdev or stapusr groups.

SECUREBOOT
If the current system has SecureBoot turned on in the UEFI firmware, all kernel modules
must be signed. (Some kernels may allow disabling SecureBoot long after booting with a
key sequence such as SysRq-X, making it unnecessary to sign modules.) The systemtap
compile server can sign modules with a MOK (Machine Owner Key) that it has in common with
a client system. See the following wiki page for more details:

https://sourceware.org/systemtap/wiki/SecureBoot

RESOURCE LIMITS
Many resource use limits are set by macros in the generated C code. These may be
overridden with -D flags. A selection of these is as follows:

MAXNESTING
Maximum number of nested function calls. Default determined by script analysis,
with a bonus 10 slots added for recursive scripts.

MAXSTRINGLEN
Maximum length of strings, default 128.

MAXTRYLOCK
Maximum number of iterations to wait for locks on global variables before declaring
possible deadlock and skipping the probe, default 1000.

MAXACTION
Maximum number of statements to execute during any single probe hit (with
interrupts disabled), default 1000. Note that for straight-through probe handlers
lacking loops or recursion, due to optimization, this parameter may be interpreted
too conservatively.

MAXACTION_INTERRUPTIBLE
Maximum number of statements to execute during any single probe hit which is
executed with interrupts enabled (such as begin/end probes), default (MAXACTION *
10).

MAXBACKTRACE
Maximum number of stack frames that will be be processed by the stap runtime
unwinder as produced by the backtrace functions in the [u]context-unwind.stp
tapsets, default 20.

MAXMAPENTRIES
Default maximum number of rows in any single global array, default 2048.
Individual arrays may be declared with a larger or smaller limit instead:

global big[10000],little[5]

or denoted with % to make them wrap-around automatically.

MAXERRORS
Maximum number of soft errors before an exit is triggered, default 0, which means
that the first error will exit the script. Note that with the
--suppress-handler-errors option, this limit is not enforced.

MAXSKIPPED
Maximum number of skipped probes before an exit is triggered, default 100. Running
systemtap with -t (timing) mode gives more details about skipped probes. With the
default -DINTERRUPTIBLE=1 setting, probes skipped due to reentrancy are not
accumulated against this limit. Note that with the --suppress-handler-errors
option, this limit is not enforced.

MINSTACKSPACE
Minimum number of free kernel stack bytes required in order to run a probe handler,
default 1024. This number should be large enough for the probe handler's own
needs, plus a safety margin.

MAXUPROBES
Maximum number of concurrently armed user-space probes (uprobes), default somewhat
larger than the number of user-space probe points named in the script. This pool
needs to be potentially large because individual uprobe objects (about 64 bytes
each) are allocated for each process for each matching script-level probe.

STP_MAXMEMORY
Maximum amount of memory (in kilobytes) that the systemtap module should use,
default unlimited. The memory size includes the size of the module itself, plus
any additional allocations. This only tracks direct allocations by the systemtap
runtime. This does not track indirect allocations (as done by kprobes/uprobes/etc.
internals).

STP_OVEROAD_THRESHOLD, STP_OVERLOAD_INTERVAL
Maximum number of machine cycles spent in probes on any cpu per given interval,
before an overload condition is declared and the script shut down. The defaults
are 500 million and 1 billion, so as to limit stap script cpu consumption at around
50%.

STP_PROCFS_BUFSIZE
Size of procfs probe read buffers (in bytes). Defaults to MAXSTRINGLEN. This
value can be overridden on a per-procfs file basis using the procfs read probe
.maxsize(MAXSIZE) parameter.

With scripts that contain probes on any interrupt path, it is possible that those
interrupts may occur in the middle of another probe handler. The probe in the interrupt
handler would be skipped in this case to avoid reentrance. To work around this issue,
execute stap with the option -DINTERRUPTIBLE=0 to mask interrupts throughout the probe
handler. This does add some extra overhead to the probes, but it may prevent reentrance
for common problem cases. However, probes in NMI handlers and in the callpath of the stap
runtime may still be skipped due to reentrance.

Multiple scripts can write data into a relay buffer concurrently. A host script provides
an interface for accessing its relay buffer to guest scripts. Then, the output of the
guests are merged into the output of the host. To run a script as a host, execute stap
with -DRELAYHOST[=name] option. The name identifies your host script among several hosts.
While running the host, execute stap with -DRELAYGUEST[=name] to add a guest script to the
host. Note that you must unload guests before unloading a host. If there are some guests
connected to the host, unloading the host will be failed.

In case something goes wrong with stap or staprun after a probe has already started
running, one may safely kill both user processes, and remove the active probe kernel
module with rmmod. Any pending trace messages may be lost.

UNPRIVILEGED USERS

Systemtap exposes kernel internal data structures and potentially private user
information. Because of this, use of systemtap's full capabilities are restricted to root
and to users who are members of the groups stapdev and stapusr.

However, a restricted set of systemtap's features can be made available to trusted,
unprivileged users. These users are members of the group stapusr only, or members of the
groups stapusr and stapsys. These users can load systemtap modules which have been
compiled and certified by a trusted systemtap compile-server. See the descriptions of the
options --privilege and --use-server. See README.unprivileged in the systemtap source code
for information about setting up a trusted compile server.

The restrictions enforced when --privilege=stapsys is specified are designed to prevent
unprivileged users from:

· harming the system maliciously.

The restrictions enforced when --privilege=stapusr is specified are designed to prevent
unprivileged users from:

· harming the system maliciously.

· gaining access to information which would not normally be available to an
unprivileged user.

· disrupting the performance of processes owned by other users of the system.
Some overhead to the system in general is unavoidable since the unprivileged
user's probes will be triggered at the appropriate times. What we would like to
avoid is targeted interruption of another user's processes which would not
normally be possible by an unprivileged user.

PROBE RESTRICTIONS
A member of the groups stapusr and stapsys may use all probe points.

A member of only the group stapusr may use only the following probes:

· begin, begin(n)

· end, end(n)

· error(n)

· never

· process.*, where the target process is owned by the user.

· timer.{jiffies,s,sec,ms,msec,us,usec,ns,nsec}(n)*

· timer.hz(n)

SCRIPT LANGUAGE RESTRICTIONS
The following scripting language features are unavailable to all unprivileged users:

· any feature enabled by the Guru Mode (-g) option.

· embedded C code.

RUNTIME RESTRICTIONS
The following runtime restrictions are placed upon all unprivileged users:

· Only the default runtime code (see -R) may be used.

Additional restrictions are placed on members of only the group stapusr:

· Probing of processes owned by other users is not permitted.

· Access of kernel memory (read and write) is not permitted.

COMMAND LINE OPTION RESTRICTIONS
Some command line options provide access to features which must not be available to all
unprivileged users:

· -g may not be specified.

· The following options may not be used by the compile-server client:

-a, -B, -D, -I, -r, -R

ENVIRONMENT RESTRICTIONS
The following environment variables must not be set for all unprivileged users:

SYSTEMTAP_RUNTIME
SYSTEMTAP_TAPSET
SYSTEMTAP_DEBUGINFO_PATH

TAPSET RESTRICTIONS
In general, tapset functions are only available for members of the group stapusr when they
do not gather information that an ordinary program running with that user's privileges
would be denied access to.

There are two categories of unprivileged tapset functions. The first category consists of
utility functions that are unconditionally available to all users; these include such
things as:

cpu:long ()
exit ()
str_replace:string (prnt_str:string, srch_str:string, rplc_str:string)

The second category consists of so-called myproc-unprivileged functions that can only
gather information within their own processes. Scripts that wish to use these functions
must test the result of the tapset function is_myproc and only call these functions if the
result is 1. The script will exit immediately if any of these functions are called by an
unprivileged user within a probe within a process which is not owned by that user.
Examples of myproc-unprivileged functions include:

print_usyms (stk:string)
user_int:long (addr:long)
usymname:string (addr:long)

A compile error is triggered when any function not in either of the above categories is
used by members of only the group stapusr.

No other built-in tapset functions may be used by members of only the group stapusr.

ALTERNATE RUNTIMES

As described above, systemtap's default runtime mode involves building and loading kernel
modules, with various security tradeoffs presented. Systemtap now includes a new
prototype backend, selected with --runtime=dyninst, which uses Dyninst to instrument a
user's own processes at runtime. This backend does not use kernel modules, and does not
require root privileges, but is restricted with respect to the kinds of probes and other
constructs that a script may use.

The dyninst runtime operates in target-attach mode, so it does require a -c COMMAND or -x
PID process. For example:

stap --runtime=dyninst -c 'stap -V'
-e 'probe process.function("main")
{ println("hi from dyninst!") }'

It may be necessary to disable a conflicting selinux check with

# setsebool allow_execstack 1

EXIT STATUS

The systemtap translator generally returns with a success code of 0 if the requested
script was processed and executed successfully through the requested pass. Otherwise,
errors may be printed to stderr and a failure code is returned. Use -v or -vp N to
increase (global or per-pass) verbosity to identify the source of the trouble.

In listings mode (-l and -L), error messages are normally suppressed. A success code of 0
is returned if at least one matching probe was found.

A script executing in pass 5 that is interrupted with ^C / SIGINT is considered to be
successful.

DEPRECATION

Over time, some features of the script language and the tapset library may undergo
incompatible changes, so that a script written against an old version of systemtap may no
longer run. In these cases, it may help to run systemtap with the --compatible VERSION
flag, specifying the last known working version. Running systemtap with the
--check-version flag will output a warning if any possible incompatible elements have been
parsed. Deprecation historical details may be found in the NEWS file.

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