Provided by: userv_1.2.0_amd64 
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.
FILES
/var/run/userv/socket UNIX-domain socket used for communication between userv and uservd.
/var/run/userv/%x.%x.%x Pipes used for connecting file descriptors in the client and the service.
SEE ALSO
uservd(8)
Ian Jackson, User service daemon and client specification.
COPYRIGHT
GNU userv is Copyright 1996-2017 Ian Jackson; Copyright 2000 Ben Harris; and Copyright 2016-2017 Peter
Benie.
GNU userv is licensed under the terms of the GNU General Public Licence, version 3 or (at your option) any
later version, and it comes with NO WARRANTY, not even the implied warranty of MERCHANTABILITY or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU General Public License for details.
You should have received a copy of the GNU General Public License along with userv, if not, see
http://www.gnu.org/licenses/
HISTORY
userv was initially written in 1996 by Ian Jackson. It became GNU userv in 1999, and version 1.0 was
released in 2000.