prctl.2: Rework the PR_SET_PDEATHSIG description a little, for easier readability

[thirdparty/man-pages.git] / man2 / prctl.2

.\" Copyright (C) 1998 Andries Brouwer (aeb@cwi.nl)
.\" and Copyright (C) 2002, 2006, 2008, 2012, 2013 Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright Guillem Jover <guillem@hadrons.org>
.\" and Copyright (C) 2014 Dave Hansen / Intel
.\"
.\" %%%LICENSE_START(VERBATIM)
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.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" 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.\" manual page may be incorrect or out-of-date.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
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.\" %%%LICENSE_END
.\"
.\" 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
.\" 2012-04 Cyrill Gorcunov, Document PR_SET_MM
.\" 2012-04-25 Michael Kerrisk, Document PR_TASK_PERF_EVENTS_DISABLE and
.\"				PR_TASK_PERF_EVENTS_ENABLE
.\" 2012-09-20 Kees Cook, update PR_SET_SECCOMP for mode 2
.\" 2012-09-20 Kees Cook, document PR_SET_NO_NEW_PRIVS, PR_GET_NO_NEW_PRIVS
.\" 2012-10-25 Michael Kerrisk, Document PR_SET_TIMERSLACK and
.\"                             PR_GET_TIMERSLACK
.\" 2013-01-10 Kees Cook, document PR_SET_PTRACER
.\" 2012-02-04 Michael Kerrisk, document PR_{SET,GET}_CHILD_SUBREAPER
.\" 2014-11-10 Dave Hansen, document PR_MPX_{EN,DIS}ABLE_MANAGEMENT
.\"
.\"
.TH PRCTL 2 2018-02-02 "Linux" "Linux Programmer's Manual"
.SH NAME
prctl \- operations on a process
.SH SYNOPSIS
.nf
.B #include <sys/prctl.h>
.PP
.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_CAP_AMBIENT " (since Linux 4.3)"
.\" commit 58319057b7847667f0c9585b9de0e8932b0fdb08
Reads or changes the ambient capability set of the calling thread,
according to the value of
.IR arg2 ,
which must be one of the following:
.RS
.\"
.TP
.B PR_CAP_AMBIENT_RAISE
The capability specified in
.I arg3
is added to the ambient set.
The specified capability must already be present in
both the permitted and the inheritable sets of the process.
This operation is not permitted if the
.B SECBIT_NO_CAP_AMBIENT_RAISE
securebit is set.
.TP
.B PR_CAP_AMBIENT_LOWER
The capability specified in
.I arg3
is removed from the ambient set.
.TP
.B PR_CAP_AMBIENT_IS_SET
The
.BR prctl ()
call returns 1 if the capability in
.I arg3
is in the ambient set and 0 if it is not.
.TP
.BR PR_CAP_AMBIENT_CLEAR_ALL
All capabilities will be removed from the ambient set.
This operation requires setting
.I arg3
to zero.
.RE
.IP
In all of the above operations,
.I arg4
and
.I arg5
must be specified as 0.
.IP
Higher-level interfaces layered on top of the above operations are
provided in the
.BR libcap (3)
library in the form of
.BR cap_get_ambient (3),
.BR cap_set_ambient (3),
and
.BR cap_reset_ambient (3).
.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).
.IP
If the capability specified in
.I arg2
is not valid, then the call fails with the error
.BR EINVAL .
.IP
A higher-level interface layered on top of this operation is provided in the
.BR libcap (3)
library in the form of
.BR cap_get_bound (3).
.TP
.BR PR_CAPBSET_DROP " (since Linux 2.6.25)"
If the calling thread has the
.B CAP_SETPCAP
capability within its user namespace, 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.
.IP
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.
.IP
A higher-level interface layered on top of this operation is provided in the
.BR libcap (3)
library in the form of
.BR cap_drop_bound (3).
.TP
.BR PR_SET_CHILD_SUBREAPER " (since Linux 3.4)"
.\" commit ebec18a6d3aa1e7d84aab16225e87fd25170ec2b
If
.I arg2
is nonzero,
set the "child subreaper" attribute of the calling process;
if
.I arg2
is zero, unset the attribute.
.IP
A subreaper fulfills the role of
.BR init (1)
for its descendant processes.
When a process becomes orphaned
(i.e., its immediate parent terminates)
then that process will be reparented to
the nearest still living ancestor subreaper.
Subsequently, calls to
.BR getppid ()
in the orphaned process will now return the PID of the subreaper process,
and when the orphan terminates, it is the subreaper process that
will receive a
.BR SIGCHLD
signal and will be able to
.BR wait (2)
on the process to discover its termination status.
.IP
The setting of this bit is not inherited by children created by
.BR fork (2)
and
.BR clone (2).
The setting is preserved across
.BR execve (2).
.IP
Establishing a subreaper process is useful in session management frameworks
where a hierarchical group of processes is managed by a subreaper process
that needs to be informed when one of the processes\(emfor example,
a double-forked daemon\(emterminates
(perhaps so that it can restart that process).
Some
.BR init (1)
frameworks (e.g.,
.BR systemd (1))
employ a subreaper process for similar reasons.
.TP
.BR PR_GET_CHILD_SUBREAPER " (since Linux 3.4)"
Return the "child subreaper" setting of the caller,
in the location pointed to by
.IR "(int\ *) arg2" .
.TP
.BR PR_SET_DUMPABLE " (since Linux 2.3.20)"
Set the state of the "dumpable" flag,
which determines whether core dumps are produced for the calling process
upon delivery of a signal whose default behavior is to produce a core dump.
.IP
In kernels up to and including 2.6.12,
.I arg2
must be either 0
.RB ( SUID_DUMP_DISABLE ,
process is not dumpable) or 1
.RB ( SUID_DUMP_USER ,
process is dumpable).
Between kernels 2.6.13 and 2.6.17,
.\" commit abf75a5033d4da7b8a7e92321d74021d1fcfb502
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).)
.IP
Normally, this flag is set to 1.
However, it is reset to the current value contained in the file
.IR /proc/sys/fs/\:suid_dumpable
(which by default has the value 0),
in the following circumstances:
.\" See kernel/cred.c::commit_creds() (Linux 3.18 sources)
.RS
.IP * 3
The process's effective user or group ID is changed.
.IP *
The process's filesystem user or group ID is changed (see
.BR credentials (7)).
.IP *
The process executes
.RB ( execve (2))
a set-user-ID or set-group-ID program, resulting in a change
of either the effective user ID or the effective group ID.
.IP *
The process executes
.RB ( execve (2))
a program that has file capabilities (see
.BR capabilities (7)),
.\" See kernel/cred.c::commit_creds()
but only if the permitted capabilities
gained exceed those already permitted for the process.
.\" Also certain namespace operations;
.RE
.IP
Processes that are not dumpable can not be attached via
.BR ptrace (2)
.BR PTRACE_ATTACH ;
see
.BR ptrace (2)
for further details.
.IP
If a process is not dumpable,
the ownership of files in the process's
.IR /proc/[pid]
directory is affected as described 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_FP_MODE " (since Linux 4.0, only on MIPS)"
.\" commit 9791554b45a2acc28247f66a5fd5bbc212a6b8c8
On the MIPS architecture,
user-space code can be built using an ABI which permits linking
with code that has more restrictive floating-point (FP) requirements.
For example, user-space code may be built to target the O32 FPXX ABI
and linked with code built for either one of the more restrictive
FP32 or FP64 ABIs.
When more restrictive code is linked in,
the overall requirement for the process is to use the more
restrictive floating-point mode.
.IP
Because the kernel has no means of knowing in advance
which mode the process should be executed in,
and because these restrictions can
change over the lifetime of the process, the
.B PR_SET_FP_MODE
operation is provided to allow control of the floating-point mode
from user space.
.IP
.\" https://dmz-portal.mips.com/wiki/MIPS_O32_ABI_-_FR0_and_FR1_Interlinking
The
.I (unsigned int) arg2
argument is a bit mask describing the floating-point mode used:
.RS
.TP
.BR PR_FP_MODE_FR
When this bit is
.I unset
(so called
.BR FR=0 " or " FR0
mode), the 32 floating-point registers are 32 bits wide,
and 64-bit registers are represented as a pair of registers
(even- and odd- numbered,
with the even-numbered register containing the lower 32 bits,
and the odd-numbered register containing the higher 32 bits).
.IP
When this bit is
.I set
(on supported hardware),
the 32 floating-point registers are 64 bits wide (so called
.BR FR=1 " or " FR1
mode).
Note that modern MIPS implementations (MIPS R6 and newer) support
.B FR=1
mode only.
.IP
.IP
Applications that use the O32 FP32 ABI can operate only when this bit is
.I unset
.RB ( FR=0 ;
or they can be used with FRE enabled, see below).
Applications that use the O32 FP64 ABI
(and the O32 FP64A ABI, which exists to
provide the ability to operate with existing FP32 code; see below)
can operate only when this bit is
.I set
.RB ( FR=1 ).
Applications that use the O32 FPXX ABI can operate with either
.BR FR=0
or
.BR FR=1 .
.TP
.BR PR_FP_MODE_FRE
Enable emulation of 32-bit floating-point mode.
When this mode is enabled,
it emulates 32-bit floating-point operations
by raising a reserved-instruction exception
on every instruction that uses 32-bit formats and
the kernel then handles the instruction in software.
(The problem lies in the discrepancy of handling odd-numbered registers
which are the high 32 bits of 64-bit registers with even numbers in
.B FR=0
mode and the lower 32-bit parts of odd-numbered 64-bit registers in
.B FR=1
mode.)
Enabling this bit is necessary when code with the O32 FP32 ABI should operate
with code with compatible the O32 FPXX or O32 FP64A ABIs (which require
.B FR=1
FPU mode) or when it is executed on newer hardware (MIPS R6 onwards)
which lacks
.B FR=0
mode support when a binary with the FP32 ABI is used.
.IP
Note that this mode makes sense only when the FPU is in 64-bit mode
.RB ( FR=1 ).
.IP
Note that the use of emulation inherently has a significant performance hit
and should be avoided if possible.
.RE
.IP
In the N32/N64 ABI, 64-bit floating-point mode is always used,
so FPU emulation is not required and the FPU always operates in
.B FR=1
mode.
.IP
This option is mainly intended for use by the dynamic linker
.RB ( ld.so (8)).
.IP
The arguments
.IR arg3 ,
.IR arg4 ,
and
.IR arg5
are ignored.
.TP
.BR PR_GET_FP_MODE " (since Linux 4.0, only on MIPS)"
Get the current floating-point mode (see the description of
.B PR_SET_FP_MODE
for details).
.IP
On success,
the call returns a bit mask which represents the current floating-point mode.
.IP
The arguments
.IR arg2 ,
.IR arg3 ,
.IR arg4 ,
and
.IR arg5
are ignored.
.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
.B PR_FPEMU_NOPRINT
to silently emulate floating-point operation accesses, or
.B PR_FPEMU_SIGFPE
to not emulate floating-point 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 calling thread's "keep capabilities" flag.
The effect if this flag is described in
.BR capabilities (7).
.I arg2
must be either 0 (clear the flag)
or 1 (set the 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 thread's
"keep capabilities" flag.
See
.BR capabilities (7)
for a description of this flag.
.TP
.BR PR_MCE_KILL " (since Linux 2.6.32)"
Set the machine check memory corruption kill policy for the calling 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 thread receives a
.B SIGBUS
signal as soon as hardware memory corruption is detected inside
its address space.
In late kill mode, the process is killed only when it accesses a corrupted page.
See
.BR 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.
.TP
.BR PR_SET_MM " (since Linux 3.3)"
.\" commit 028ee4be34a09a6d48bdf30ab991ae933a7bc036
Modify certain kernel memory map descriptor fields
of the calling process.
Usually these fields are set by the kernel and dynamic loader (see
.BR ld.so (8)
for more information) and a regular application should not use this feature.
However, there are cases, such as self-modifying programs,
where a program might find it useful to change its own memory map.
.IP
The calling process must have the
.BR CAP_SYS_RESOURCE
capability.
The value in
.I arg2
is one of the options below, while
.I arg3
provides a new value for the option.
The
.I arg4
and
.I arg5
arguments must be zero if unused.
.IP
Before Linux 3.10,
.\" commit 52b3694157e3aa6df871e283115652ec6f2d31e0
this feature is available only if the kernel is built with the
.BR CONFIG_CHECKPOINT_RESTORE
option enabled.
.RS
.TP
.BR PR_SET_MM_START_CODE
Set the address above which the program text can run.
The corresponding memory area must be readable and executable,
but not writable or shareable (see
.BR mprotect (2)
and
.BR mmap (2)
for more information).
.TP
.BR PR_SET_MM_END_CODE
Set the address below which the program text can run.
The corresponding memory area must be readable and executable,
but not writable or shareable.
.TP
.BR PR_SET_MM_START_DATA
Set the address above which initialized and
uninitialized (bss) data are placed.
The corresponding memory area must be readable and writable,
but not executable or shareable.
.TP
.B PR_SET_MM_END_DATA
Set the address below which initialized and
uninitialized (bss) data are placed.
The corresponding memory area must be readable and writable,
but not executable or shareable.
.TP
.BR PR_SET_MM_START_STACK
Set the start address of the stack.
The corresponding memory area must be readable and writable.
.TP
.BR PR_SET_MM_START_BRK
Set the address above which the program heap can be expanded with
.BR brk (2)
call.
The address must be greater than the ending address of
the current program data segment.
In addition, the combined size of the resulting heap and
the size of the data segment can't exceed the
.BR RLIMIT_DATA
resource limit (see
.BR setrlimit (2)).
.TP
.BR PR_SET_MM_BRK
Set the current
.BR brk (2)
value.
The requirements for the address are the same as for the
.BR PR_SET_MM_START_BRK
option.
.PP
The following options are available since Linux 3.5.
.\" commit fe8c7f5cbf91124987106faa3bdf0c8b955c4cf7
.TP
.BR PR_SET_MM_ARG_START
Set the address above which the program command line is placed.
.TP
.BR PR_SET_MM_ARG_END
Set the address below which the program command line is placed.
.TP
.BR PR_SET_MM_ENV_START
Set the address above which the program environment is placed.
.TP
.BR PR_SET_MM_ENV_END
Set the address below which the program environment is placed.
.IP
The address passed with
.BR PR_SET_MM_ARG_START ,
.BR PR_SET_MM_ARG_END ,
.BR PR_SET_MM_ENV_START ,
and
.BR PR_SET_MM_ENV_END
should belong to a process stack area.
Thus, the corresponding memory area must be readable, writable, and
(depending on the kernel configuration) have the
.BR MAP_GROWSDOWN
attribute set (see
.BR mmap (2)).
.TP
.BR PR_SET_MM_AUXV
Set a new auxiliary vector.
The
.I arg3
argument should provide the address of the vector.
The
.I arg4
is the size of the vector.
.TP
.BR PR_SET_MM_EXE_FILE
.\" commit b32dfe377102ce668775f8b6b1461f7ad428f8b6
Supersede the
.IR /proc/pid/exe
symbolic link with a new one pointing to a new executable file
identified by the file descriptor provided in
.I arg3
argument.
The file descriptor should be obtained with a regular
.BR open (2)
call.
.IP
To change the symbolic link, one needs to unmap all existing
executable memory areas, including those created by the kernel itself
(for example the kernel usually creates at least one executable
memory area for the ELF
.IR \.text
section).
.IP
In Linux 4.9 and earlier, the
.\" commit 3fb4afd9a504c2386b8435028d43283216bf588e
.BR PR_SET_MM_EXE_FILE
operation can be performed only once in a process's lifetime;
attempting to perform the operation a second time results in the error
.BR EPERM .
This restriction was enforced for security reasons that were subsequently
deemed specious,
and the restriction was removed in Linux 4.10 because some
user-space applications needed to perform this operation more than once.
.PP
The following options are available since Linux 3.18.
.\" commit f606b77f1a9e362451aca8f81d8f36a3a112139e
.TP
.BR PR_SET_MM_MAP
Provides one-shot access to all the addresses by passing in a
.I struct prctl_mm_map
(as defined in \fI<linux/prctl.h>\fP).
The
.I arg4
argument should provide the size of the struct.
.IP
This feature is available only if the kernel is built with the
.BR CONFIG_CHECKPOINT_RESTORE
option enabled.
.TP
.BR PR_SET_MM_MAP_SIZE
Returns the size of the
.I struct prctl_mm_map
the kernel expects.
This allows user space to find a compatible struct.
The
.I arg4
argument should be a pointer to an unsigned int.
.IP
This feature is available only if the kernel is built with the
.BR CONFIG_CHECKPOINT_RESTORE
option enabled.
.RE
.TP
.BR PR_MPX_ENABLE_MANAGEMENT ", " PR_MPX_DISABLE_MANAGEMENT " (since Linux 3.19) "
.\" commit fe3d197f84319d3bce379a9c0dc17b1f48ad358c
.\" See also http://lwn.net/Articles/582712/
.\" See also https://gcc.gnu.org/wiki/Intel%20MPX%20support%20in%20the%20GCC%20compiler
Enable or disable kernel management of Memory Protection eXtensions (MPX)
bounds tables.
The
.IR arg2 ,
.IR arg3 ,
.IR arg4 ,
and
.IR arg5
.\" commit e9d1b4f3c60997fe197bf0243cb4a41a44387a88
arguments must be zero.
.IP
MPX is a hardware-assisted mechanism for performing bounds checking on
pointers.
It consists of a set of registers storing bounds information
and a set of special instruction prefixes that tell the CPU on which
instructions it should do bounds enforcement.
There is a limited number of these registers and
when there are more pointers than registers,
their contents must be "spilled" into a set of tables.
These tables are called "bounds tables" and the MPX
.BR prctl ()
operations control
whether the kernel manages their allocation and freeing.
.IP
When management is enabled, the kernel will take over allocation
and freeing of the bounds tables.
It does this by trapping the #BR exceptions that result
at first use of missing bounds tables and
instead of delivering the exception to user space,
it allocates the table and populates the bounds directory
with the location of the new table.
For freeing, the kernel checks to see if bounds tables are
present for memory which is not allocated, and frees them if so.
.IP
Before enabling MPX management using
.BR PR_MPX_ENABLE_MANAGEMENT ,
the application must first have allocated a user-space buffer for
the bounds directory and placed the location of that directory in the
.I bndcfgu
register.
.IP
These calls fail if the CPU or kernel does not support MPX.
Kernel support for MPX is enabled via the
.BR CONFIG_X86_INTEL_MPX
configuration option.
You can check whether the CPU supports MPX by looking for the 'mpx'
CPUID bit, like with the following command:
.IP
	cat /proc/cpuinfo | grep ' mpx '
.IP
A thread may not switch in or out of long (64-bit) mode while MPX is
enabled.
.IP
All threads in a process are affected by these calls.
.IP
The child of a
.BR fork (2)
inherits the state of MPX management.
During
.BR execve (2),
MPX management is reset to a state as if
.BR PR_MPX_DISABLE_MANAGEMENT
had been called.
.IP
For further information on Intel MPX, see the kernel source file
.IR Documentation/x86/intel_mpx.txt .
.TP
.BR PR_SET_NAME " (since Linux 2.6.9)"
Set the name of the calling thread,
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
including the terminating null byte.
(If the length of the string, including the terminating null byte,
exceeds 16 bytes, the string is silently truncated.)
This is the same attribute that can be set via
.BR pthread_setname_np (3)
and retrieved using
.BR pthread_getname_np (3).
The attribute is likewise accessible via
.IR /proc/self/task/[tid]/comm ,
where
.I tid
is the name of the calling thread.
.TP
.BR PR_GET_NAME " (since Linux 2.6.11)"
Return the name of the calling thread,
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.
.TP
.BR PR_SET_NO_NEW_PRIVS " (since Linux 3.5)"
Set the calling thread's
.I no_new_privs
bit to the value in
.IR arg2 .
With
.I no_new_privs
set to 1,
.BR execve (2)
promises not to grant privileges to do anything
that could not have been done without the
.BR execve (2)
call (for example,
rendering the set-user-ID and set-group-ID mode bits,
and file capabilities non-functional).
Once set, this bit cannot be unset.
The setting of this bit is inherited by children created by
.BR fork (2)
and
.BR clone (2),
and preserved across
.BR execve (2).
.IP
Since Linux 4.10,
the value of a thread's
.I no_new_privs
bit can be viewed via the
.I NoNewPrivs
field in the
.IR /proc/[pid]/status
file.
.IP
For more information, see the kernel source file
.IR Documentation/userspace\-api/no_new_privs.rst
.\" commit 40fde647ccb0ae8c11d256d271e24d385eed595b
(or
.IR Documentation/prctl/no_new_privs.txt
before Linux 4.13).
See also
.BR seccomp (2).
.TP
.BR PR_GET_NO_NEW_PRIVS " (since Linux 3.5)"
Return (as the function result) the value of the
.I no_new_privs
bit for the calling thread.
A value of 0 indicates the regular
.BR execve (2)
behavior.
A value of 1 indicates
.BR execve (2)
will operate in the privilege-restricting mode described above.
.TP
.BR PR_SET_PDEATHSIG " (since Linux 2.1.57)"
Set the parent 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.
.IP
.IR Warning :
.\" https://bugzilla.kernel.org/show_bug.cgi?id=43300
the "parent" in this case is considered to be the
.I thread
that created this process.
In other words, the signal will be sent when that thread terminates
(via, for example,
.BR pthread_exit (3)),
rather than after all of the threads in the parent process terminate.
.IP
The parent death signal setting is cleared for the child of a
.BR fork (2).
It is also
(since Linux 2.4.36 / 2.6.23)
.\" commit d2d56c5f51028cb9f3d800882eb6f4cbd3f9099f
cleared when executing a set-user-ID or set-group-ID binary,
or a binary that has associated capabilities (see
.BR capabilities (7));
otherwise, this value is preserved across
.BR execve (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_PTRACER " (since Linux 3.4)"
.\" commit 2d514487faf188938a4ee4fb3464eeecfbdcf8eb
.\" commit bf06189e4d14641c0148bea16e9dd24943862215
This is meaningful only when the Yama LSM is enabled and in mode 1
("restricted ptrace", visible via
.IR /proc/sys/kernel/yama/ptrace_scope ).
When a "ptracer process ID" is passed in \fIarg2\fP,
the caller is declaring that the ptracer process can
.BR ptrace (2)
the calling process as if it were a direct process ancestor.
Each
.B PR_SET_PTRACER
operation replaces the previous "ptracer process ID".
Employing
.B PR_SET_PTRACER
with
.I arg2
set to 0 clears the caller's "ptracer process ID".
If
.I arg2
is
.BR PR_SET_PTRACER_ANY ,
the ptrace restrictions introduced by Yama are effectively disabled for the
calling process.
.IP
For further information, see the kernel source file
.IR Documentation/admin\-guide/LSM/Yama.rst
.\" commit 90bb766440f2147486a2acc3e793d7b8348b0c22
(or
.IR Documentation/security/Yama.txt
before Linux 4.13).
.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 (seccomp) mode for the calling thread, to limit
the available system calls.
The more recent
.BR seccomp (2)
system call provides a superset of the functionality of
.BR PR_SET_SECCOMP .
.IP
The seccomp mode is selected via
.IR arg2 .
(The seccomp constants are defined in
.IR <linux/seccomp.h> .)
.IP
With
.IR arg2
set to
.BR SECCOMP_MODE_STRICT ,
the only system calls that the thread is permitted to make are
.BR read (2),
.BR write (2),
.BR _exit (2)
(but not
.BR exit_group (2)),
and
.BR sigreturn (2).
Other system calls result in the delivery of a
.BR SIGKILL
signal.
Strict 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 available only
if the kernel is configured with
.B CONFIG_SECCOMP
enabled.
.IP
With
.IR arg2
set to
.BR SECCOMP_MODE_FILTER " (since Linux 3.5),"
the system calls allowed are defined by a pointer
to a Berkeley Packet Filter passed in
.IR arg3 .
This argument is a pointer to
.IR "struct sock_fprog" ;
it can be designed to filter
arbitrary system calls and system call arguments.
This mode is available only if the kernel is configured with
.B CONFIG_SECCOMP_FILTER
enabled.
.IP
If
.BR SECCOMP_MODE_FILTER
filters permit
.BR fork (2),
then the seccomp mode is inherited by children created by
.BR fork (2);
if
.BR execve (2)
is permitted, then the seccomp mode is preserved across
.BR execve (2).
If the filters permit
.BR prctl ()
calls, then additional filters can be added;
they are run in order until the first non-allow result is seen.
.IP
For further information, see the kernel source file
.IR Documentation/userspace\-api/seccomp_filter.rst
.\" commit c061f33f35be0ccc80f4b8e0aea5dfd2ed7e01a3
(or
.IR Documentation/prctl/seccomp_filter.txt
before Linux 4.13).
.TP
.BR PR_GET_SECCOMP " (since Linux 2.6.23)"
Return (as the function result)
the secure computing mode of the calling thread.
If the caller is not in secure computing mode, this operation returns 0;
if the caller is in strict secure computing mode, then the
.BR prctl ()
call will cause a
.B SIGKILL
signal to be sent to the process.
If the caller is in filter mode, and this system call is allowed by the
seccomp filters, it returns 2; otherwise, the process is killed with a
.BR SIGKILL
signal.
This operation is available only
if the kernel is configured with
.B CONFIG_SECCOMP
enabled.
.IP
Since Linux 3.8, the
.IR Seccomp
field of the
.IR /proc/[pid]/status
file provides a method of obtaining the same information,
without the risk that the process is killed; see
.BR proc (5).
.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_GET_SPECULATION_CTRL " (since Linux 4.17)"
Returns the state of the speculation misfeature specified in
.IR arg2 .
Currently, the only permitted value for this argument is
.BR PR_SPEC_STORE_BYPASS
(otherwise the call fails with the error
.BR ENODEV ).
.IP
The return value uses bits 0-3 with the following meaning:
.RS
.TP
.BR PR_SPEC_PRCTL
Mitigation can be controlled per thread by
.B PR_SET_SPECULATION_CTRL
.TP
.BR PR_SPEC_ENABLE
The speculation feature is enabled, mitigation is disabled.
.TP
.BR PR_SPEC_DISABLE
The speculation feature is disabled, mitigation is enabled
.TP
.BR PR_SPEC_FORCE_DISABLE
Same as
.B PR_SPEC_DISABLE
but cannot be undone.
.RE
.IP
If all bits are 0,
then the CPU is not affected by the speculation misfeature.
.IP
If
.B PR_SPEC_PRCTL
is set, then per-thread control of the mitigation is available.
If not set,
.BR prctl ()
for the speculation misfeature will fail.
.IP
The
.IR arg3 ,
.IR arg4 ,
and
.I arg5
arguments must be specified as 0; otherwise the call fails with the error
.BR EINVAL .
.TP
.BR PR_SET_SPECULATION_CTRL " (since Linux 4.17)"
.\" commit b617cfc858161140d69cc0b5cc211996b557a1c7
.\" commit 356e4bfff2c5489e016fdb925adbf12a1e3950ee
Sets the state of the speculation misfeature specified in
.IR arg2 .
Currently, the only permitted value for this argument is
.B PR_SPEC_STORE_BYPASS
(otherwise the call fails with the error
.BR ENODEV ).
This setting is a per-thread attribute.
The
.IR arg3
argument is used to hand in the control value,
which is one of the following:
.RS
.TP
.BR PR_SPEC_ENABLE
The speculation feature is enabled, mitigation is disabled.
.TP
.BR PR_SPEC_DISABLE
The speculation feature is disabled, mitigation is enabled
.TP
.BR PR_SPEC_FORCE_DISABLE
Same as
.B PR_SPEC_DISABLE
but cannot be undone.
A subsequent
.B
prctl(..., PR_SPEC_ENABLE)
will fail with the error
.BR EPERM .
.RE
.IP
Any other value in
.IR arg3
will result in the call failing with the error
.BR ERANGE .
.IP
The
.I arg4
and
.I arg5
arguments must be specified as 0; otherwise the call fails with the error
.BR EINVAL .
.IP
The speculation feature can also be controlled by the
.B spec_store_bypass_disable
boot parameter.
This parameter may enforce a read-only policy which will result in the
.BR prctl (2)
call failing with the error
.BR ENXIO .
For further details, see the kernel source file
.IR Documentation/admin-guide/kernel-parameters.txt .
.TP
.BR PR_SET_THP_DISABLE " (since Linux 3.15)"
.\" commit a0715cc22601e8830ace98366c0c2bd8da52af52
Set the state of the "THP disable" flag for the calling thread.
If
.I arg2
has a nonzero value, the flag is set, otherwise it is cleared.
Setting this flag provides a method
for disabling transparent huge pages
for jobs where the code cannot be modified, and using a malloc hook with
.BR madvise (2)
is not an option (i.e., statically allocated data).
The setting of the "THP disable" flag is inherited by a child created via
.BR fork (2)
and is preserved across
.BR execve (2).
.\"
.TP
.BR PR_TASK_PERF_EVENTS_DISABLE " (since Linux 2.6.31)"
Disable all performance counters attached to the calling process,
regardless of whether the counters were created by
this process or another process.
Performance counters created by the calling process for other
processes are unaffected.
For more information on performance counters, see the Linux kernel source file
.IR tools/perf/design.txt .
.IP
Originally called
.BR PR_TASK_PERF_COUNTERS_DISABLE ;
.\" commit 1d1c7ddbfab358445a542715551301b7fc363e28
renamed (retaining the same numerical value)
in Linux 2.6.32.
.\"
.TP
.BR PR_TASK_PERF_EVENTS_ENABLE " (since Linux 2.6.31)"
The converse of
.BR PR_TASK_PERF_EVENTS_DISABLE ;
enable performance counters attached to the calling process.
.IP
Originally called
.BR PR_TASK_PERF_COUNTERS_ENABLE ;
.\" commit 1d1c7ddbfab358445a542715551301b7fc363e28
renamed
.\" commit cdd6c482c9ff9c55475ee7392ec8f672eddb7be6
in Linux 2.6.32.
.\"
.TP
.BR PR_GET_THP_DISABLE " (since Linux 3.15)"
Return (via the function result) the current setting of the "THP disable"
flag for the calling thread:
either 1, if the flag is set, or 0, if it is not.
.TP
.BR PR_GET_TID_ADDRESS " (since Linux 3.5)"
.\" commit 300f786b2683f8bb1ec0afb6e1851183a479c86d
Retrieve the
.I clear_child_tid
address set by
.BR set_tid_address (2)
and the
.BR clone (2)
.B CLONE_CHILD_CLEARTID
flag, in the location pointed to by
.IR "(int\ **)\ arg2" .
This feature is available only if the kernel is built with the
.BR CONFIG_CHECKPOINT_RESTORE
option enabled.
Note that since the
.BR prctl ()
system call does not have a compat implementation for
the AMD64 x32 and MIPS n32 ABIs,
and the kernel writes out a pointer using the kernel's pointer size,
this operation expects a user-space buffer of 8 (not 4) bytes on these ABIs.
.TP
.BR PR_SET_TIMERSLACK " (since Linux 2.6.28)"
.\" See https://lwn.net/Articles/369549/
.\" commit 6976675d94042fbd446231d1bd8b7de71a980ada
Each thread has two associated timer slack values:
a "default" value, and a "current" value.
This operation sets the "current" timer slack value for the calling thread.
If the nanosecond value supplied in
.IR arg2
is greater than zero, then the "current" value is set to this value.
If
.I arg2
is less than or equal to zero,
.\" It seems that it's not possible to set the timer slack to zero;
.\" The minimum value is 1? Seems a little strange.
the "current" timer slack is reset to the
thread's "default" timer slack value.
.IP
The "current" timer slack is used by the kernel to group timer expirations
for the calling thread that are close to one another;
as a consequence, timer expirations for the thread may be
up to the specified number of nanoseconds late (but will never expire early).
Grouping timer expirations can help reduce system power consumption
by minimizing CPU wake-ups.
.IP
The timer expirations affected by timer slack are those set by
.BR select (2),
.BR pselect (2),
.BR poll (2),
.BR ppoll (2),
.BR epoll_wait (2),
.BR epoll_pwait (2),
.BR clock_nanosleep (2),
.BR nanosleep (2),
and
.BR futex (2)
(and thus the library functions implemented via futexes, including
.\" List obtained by grepping for futex usage in glibc source
.BR pthread_cond_timedwait (3),
.BR pthread_mutex_timedlock (3),
.BR pthread_rwlock_timedrdlock (3),
.BR pthread_rwlock_timedwrlock (3),
and
.BR sem_timedwait (3)).
.IP
Timer slack is not applied to threads that are scheduled under
a real-time scheduling policy (see
.BR sched_setscheduler (2)).
.IP
When a new thread is created,
the two timer slack values are made the same as the "current" value
of the creating thread.
Thereafter, a thread can adjust its "current" timer slack value via
.BR PR_SET_TIMERSLACK .
The "default" value can't be changed.
The timer slack values of
.IR init
(PID 1), the ancestor of all processes,
are 50,000 nanoseconds (50 microseconds).
The timer slack values are preserved across
.BR execve (2).
.IP
Since Linux 4.6, the "current" timer slack value of any process
can be examined and changed via the file
.IR /proc/[pid]/timerslack_ns .
See
.BR proc (5).
.TP
.BR PR_GET_TIMERSLACK " (since Linux 2.6.28)"
Return (as the function result)
the "current" timer slack value of the calling thread.
.TP
.BR PR_SET_TIMING " (since Linux 2.6.0)"
.\" Precisely: 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)"
.\" Precisely: 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;
.\" sh: 94ea5e449ae834af058ef005d16a8ad44fcf13d6
.\" tile: 2f9ac29eec71a696cb0dcc5fb82c0f8d4dac28c9
sh, since Linux 2.6.34; tile, since Linux 3.12)
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.
Alpha also supports an additional flag with the value
of 4 and no corresponding named constant,
which instructs kernel to not fix up
unaligned accesses (it is analogous to providing the
.BR UAC_NOFIX
flag in
.BR SSI_NVPAIRS
operation of the
.BR setsysinfo ()
system call on Tru64).
.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 "(unsigned int\ *) arg2" .
.SH RETURN VALUE
On success,
.BR PR_GET_DUMPABLE ,
.BR PR_GET_KEEPCAPS ,
.BR PR_GET_NO_NEW_PRIVS ,
.BR PR_GET_THP_DISABLE ,
.BR PR_CAPBSET_READ ,
.BR PR_GET_TIMING ,
.BR PR_GET_TIMERSLACK ,
.BR PR_GET_SECUREBITS ,
.BR PR_MCE_KILL_GET ,
.BR PR_CAP_AMBIENT + PR_CAP_AMBIENT_IS_SET ,
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 EACCES
.I option
is
.BR PR_SET_SECCOMP
and
.I arg2
is
.BR SECCOMP_MODE_FILTER ,
but the process does not have the
.BR CAP_SYS_ADMIN
capability or has not set the
.IR no_new_privs
attribute (see the discussion of
.BR PR_SET_NO_NEW_PRIVS
above).
.TP
.B EACCES
.I option
is
.BR PR_SET_MM ,
and
.I arg3
is
.BR PR_SET_MM_EXE_FILE ,
the file is not executable.
.TP
.B EBADF
.I option
is
.BR PR_SET_MM ,
.I arg3
is
.BR PR_SET_MM_EXE_FILE ,
and the file descriptor passed in
.I arg4
is not valid.
.TP
.B EBUSY
.I option
is
.BR PR_SET_MM ,
.I arg3
is
.BR PR_SET_MM_EXE_FILE ,
and this the second attempt to change the
.I /proc/pid/exe
symbolic link, which is prohibited.
.TP
.B EFAULT
.I arg2
is an invalid address.
.TP
.B EFAULT
.I option
is
.BR PR_SET_SECCOMP ,
.I arg2
is
.BR SECCOMP_MODE_FILTER ,
the system was built with
.BR CONFIG_SECCOMP_FILTER ,
and
.I arg3
is an invalid address.
.TP
.B EINVAL
The value of
.I option