doc/capsh.1

.TH CAPSH 1 "2021-10-22" "libcap" "User Commands"
.SH NAME
capsh \- capability shell wrapper
.SH SYNOPSIS
.B capsh
[\fIOPTION\fR]...
.SH DESCRIPTION
Linux capability support and use can be explored and constrained with
this tool. This tool provides a handy wrapper for certain types
of capability testing and environment creation. It also provides some
debugging features useful for summarizing capability state.
.SH OPTIONS
.B capsh
takes a number of optional arguments, acting on them in the
order they are provided. They are as follows:
.TP
.B \-\-help
Display the list of commands supported by
.BR capsh .
.TP
.B \-\-print
Display prevailing capability and related state.
.TP
.B \-\-current
Display prevailing capability state, 1e capabilities and IAB vector.
.TP
.BI \-\- " [args]"
Execute
.B /bin/bash
with trailing arguments. Note, you can use
.B \-c 'command to execute'
for specific commands.
.TP
.BI \-\+ " [args]"
Uses \fBcap_launch\fP(3) to fork a child to execute the shell. When
the child exits, \fBcapsh\fP exits with the status of the child or 1
in the case that the child was terminated by a signal.
.TP
.BI == " [args]"
Execute
.B capsh
again with the remaining arguments. Useful for testing
.BR exec ()
behavior. Note, PATH is searched when the running
.B capsh
was found via the shell's PATH searching. If the
.B exec
occurs after a
.BI \-\-chroot= /some/path
argument the PATH located binary may not be resolve to the same binary
as that running initially. This behavior is an intended feature as it
can complete the chroot transition.
.TP
.BI =\+ " [args]"
Uses \fBcap_launch\fP(3) to fork a child to re-execute
\fBcapsh\fP. When this child exits, \fBcapsh\fP exits with the status
of the child or 1 in the case that the child was terminated by a
signal.
.TP
.BI \-\-caps= cap-set
Set the prevailing process capabilities to those specified by
.IR cap-set .
Where
.I cap-set
is a text-representation of capability state as per
.BR cap_from_text (3).
.TP
.BI \-\-drop= cap-list
Remove the listed capabilities from the prevailing bounding set. The
capabilities are a comma-separated list of capabilities as recognized
by the
.BR cap_from_name (3)
function. Use of this feature requires that
.B capsh
is operating with
.B CAP_SETPCAP
in its effective set.
.TP
.BI \-\-inh= cap-list
Set the inheritable set of capabilities for the current process to
equal those provided in the comma separated list. For this action to
succeed, the prevailing process should already have each of these
capabilities in the union of the current inheritable and permitted
capability sets, or
.B capsh
should be operating with
.B CAP_SETPCAP
in its effective set.
.TP
.BI \-\-user= username
Assume the identity of the named user. That is, look up the user's
UID and GID
with
.BR getpwuid (3)
and their group memberships with
.BR getgrouplist (3)
and set them all using
.BR cap_setuid (3)
and
.BR cap_setgroups (3).
Following this command, the effective capabilities will be cleared,
but the permitted set will not be, so the running program is still
privileged.
.TP
.B \-\-mode
Display the prevailing libcap mode as guessed by the
.BR cap_get_mode (3)
function.
.TP
.BR \-\-mode= <mode>
Force the program into a
.BR cap_set_mode (3)
security mode. This is a set of securebits and prevailing capability
arrangement recommended for its pre-determined security stance.
.TP
.B \-\-modes
Lists all of the libcap modes supported by
.BR \-\-mode= <mode>.
.TP
.BR \-\-inmode= <mode>
Confirm that the prevailing mode is that specified in
.IR <mode> ,
or exit with a status 1.
.TP
.BI \-\-uid= id
Force all
UID
values to equal
.I id
using the
.BR setuid (2)
system call. This argument may require explicit preparation of the
effective set.
.TP
.BR \-\-cap\-uid= <uid>
use the
.BR cap_setuid (3)
function to set the UID of the current process. This performs all
preparations for setting the UID without dropping capabilities in the
process. Following this command the prevailing effective capabilities
will be lowered.
.TP
.BI \-\-is\-uid= <id>
Exit with status 1 unless the current
UID equals
.IR <id> .
.TP
.BI \-\-gid= <id>
Force all
GID
values to equal
.I id
using the
.BR setgid (2)
system call.
.TP
.BI \-\-is\-gid= <id>
Exit with status 1 unless the current
GIQ equals
.IR <id> .
.TP
.BI \-\-groups= <gid-list>
Set the supplementary groups to the numerical list provided. The
groups are set with the
.BR setgroups (2)
system call. See
.B \-\-user
for a more convenient way of doing this.
.TP
.BI \-\-keep= <0|1>
In a non-pure capability mode, the kernel provides liberal privilege
to the super-user. However, it is normally the case that when the
super-user changes
UID
to some lesser user, then capabilities are dropped. For these
situations, the kernel can permit the process to retain its
capabilities after a
.BR setuid (2)
system call. This feature is known as
.I keep-caps
support. The way to activate it using this program is with this
argument. Setting the value to 1 will cause
.I keep-caps
to be active. Setting it to 0 will cause keep-caps to deactivate for
the current process. In all cases,
.I keep-caps
is deactivated when an
.BR exec ()
is performed. See
.BR \-\-secbits and \-\-mode
for ways to disable this feature.
.TP
.BI \-\-secbits= N
Set the security-bits for the program.
This is done using the
.BR prctl (2)
.B PR_SET_SECUREBITS
operation.  The list of supported bits and their meaning can be found
in the
.B <sys/secbits.h>
header file. The program will list these bits via the
.B \-\-print
command.  The argument is expressed as a numeric bitmask, in any of
the formats permitted by
.BR strtoul (3).
An alternative to this bit-twiddling is embedded in the
.B \-\-mode*
commandline arguments.
.TP
.BI \-\-chroot= /some/path
Execute the
.BR chroot (2)
system call with the new root-directory (/) equal to
.IR path .
This operation requires
.B CAP_SYS_CHROOT
to be in effect.
.TP
.BI \-\-forkfor= sec
This command causes the program to fork a child process for so many
seconds. The child will sleep that long and then exit with status
0. The purpose of this command is to support exploring the way
processes are killable in the face of capability changes. See the
.B \-\-killit
command. Only one fork can be active at a time.
.TP
.BI \-\-killit= sig
This commands causes a
.B \-\-forkfor
child to be
.BR kill (2)d
with the specified signal. The command then waits for the child to exit.
If the exit status does not match the signal being used to kill it, the
.B capsh
program exits with status 1.
.TP
.BI \-\-explain= cap_xxx
Give a brief textual description of what privileges the specified
capability makes available to a running program. Note, instead of
\fIcap_xxx\fP, one can provide a decimal number and \fBcapsh\fP will
look up the corresponding capability's description.
.TP
.BI \-\-shell =/full/path
This option changes the shell that is invoked when the argument
\fB==\fP is encountered.
.TP
.BI \-\-strict
This option toggles the suppression of subsequent attempts to fixup
\fB\-\-caps=\fP and \fB\-\-inh=\fP arguments. That is, when the
prevailing Effective flag does not contain \fBCAP_SETPCAP\fB the to be
raised Inheritable Flag values (in strict mode) are limited to those
in the Permitted set. The strict mode defaults to off. Supplying this
argument an even number of times restores this default behavior.
.TP
.BI \-\-suggest= phrase
Scan each of the textual descriptions of capabilities, known to
\fBcapsh\fP, and display all descriptions that include \fIphrase\fP.
.TP
.BI \-\-decode= N
This is a convenience feature. If you look at
.B /proc/1/status
there are some capability related fields of the following form:
.nf

CapInh:	0000000000000000
CapPrm:	0000003fffffffff
CapEff:	0000003fffffffff
CapBnd:	0000003fffffffff
CapAmb:	0000000000000000

.fi
This option provides a quick way to decode a capability vector
represented in this hexadecimal form.
Here's an example that decodes the two lowest capability bits:
.IP
.nf
$ \fBcapsh \-\-decode=3\fP
0x0000000000000003=cap_chown,cap_dac_override
.fi
.TP
.BI \-\-supports= xxx
As the kernel evolves, more capabilities are added. This option can be used
to verify the existence of a capability on the system. For example,
.BI \-\-supports= cap_syslog
will cause
.B capsh
to promptly exit with a status of 1 when run on
kernel 2.6.27.  However, when run on kernel 2.6.38 it will silently
succeed.
.TP
.BI \-\-has\-p= xxx
Exit with status 1 unless the
.I permitted
vector has capability
.B xxx
raised.
.TP
.B \-\-has\-ambient
Performs a check to see if the running kernel supports ambient
capabilities. If not,
.B capsh
exits with status 1.
.TP
.BI \-\-has\-a= xxx
Exit with status 1 unless the
.I ambient
vector has capability
.B xxx
raised.
.TP
.BI \-\-has\-b= xxx
Exit with status 1 unless the
.I bounding
vector has capability
.B xxx
in its (default) non-blocked state.
.TP
.BI \-\-iab= xxx
Attempts to set the IAB tuple of inheritable capability vectors.
The text conventions used for \fIxxx\fP are those of
.BR cap_iab_from_text (3).
.TP
.BI \-\-addamb= xxx
Adds the specified ambient capability to the running process.
.TP
.BI \-\-delamb= xxx
Removes the specified ambient capability from the running process.
.TP
.B \-\-noamb
Drops all ambient capabilities from the running process.
.TP
.B \-\-quiet
This argument is ignored unless it is the first one. If present, it
suppresses the capsh runtime check to confirm the running libcap is
recent enough that it can name all of the kernel supported capability
values.
.SH "EXIT STATUS"
Following successful execution,
.B capsh
exits with status 0. Following
an error,
.B capsh
immediately exits with status 1.
.SH AUTHOR
Written by Andrew G. Morgan <morgan@kernel.org>.
.SH "REPORTING BUGS"
Please report bugs via:
.TP
https://bugzilla.kernel.org/buglist.cgi?component=libcap&list_id=1090757
.SH "SEE ALSO"
.BR libcap (3),
.BR cap_from_text (3),
.BR cap_iab (3)
.BR capabilities (7),
.BR captree (8),
.BR getcap (8),
.BR getpcaps (8),
and
.BR setcap (8).