.\" Hey Emacs! This file is -*- nroff -*- source.
.\"
.\" Copyright (C) 1998 Andries Brouwer (aeb@cwi.nl)
.\" and Copyright (C) 2002 Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright Guillem Jover <guillem@hadrons.org>
.\"
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.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
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.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" Modified Thu Nov 11 04:19:42 MET 1999, aeb: added PR_GET_PDEATHSIG
.\" Modified 27 Jun 02, Michael Kerrisk
.\" 	Added PR_SET_DUMPABLE, PR_GET_DUMPABLE,
.\"	PR_SET_KEEPCAPS, PR_GET_KEEPCAPS
.\" Modified 2006-08-30 Guillem Jover <guillem@hadrons.org>
.\"	Updated Linux versions where the options where introduced.
.\"	Added PR_SET_TIMING, PR_GET_TIMING, PR_SET_NAME, PR_GET_NAME,
.\"	PR_SET_UNALIGN, PR_GET_UNALIGN, PR_SET_FPEMU, PR_GET_FPEMU,
.\"	PR_SET_FPEXC, PR_GET_FPEXC
.\" 2008-04-29 Serge Hallyn, Document PR_CAPBSET_READ and PR_CAPBSET_DROP
.\" 2008-06-13 Erik Bosman, <ejbosman@cs.vu.nl>
.\"     Document PR_GET_TSC and PR_SET_TSC.
.\" 2008-06-15 mtk, Document PR_SET_SECCOMP, PR_GET_SECCOMP
.\" 2009-10-03 Andi Kleen, document PR_MCE_KILL_*
.\"
.\" FIXME: Document PR_SET_TIMERSLACK and PR_GET_TIMERSLACK (new in 2.6.28)
.\" FIXME: Document PR_TASK_PERF_EVENTS_DISABLE and
.\"        PR_TASK_PERF_EVENTS_ENABLE (new in 2.6.32)
.\"
.TH PRCTL 2 2010-05-13 "Linux" "Linux Programmer's Manual"
.SH NAME
prctl \- operations on a process
.SH SYNOPSIS
.nf
.B #include <sys/prctl.h>
.sp
.BI "int prctl(int " option ", unsigned long " arg2 ", unsigned long " arg3 ,
.BI "          unsigned long " arg4 ", unsigned long " arg5 );
.fi
.SH DESCRIPTION
.BR prctl ()
is called with a first argument describing what to do
(with values defined in \fI<linux/prctl.h>\fP), and further
arguments with a significance depending on the first one.
The first argument can be:
.TP
.BR PR_CAPBSET_READ " (since Linux 2.6.25)
Return (as the function result) 1 if the capability specified in
.I arg2
is in the calling thread's capability bounding set,
or 0 if it is not.
(The capability constants are defined in
.IR <linux/capability.h> .)
The capability bounding set dictates
whether the process can receive the capability through a
file's permitted capability set on a subsequent call to
.BR execve (2).

If the capability specified in
.I arg2
is not valid, then the call fails with the error
.BR EINVAL .
.TP
.BR PR_CAPBSET_DROP " (since Linux 2.6.25)"
If the calling thread has the
.B CAP_SETPCAP
capability, then drop the capability specified by
.I arg2
from the calling thread's capability bounding set.
Any children of the calling thread will inherit the newly
reduced bounding set.

The call fails with the error:
.B EPERM
if the calling thread does not have the
.BR CAP_SETPCAP ;
.BR EINVAL
if
.I arg2
does not represent a valid capability; or
.BR EINVAL
if file capabilities are not enabled in the kernel,
in which case bounding sets are not supported.
.TP
.BR PR_SET_DUMPABLE " (since Linux 2.3.20)"
Set the state of the flag determining whether core dumps are produced
for this process upon delivery of a signal whose default behavior is
to produce a core dump.
(Normally this flag is set for a process by default, but it is cleared
when a set-user-ID or set-group-ID program is executed and also by
various system calls that manipulate process UIDs and GIDs).
In kernels up to and including 2.6.12,
.I arg2
must be either 0 (process is not dumpable) or 1 (process is dumpable).
Between kernels 2.6.13 and 2.6.17, the value 2 was also permitted,
which caused any binary which normally would not be dumped
to be dumped readable by root only;
for security reasons, this feature has been removed.
.\" See http://marc.theaimsgroup.com/?l=linux-kernel&m=115270289030630&w=2
.\" Subject:    Fix prctl privilege escalation (CVE-2006-2451)
.\" From:       Marcel Holtmann <marcel () holtmann ! org>
.\" Date:       2006-07-12 11:12:00
(See also the description of
.I /proc/sys/fs/suid_dumpable
in
.BR proc (5).)
.TP
.BR PR_GET_DUMPABLE " (since Linux 2.3.20)"
Return (as the function result) the current state of the calling
process's dumpable flag.
.\" Since Linux 2.6.13, the dumpable flag can have the value 2,
.\" but in 2.6.13 PR_GET_DUMPABLE simply returns 1 if the dumpable
.\" flags has a nonzero value.  This was fixed in 2.6.14.
.TP
.BR PR_SET_ENDIAN " (since Linux 2.6.18, PowerPC only)"
Set the endian-ness of the calling process to the value given
in \fIarg2\fP, which should be one of the following:
.\" Respectively 0, 1, 2
.BR PR_ENDIAN_BIG ,
.BR PR_ENDIAN_LITTLE ,
or
.B PR_ENDIAN_PPC_LITTLE
(PowerPC pseudo little endian).
.TP
.BR PR_GET_ENDIAN " (since Linux 2.6.18, PowerPC only)"
Return the endian-ness of the calling process,
in the location pointed to by
.IR "(int\ *) arg2" .
.TP
.BR PR_SET_FPEMU " (since Linux 2.4.18, 2.5.9, only on ia64)"
Set floating-point emulation control bits to \fIarg2\fP.
Pass \fBPR_FPEMU_NOPRINT\fP to silently emulate fp operations accesses, or
\fBPR_FPEMU_SIGFPE\fP to not emulate fp operations and send
.B SIGFPE
instead.
.TP
.BR PR_GET_FPEMU " (since Linux 2.4.18, 2.5.9, only on ia64)"
Return floating-point emulation control bits,
in the location pointed to by
.IR "(int\ *) arg2" .
.TP
.BR PR_SET_FPEXC " (since Linux 2.4.21, 2.5.32, only on PowerPC)"
Set floating-point exception mode to \fIarg2\fP.
Pass \fBPR_FP_EXC_SW_ENABLE\fP to use FPEXC for FP exception enables,
\fBPR_FP_EXC_DIV\fP for floating-point divide by zero,
\fBPR_FP_EXC_OVF\fP for floating-point overflow,
\fBPR_FP_EXC_UND\fP for floating-point underflow,
\fBPR_FP_EXC_RES\fP for floating-point inexact result,
\fBPR_FP_EXC_INV\fP for floating-point invalid operation,
\fBPR_FP_EXC_DISABLED\fP for FP exceptions disabled,
\fBPR_FP_EXC_NONRECOV\fP for async nonrecoverable exception mode,
\fBPR_FP_EXC_ASYNC\fP for async recoverable exception mode,
\fBPR_FP_EXC_PRECISE\fP for precise exception mode.
.TP
.BR PR_GET_FPEXC " (since Linux 2.4.21, 2.5.32, only on PowerPC)"
Return floating-point exception mode,
in the location pointed to by
.IR "(int\ *) arg2" .
.TP
.BR PR_SET_KEEPCAPS " (since Linux 2.2.18)"
Set the state of the thread's "keep capabilities" flag,
which determines whether the threads's permitted
capability set is cleared when a change is made to the threads's user IDs
such that the threads's real UID, effective UID, and saved set-user-ID
all become nonzero when at least one of them previously had the value 0.
By default, the permitted capability set is cleared when such a change is made;
setting the "keep capabilities" flag prevents it from being cleared.
.I arg2
must be either 0 (permitted capabilities are cleared)
or 1 (permitted capabilities are kept).
(A thread's
.I effective
capability set is always cleared when such a credential change is made,
regardless of the setting of the "keep capabilities" flag.)
The "keep capabilities" value will be reset to 0 on subsequent calls to
.BR execve (2).
.TP
.BR PR_GET_KEEPCAPS " (since Linux 2.2.18)"
Return (as the function result) the current state of the calling threads's
"keep capabilities" flag.
.TP
.BR PR_SET_NAME " (since Linux 2.6.9)"
Set the process name for the calling process,
using the value in the location pointed to by
.IR "(char\ *) arg2" .
The name can be up to 16 bytes long,
.\" TASK_COMM_LEN in include/linux/sched.h
and should be null-terminated if it contains fewer bytes.
.TP
.BR PR_GET_NAME " (since Linux 2.6.11)"
Return the process name for the calling process,
in the buffer pointed to by
.IR "(char\ *) arg2" .
The buffer should allow space for up to 16 bytes;
the returned string will be null-terminated if it is shorter than that.
.TP
.BR PR_SET_PDEATHSIG " (since Linux 2.1.57)"
Set the parent process death signal
of the calling process to \fIarg2\fP (either a signal value
in the range 1..maxsig, or 0 to clear).
This is the signal that the calling process will get when its
parent dies.
This value is cleared for the child of a
.BR fork (2).
.TP
.BR PR_GET_PDEATHSIG " (since Linux 2.3.15)"
Return the current value of the parent process death signal,
in the location pointed to by
.IR "(int\ *) arg2" .
.TP
.BR PR_SET_SECCOMP " (since Linux 2.6.23)"
.\" See http://thread.gmane.org/gmane.linux.kernel/542632
.\" [PATCH 0 of 2] seccomp updates
.\" andrea@cpushare.com
Set the secure computing mode for the calling thread.
In the current implementation,
.IR arg2
must be 1.
After the secure computing mode has been set to 1,
the only system calls that the thread is permitted to make are
.BR read (2),
.BR write (2),
.BR _exit (2),
and
.BR sigreturn (2).
Other system calls result in the delivery of a
.BR SIGKILL
signal.
Secure computing mode is useful for number-crunching applications
that may need to execute untrusted byte code,
perhaps obtained by reading from a pipe or socket.
This operation is only available
if the kernel is configured with CONFIG_SECCOMP enabled.
.TP
.BR PR_GET_SECCOMP " (since Linux 2.6.23)"
Return the secure computing mode of the calling thread.
Not very useful for the current implementation (mode equals 1),
but may be useful for other possible future modes:
if the caller is not in secure computing mode, this operation returns 0;
if the caller is in secure computing mode, then the
.BR prctl ()
call will cause a
.B SIGKILL
signal to be sent to the process.
This operation is only available
if the kernel is configured with CONFIG_SECCOMP enabled.
.TP
.BR PR_SET_SECUREBITS " (since Linux 2.6.26)"
Set the "securebits" flags of the calling thread to the value supplied in
.IR arg2 .
See
.BR capabilities (7).
.TP
.BR PR_GET_SECUREBITS " (since Linux 2.6.26)"
Return (as the function result)
the "securebits" flags of the calling thread.
See
.BR capabilities (7).
.TP
.BR PR_SET_TIMING " (since Linux 2.6.0-test4)"
Set whether to use (normal, traditional) statistical process timing or
accurate timestamp based process timing, by passing
.B PR_TIMING_STATISTICAL
.\" 0
or
.B PR_TIMING_TIMESTAMP
.\" 1
to \fIarg2\fP.
.B PR_TIMING_TIMESTAMP
is not currently implemented
(attempting to set this mode will yield the error
.BR EINVAL ).
.\" PR_TIMING_TIMESTAMP doesn't do anything in 2.6.26-rc8,
.\" and looking at the patch history, it appears
.\" that it never did anything.
.TP
.BR PR_GET_TIMING " (since Linux 2.6.0-test4)"
Return (as the function result) which process timing method is currently
in use.
.TP
.BR PR_SET_TSC " (since Linux 2.6.26, x86 only)"
Set the state of the flag determining whether the timestamp counter
can be read by the process.
Pass
.B PR_TSC_ENABLE
to
.I arg2
to allow it to be read, or
.B PR_TSC_SIGSEGV
to generate a
.B SIGSEGV
when the process tries to read the timestamp counter.
.TP
.BR PR_GET_TSC " (since Linux 2.6.26, x86 only)"
Return the state of the flag determining whether the timestamp counter
can be read,
in the location pointed to by
.IR "(int\ *) arg2" .
.TP
.B PR_SET_UNALIGN
(Only on: ia64, since Linux 2.3.48; parisc, since Linux 2.6.15;
PowerPC, since Linux 2.6.18; Alpha, since Linux 2.6.22)
Set unaligned access control bits to \fIarg2\fP.
Pass
\fBPR_UNALIGN_NOPRINT\fP to silently fix up unaligned user accesses,
or \fBPR_UNALIGN_SIGBUS\fP to generate
.B SIGBUS
on unaligned user access.
.TP
.B PR_GET_UNALIGN
(see
.B PR_SET_UNALIGN
for information on versions and architectures)
Return unaligned access control bits, in the location pointed to by
.IR "(int\ *) arg2" .
.TP
.BR PR_MCE_KILL " (since Linux 2.6.32)"
Set the machine check memory corruption kill policy for the current thread.
If
.I arg2
is
.BR PR_MCE_KILL_CLEAR ,
clear the thread memory corruption kill policy and use the system-wide default.
(The system-wide default is defined by
.IR /proc/sys/vm/memory_failure_early_kill ;
see
.BR proc (5).)
If
.I arg2
is
.BR PR_MCE_KILL_SET ,
use a thread-specific memory corruption kill policy.
In this case,
.I arg3
defines whether the policy is
.I early kill
.RB ( PR_MCE_KILL_EARLY ),
.I late kill
.RB ( PR_MCE_KILL_LATE ),
or the system-wide default
.RB ( PR_MCE_KILL_DEFAULT ).
Early kill means that the task receives a
.B SIGBUS
signal as soon as hardware memory corruption is detected inside
its address space.
In late kill mode, the process is only killed when it accesses a corrupted page.
See
.I sigaction(2)
for more information on the
.BR SIGBUS
signal.
The policy is inherited by children.
The remaining unused
.BR prctl ()
arguments must be zero for future compatibility.
.TP
.BR PR_MCE_KILL_GET " (since Linux 2.6.32)"
Return the current per-process machine check kill policy.
All unused
.BR prctl ()
arguments must be zero.
.SH "RETURN VALUE"
On success,
.BR PR_GET_DUMPABLE ,
.BR PR_GET_KEEPCAPS ,
.BR PR_CAPBSET_READ ,
.BR PR_GET_TIMING ,
.BR PR_GET_SECUREBITS ,
.BR PR_MCE_KILL_GET ,
and (if it returns)
.BR PR_GET_SECCOMP
return the nonnegative values described above.
All other
.I option
values return 0 on success.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EFAULT
.I arg2
is an invalid address.
.TP
.B EINVAL
The value of
.I option
is not recognized.
.TP
.B EINVAL
.I option
is
.BR PR_MCE_KILL
or
.BR PR_MCE_KILL_GET ,
and unused
.BR prctl ()
arguments were not specified as zero.
.TP
.B EINVAL
.I arg2
is not valid value for this
.IR option .
.TP
.B EINVAL
.I option
is
.BR PR_SET_SECCOMP
or
.BR PR_SET_SECCOMP ,
and the kernel was not configured with
.BR CONFIG_SECCOMP .
.TP
.B EPERM
.I option
is
.BR PR_SET_SECUREBITS ,
and the caller does not have the
.B CAP_SETPCAP
capability,
or tried to unset a "locked" flag,
or tried to set a flag whose corresponding locked flag was set
(see
.BR capabilities (7)).
.TP
.B EPERM
.I option
is
.BR PR_SET_KEEPCAPS ,
and the callers's
.B SECURE_KEEP_CAPS_LOCKED
flag is set
(see
.BR capabilities (7)).
.TP
.B EPERM
.I option
is
.BR PR_CAPBSET_DROP ,
and the caller does not have the
.B CAP_SETPCAP
capability.
.\" The following can't actually happen, because prctl() in
.\" seccomp mode will cause SIGKILL.
.\" .TP
.\" .B EPERM
.\" .I option
.\" is
.\" .BR PR_SET_SECCOMP ,
.\" and secure computing mode is already 1.
.SH VERSIONS
The
.BR prctl ()
system call was introduced in Linux 2.1.57.
.\" The library interface was added in glibc 2.0.6
.SH "CONFORMING TO"
This call is Linux-specific.
IRIX has a
.BR prctl ()
system call (also introduced in Linux 2.1.44
as irix_prctl on the MIPS architecture),
with prototype
.sp
.BI "ptrdiff_t prctl(int " option ", int " arg2 ", int " arg3 );
.sp
and options to get the maximum number of processes per user,
get the maximum number of processors the calling process can use,
find out whether a specified process is currently blocked,
get or set the maximum stack size, etc.
.SH "SEE ALSO"
.BR signal (2),
.BR core (5)
.SH COLOPHON
This page is part of release 3.27 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.