userv - Online in the Cloud

PROGRAM:

NAME

userv — request user services

SYNOPSIS

userv [option ...] [--] service-user service-name [argument ...]
userv [option ...] -B | --builtin [--] builtin-service [info-argument ...]

DESCRIPTION

userv is used to have a task performed under different userid while maintaining limited
trust between caller and callee.

service-user specifies which user account is to perform the task. The user may be a login
name or a numeric uid, or ‘-’ to indicate that the service user is to be the same as the
calling user.

The service name is interpreted by the userv daemon on behalf of the service user. This is
controlled by configuration files in the service user's filespace; consult the userv
specification for details.

OPTIONS

Single-letter options may be combined as is usual with Unix programs, and the value for such
an option may appear in the same argument or in the next.

-B | --builtin
Requests that a builtin service be provided. This is equivalent to using the
--override option to specify a string consisting of ‘execute-builtin’ followed
by the builtin-service requested, and requesting a service user of ‘-’
(indicating the calling user).

If the builtin service being requested requires a service-argument then this
must be supplied to the client in the same argument as the builtin-service. See
the specification, or the output of
userv -B help
for details of the builtin services available, and below for details of the
--override options.

The actual service name passed will be the builtin-service; note that this
actual service name (as opposed to the override data) and the info-arguments
supplied will be ignored by most builtin services; the override mechanism and
‘execute-builtin’ will be used to ensure that the right builtin service is
called with the right service-arguments.

-f | --file fd[fdmodifiers]=filename
Requests that data be copied in and out of the service using pipes. For each
file or descriptor this will be done by creating a pipe, one end of which is
passed to the service program and the other end of which is passed to a copy of
cat invoked by the client; the other file descriptor passed to cat will be one
inherited by the client program from the caller or one opened by the client
program on behalf of the caller.

The descriptor in the service program that should be connected must be specified
as fd, either as a decimal number or as one of the strings ‘stdin’, ‘stdout’ or
‘stderr’. The next argument is a filename which will be opened by the client
with the privileges of the calling user.

modifiers is used to specify whether the file or descriptor is to be read from
or written to. It consists of a series of words separated by commas. A comma
may separate the modifiers from the fd and is required if fd is not numeric.
The modifier words are:

read O_RDONLY: Allow reading and not writing. May not be used with
‘write’ or things that imply it.

write O_WRONLY: Allow writing and not reading. Doesn't truncate or
create without ‘truncate’ or ‘create’. ‘write’ or things that
imply it may not be used with ‘read’.

overwrite Equivalent to ‘write,create,truncate’.

create, creat O_CREAT: Creates the file if necessary. Implies ‘write’.

exclusive, excl O_EXCL: Fails if the file already exists. Implies write and
create. May not be used with ‘truncate’.

truncate, trunc O_TRUNC: Truncate any existing file. Implies ‘write’. May
not be used with ‘exclusive’.

append O_APPEND: All writes will append to the file. Implies ‘write’
(but not ‘create’).

sync O_SYNC: Do writes synchronously. Implies ‘write’.

wait, nowait, close
These modifiers control the behaviour of the client, with
respect to the pipes carrying data to and from the service,
when the service terminates. See below.

fd The filename is not a filename but a numeric file descriptor.
One or both of ‘read’ and ‘write’ must be specified, and no
other words are allowed. The filename may also be ‘stdin’,
‘stdout’ or ‘stderr’ for file descriptor 0, 1 or 2
respectively.

If no modifiers which imply ‘read’ or ‘write’ are used it is as if ‘write’ had
been specified, except that if the filedescriptor 0 of the service is being
opened (either specified numerically or with ‘stdin’) it is as if ‘overwrite’
had been specified (or ‘write’ if only ‘fd’ was specified).

The client will also use O_NOCTTY when opening files specified by the caller, to
avoid changing its controlling terminal.

By default stdin, stdout and stderr of the service will be connected to the
corresponding descriptors on the client. Diagnostics from the client and daemon
will also appear on stderr.

If ‘wait’ is specified, the client will wait for the pipe to be closed, and only
exit after this has happened. This means that either the receiving end of the
pipe connection was closed while data was still available at the sending end, or
that the end of file was reached on the reading file descriptor. Errors
encountered reading or writing in the client at this stage will be considered a
system error and cause the client to exit with status 255, but will not cause
disconnection at the service side since the service has already exited.

If ‘close’ is specified the client will immediately close the pipe connection by
killing the relevant copy of cat. If the service uses the descriptor it will
get SIGPIPE (or EPIPE) for a writing descriptor or end of file for a reading
one; the descriptor opened by or passed to the client will also be closed.

If ‘nowait’ is specified then the client will not wait and the connection will
remain open after the client terminates. Data may continue to be passed between
the inheritors of the relevant descriptor on the service side and the
corresponding file or descriptor on the client side until either side closes
their descriptor. This should not usually be specified for stderr (or stdout if
‘--signals stdout’ is used) since diagnostics from the service side may arrive
after the client has exited and be confused with expected output.

The default is ‘wait’ for writing file descriptors and ‘close’ for reading ones.

-w | --fdwait fd=action
Sets the action on termination of the service for the specified file descriptor;
action must be ‘wait’, ‘nowait’ or ‘close’ as described above. The file
descriptor must be specified as open when this option is encountered; this
option is overridden by any later --file or --fdwait option - even by a --file
which does not specify an action on termination (in this case the default will
be used, as described above).

-D | --defvar name=value
Set a user-defined variable name to value. These user-defined variables are
made available in the configuration language as the parameters ‘u-name’ and are
passed to the service in environment variables USERV_U_name. name may contain
only alphanumerics and underscores, and must start with a letter. If several
definitions are given for the same name then only the last is effective.

-t | --timeout seconds
Time out the service if it takes longer than seconds seconds (a positive
integer, in decimal). Timeout will produce a diagnostic on stderr and an exit
status of 255. If seconds is zero then no timeout will be implemented (this is
the default).

-S | --signals method
Affects the handling of the exit status when the service terminates due to a
signal. (The client will always finish by calling _exit(), so that only numbers
from 0 to 255 can be returned and not the full range of numbers and signal
indications which can be returned by the wait() family of system calls.)

The method may be one of the following:

status The client's exit status will be status. This will not be
distinguishable from the service really having exited with
code status. This method is the default, with a status of
254.

number, number-nocore
The client's exit status will be the number of the signal
which caused the termination of the service. If ‘number’ is
used rather than ‘number-nocore’ then 128 will be added if the
service dumped core. ‘number’ is very like the exit code
mangling done by the Bourne shell.

highbit The client's exit status will be the number of the signal with
128 added. If the service exits normally with an exit code of
greater than 127 then 127 will be returned.

stdout The service's numeric wait status as two decimal numbers (high
byte first) and a textual description of its meaning will be
printed to the client's standard output. It will be preceded
by a newline and followed by an extra newline, and the numbers
are separated from each other and from the textual description
by single spaces. The exit status of the client will be zero,
unless a system error occurs in which case no exit status and
description will be printed to stdout, and an error message
will be printed to stderr as usual.

Problems such as client usage errors, the service not being
found or permission being denied or failure of a system call
are system errors. An error message describing the problem
will be printed on the client's stderr, and the client's exit
status will be 255. If the client dies due to a signal this
should be treated as a serious system error.

-H | --hidecwd
Prevents the calling process's current directory name from being passed to the
service; the null string will be passed instead.

-P | --sigpipe
If the service program is terminated due to a SIGPIPE the exit status of the
client will be zero, even if it would have been something else according to the
exit status method specified. This option has no effect on the code and
description printed if the exit status method ‘stdout’ is in use.

-h | --help
Prints the client's usage message.

--copyright
Prints the copyright and lack of warranty notice.

SECURITY-OVERRIDING OPTIONS

There are also some options which are available for debugging and to allow the system
administrator to override a user's policy. These options are available only if the client
is called by root or if the calling user is the same as the service user.

--override configuration-data

--override-file file
Do not read the usual configuration files. Instead, the client sends
configuration-data (followed by a newline) or the contents of filename (which is
opened in the context of the client) to the daemon and the daemon uses that data
instead. The configuration-data must all be in one argument. It will have a
single newline appended so that a single directive can easily be given, but if
more than one directive is required it will have to contain one or more real
newlines.

--spoof-user user
Pretend to the service that it is being called by user (which may be a username
or a uid). This will also affect the group and supplementary groups supplied to
the service; they will be the standard group and supplementary groups for user.
The --spoof-user option will not affect which user is chosen if the service user
is specified as just ‘-’; in this case the service user will be the real calling
user.

ENVIRONMENT

LOGNAME, USER These are used to determine the name of the calling user, to be passed to
the service in USERV_USER. Their values will only be used if they
correspond to the calling UID.

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