Many pages: Fix style issues reported by `make lint-groff`

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

.\" Copyright (C) 1998 Andries Brouwer (aeb@cwi.nl)
.\" and Copyright (C) 2002, 2006, 2008, 2012, 2013, 2015 Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright Guillem Jover <guillem@hadrons.org>
.\" and Copyright (C) 2010 Andi Kleen <andi@firstfloor.org>
.\" and Copyright (C) 2012 Cyrill Gorcunov <gorcunov@openvz.org>
.\" and Copyright (C) 2014 Dave Hansen / Intel
.\" and Copyright (c) 2016 Eugene Syromyatnikov <evgsyr@gmail.com>
.\" and Copyright (c) 2018 Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
.\" and Copyright (c) 2020 Dave Martin <Dave.Martin@arm.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 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 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
prctl \- operations on a process or thread
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.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 ()
manipulates various aspects of the behavior
of the calling thread or process.
.PP
Note that careless use of some
.BR prctl ()
operations can confuse the user-space run-time environment,
so these operations should be used with care.
.PP
.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:
.\"
.\" prctl PR_CAP_AMBIENT
.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
.B 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).
.\" prctl PR_CAPBSET_READ
.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).
.\" prctl PR_CAPBSET_DROP
.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 ;
.B EINVAL
if
.I arg2
does not represent a valid capability; or
.B 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).
.\" prctl PR_SET_CHILD_SUBREAPER
.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 (2)
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
.B SIGCHLD
signal and will be able to
.BR wait (2)
on the process to discover its termination status.
.IP
The setting of the "child subreaper" attribute
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.
.\" prctl PR_GET_CHILD_SUBREAPER
.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" .
.\" prctl PR_SET_DUMPABLE
.TP
.BR PR_SET_DUMPABLE " (since Linux 2.3.20)"
Set the state of the "dumpable" attribute,
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, the "dumpable" attribute is set to 1.
However, it is reset to the current value contained in the file
.I /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).
.\" prctl PR_GET_DUMPABLE
.TP
.BR PR_GET_DUMPABLE " (since Linux 2.3.20)"
Return (as the function result) the current state of the calling
process's dumpable attribute.
.\" 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.
.\" prctl PR_SET_ENDIAN
.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).
.\" prctl PR_GET_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" .
.\" prctl PR_SET_FP_MODE
.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
.B 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
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
.B FR=0
or
.B FR=1 .
.TP
.B 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
.I arg5
are ignored.
.\" prctl PR_GET_FP_MODE
.TP
.BR PR_GET_FP_MODE " (since Linux 4.0, only on MIPS)"
Return (as the function result)
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
.I arg5
are ignored.
.\" prctl PR_SET_FPEMU
.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.
.\" prctl PR_GET_FPEMU
.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" .
.\" prctl PR_SET_FPEXC
.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.
.\" prctl PR_GET_FPEXC
.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" .
.\" prctl PR_SET_IO_FLUSHER
.TP
.BR PR_SET_IO_FLUSHER " (since Linux 5.6)"
If a user process is involved in the block layer or filesystem I/O path,
and can allocate memory while processing I/O requests it must set
\fIarg2\fP to 1.
This will put the process in the IO_FLUSHER state,
which allows it special treatment to make progress when allocating memory.
If \fIarg2\fP is 0, the process will clear the IO_FLUSHER state, and
the default behavior will be used.
.IP
The calling process must have the
.B CAP_SYS_RESOURCE
capability.
.IP
.IR arg3 ,
.IR arg4 ,
and
.I arg5
must be zero.
.IP
The IO_FLUSHER state is inherited by a child process created via
.BR fork (2)
and is preserved across
.BR execve (2).
.IP
Examples of IO_FLUSHER applications are FUSE daemons, SCSI device
emulation daemons, and daemons that perform error handling like multipath
path recovery applications.
.\" prctl PR_GET_IO_FLUSHER
.TP
.B PR_GET_IO_FLUSHER (Since Linux 5.6)
Return (as the function result) the IO_FLUSHER state of the caller.
A value of 1 indicates that the caller is in the IO_FLUSHER state;
0 indicates that the caller is not in the IO_FLUSHER state.
.IP
The calling process must have the
.B CAP_SYS_RESOURCE
capability.
.IP
.IR arg2 ,
.IR arg3 ,
.IR arg4 ,
and
.I arg5
must be zero.
.\" prctl PR_SET_KEEPCAPS
.TP
.BR PR_SET_KEEPCAPS " (since Linux 2.2.18)"
Set the state of the calling thread's "keep capabilities" flag.
The effect of 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).
.\" prctl PR_GET_KEEPCAPS
.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.
.\" prctl PR_MCE_KILL
.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
.B SIGBUS
signal.
The policy is inherited by children.
The remaining unused
.BR prctl ()
arguments must be zero for future compatibility.
.\" prctl PR_MCE_KILL_GET
.TP
.BR PR_MCE_KILL_GET " (since Linux 2.6.32)"
Return (as the function result)
the current per-process machine check kill policy.
All unused
.BR prctl ()
arguments must be zero.
.\" prctl PR_SET_MM
.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
.B 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
.B CONFIG_CHECKPOINT_RESTORE
option enabled.
.RS
.TP
.B 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
.B 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
.B 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
.B PR_SET_MM_START_STACK
Set the start address of the stack.
The corresponding memory area must be readable and writable.
.TP
.B 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
.B RLIMIT_DATA
resource limit (see
.BR setrlimit (2)).
.TP
.B PR_SET_MM_BRK
Set the current
.BR brk (2)
value.
The requirements for the address are the same as for the
.B PR_SET_MM_START_BRK
option.
.PP
The following options are available since Linux 3.5.
.\" commit fe8c7f5cbf91124987106faa3bdf0c8b955c4cf7
.TP
.B PR_SET_MM_ARG_START
Set the address above which the program command line is placed.
.TP
.B PR_SET_MM_ARG_END
Set the address below which the program command line is placed.
.TP
.B PR_SET_MM_ENV_START
Set the address above which the program environment is placed.
.TP
.B 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
.B 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
.B MAP_GROWSDOWN
attribute set (see
.BR mmap (2)).
.TP
.B 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
.B 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
.I \.text
section).
.IP
In Linux 4.9 and earlier, the
.\" commit 3fb4afd9a504c2386b8435028d43283216bf588e
.B 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
.B 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
.B CONFIG_CHECKPOINT_RESTORE
option enabled.
.TP
.B 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
.B CONFIG_CHECKPOINT_RESTORE
option enabled.
.RE
.\" prctl PR_MPX_ENABLE_MANAGEMENT
.TP
.BR PR_MPX_ENABLE_MANAGEMENT ", " PR_MPX_DISABLE_MANAGEMENT " (since Linux 3.19, removed in Linux 5.4; only on x86)"
.\" 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
.I 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
.B CONFIG_X86_INTEL_MPX
configuration option.
You can check whether the CPU supports MPX by looking for the
.I mpx
CPUID bit, like with the following command:
.IP
.in +4n
.EX
cat /proc/cpuinfo | grep \(aq mpx \(aq
.EE
.in
.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
.B 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 .
.IP
.\" commit f240652b6032b48ad7fa35c5e701cc4c8d697c0b
.\" See also https://lkml.kernel.org/r/20190705175321.DB42F0AD@viggo.jf.intel.com
Due to a lack of toolchain support,
.BR PR_MPX_ENABLE_MANAGEMENT " and " PR_MPX_DISABLE_MANAGEMENT
are not supported in Linux 5.4 and later.
.\" prctl PR_SET_NAME
.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
(see
.BR proc (5)),
where
.I tid
is the thread ID of the calling thread, as returned by
.BR gettid (2).
.\" prctl PR_GET_NAME
.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.
.\" prctl PR_SET_NO_NEW_PRIVS
.TP
.BR PR_SET_NO_NEW_PRIVS " (since Linux 3.5)"
Set the calling thread's
.I no_new_privs
attribute 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, the
.I no_new_privs
attribute cannot be unset.
The setting of this attribute 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
attribute can be viewed via the
.I NoNewPrivs
field in the
.IR /proc/ pid /status
file.
.IP
For more information, see the kernel source file
.I Documentation/userspace\-api/no_new_privs.rst
.\" commit 40fde647ccb0ae8c11d256d271e24d385eed595b
(or
.I Documentation/prctl/no_new_privs.txt
before Linux 4.13).
See also
.BR seccomp (2).
.\" prctl PR_GET_NO_NEW_PRIVS
.TP
.BR PR_GET_NO_NEW_PRIVS " (since Linux 3.5)"
Return (as the function result) the value of the
.I no_new_privs
attribute 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.
.\" prctl PR_PAC_RESET_KEYS
.\" commit ba830885656414101b2f8ca88786524d4bb5e8c1
.TP
.BR PR_PAC_RESET_KEYS " (since Linux 5.0, only on arm64)"
Securely reset the thread's pointer authentication keys
to fresh random values generated by the kernel.
.IP
The set of keys to be reset is specified by
.IR arg2 ,
which must be a logical OR of zero or more of the following:
.RS
.TP
.B PR_PAC_APIAKEY
instruction authentication key A
.TP
.B PR_PAC_APIBKEY
instruction authentication key B
.TP
.B PR_PAC_APDAKEY
data authentication key A
.TP
.B PR_PAC_APDBKEY
data authentication key B
.TP
.B PR_PAC_APGAKEY
generic authentication \(lqA\(rq key.
.IP
(Yes folks, there really is no generic B key.)
.RE
.IP
As a special case, if
.I arg2
is zero, then all the keys are reset.
Since new keys could be added in future,
this is the recommended way to completely wipe the existing keys
when establishing a clean execution context.
Note that there is no need to use
.B PR_PAC_RESET_KEYS
in preparation for calling
.BR execve (2),
since
.BR execve (2)
resets all the pointer authentication keys.
.IP
The remaining arguments
.IR arg3 ", " arg4 ", and " arg5
must all be zero.
.IP
If the arguments are invalid,
and in particular if
.I arg2
contains set bits that are unrecognized
or that correspond to a key not available on this platform,
then the call fails with error
.BR EINVAL .
.IP
.B Warning:
Because the compiler or run-time environment
may be using some or all of the keys,
a successful
.B PR_PAC_RESET_KEYS
may crash the calling process.
The conditions for using it safely are complex and system-dependent.
Don't use it unless you know what you are doing.
.IP
For more information, see the kernel source file
.I Documentation/arm64/pointer\-authentication.rst
.\"commit b693d0b372afb39432e1c49ad7b3454855bc6bed
(or
.I Documentation/arm64/pointer\-authentication.txt
before Linux 5.3).
.\" prctl PR_SET_PDEATHSIG
.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..\c
.BR NSIG "\-1" ,
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 is sent upon subsequent termination of the parent
thread and also upon termination of each subreaper process
(see the description of
.B PR_SET_CHILD_SUBREAPER
above) to which the caller is subsequently reparented.
If the parent thread and all ancestor subreapers have already terminated
by the time of the
.B PR_SET_PDEATHSIG
operation, then no parent-death signal is sent to the caller.
.IP
The parent-death signal is process-directed (see
.BR signal (7))
and, if the child installs a handler using the
.BR sigaction (2)
.B SA_SIGINFO
flag, the
.I si_pid
field of the
.I siginfo_t
argument of the handler contains the PID of the terminating parent process.
.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).
The parent-death signal setting is also cleared upon changes to
any of the following thread credentials:
.\" FIXME capability changes can also trigger this; see
.\" kernel/cred.c::commit_creds in the Linux 5.6 source.
effective user ID, effective group ID, filesystem user ID,
or filesystem group ID.
.\" prctl PR_GET_PDEATHSIG
.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" .
.\" prctl PR_SET_PTRACER
.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
.I Documentation/admin\-guide/LSM/Yama.rst
.\" commit 90bb766440f2147486a2acc3e793d7b8348b0c22
(or
.I Documentation/security/Yama.txt
before Linux 4.13).
.\" prctl PR_SET_SECCOMP
.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 ,
and is the preferred interface for new applications.
.IP
The seccomp mode is selected via
.IR arg2 .
(The seccomp constants are defined in
.IR <linux/seccomp.h> .)
The following values can be specified:
.RS
.TP
.BR SECCOMP_MODE_STRICT " (since Linux 2.6.23)"
See the description of
.B SECCOMP_SET_MODE_STRICT
in
.BR seccomp (2).
.IP
This operation is available only
if the kernel is configured with
.B CONFIG_SECCOMP
enabled.
.TP
.BR SECCOMP_MODE_FILTER " (since Linux 3.5)"
The allowed system calls 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.
See the description of
.B SECCOMP_SET_MODE_FILTER
in
.BR seccomp (2).
.IP
This operation is available only
if the kernel is configured with
.B CONFIG_SECCOMP_FILTER
enabled.
.RE
.IP
For further details on seccomp filtering, see
.BR seccomp (2).
.\" prctl PR_GET_SECCOMP
.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
.B SIGKILL
signal.
.IP
This operation is available only
if the kernel is configured with
.B CONFIG_SECCOMP
enabled.
.IP
Since Linux 3.8, the
.I 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).
.\" prctl PR_SET_SECUREBITS
.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).
.\" prctl PR_GET_SECUREBITS
.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).
.\" prctl PR_GET_SPECULATION_CTRL
.TP
.BR PR_GET_SPECULATION_CTRL " (since Linux 4.17)"
Return (as the function result)
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 ).
.IP
The return value uses bits 0-3 with the following meaning:
.RS
.TP
.B PR_SPEC_PRCTL
Mitigation can be controlled per thread by
.BR PR_SET_SPECULATION_CTRL .
.TP
.B PR_SPEC_ENABLE
The speculation feature is enabled, mitigation is disabled.
.TP
.B PR_SPEC_DISABLE
The speculation feature is disabled, mitigation is enabled.
.TP
.B PR_SPEC_FORCE_DISABLE
Same as
.B PR_SPEC_DISABLE
but cannot be undone.
.TP
.BR PR_SPEC_DISABLE_NOEXEC " (since Linux 5.1)"
Same as
.BR PR_SPEC_DISABLE ,
but the state will be cleared on
.BR execve (2).
.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 .
.\" prctl PR_SET_SPECULATION_CTRL
.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 .
The speculation-misfeature settings are per-thread attributes.
.IP
Currently,
.I arg2
must be one of:
.RS
.TP
.B PR_SPEC_STORE_BYPASS
Set the state of the speculative store bypass misfeature.
.\" commit 9137bb27e60e554dab694eafa4cca241fa3a694f
.TP
.BR PR_SPEC_INDIRECT_BRANCH " (since Linux 4.20)"
Set the state of the indirect branch speculation misfeature.
.RE
.IP
If
.I arg2
does not have one of the above values,
then the call fails with the error
.BR ENODEV .
.IP
The
.I arg3
argument is used to hand in the control value,
which is one of the following:
.RS
.TP
.B PR_SPEC_ENABLE
The speculation feature is enabled, mitigation is disabled.
.TP
.B PR_SPEC_DISABLE
The speculation feature is disabled, mitigation is enabled.
.TP
.B PR_SPEC_FORCE_DISABLE
Same as
.BR PR_SPEC_DISABLE ,
but cannot be undone.
A subsequent
.BR prctl (\c
.IR arg2 ,
.BR PR_SPEC_ENABLE )
with the same value for
.I arg2
will fail with the error
.BR EPERM .
.\" commit 71368af9027f18fe5d1c6f372cfdff7e4bde8b48
.TP
.BR PR_SPEC_DISABLE_NOEXEC " (since Linux 5.1)"
Same as
.BR PR_SPEC_DISABLE ,
but the state will be cleared on
.BR execve (2).
Currently only supported for
.I arg2
equal to
.B PR_SPEC_STORE_BYPASS.
.RE
.IP
Any unsupported value in
.I 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 ()
call failing with the error
.BR ENXIO .
For further details, see the kernel source file
.IR Documentation/admin\-guide/kernel\-parameters.txt .
.\" prctl PR_SVE_SET_VL
.\" commit 2d2123bc7c7f843aa9db87720de159a049839862
.\" linux-5.6/Documentation/arm64/sve.rst
.TP
.BR PR_SVE_SET_VL " (since Linux 4.15, only on arm64)"
Configure the thread's SVE vector length,
as specified by
.IR "(int) arg2" .
Arguments
.IR arg3 ,
.IR arg4 ,
and
.I arg5
are ignored.
.IP
The bits of
.I arg2
corresponding to
.B PR_SVE_VL_LEN_MASK
must be set to the desired vector length in bytes.
This is interpreted as an upper bound:
the kernel will select the greatest available vector length
that does not exceed the value specified.
In particular, specifying
.B SVE_VL_MAX
(defined in
.I <asm/sigcontext.h>)
for the
.B PR_SVE_VL_LEN_MASK
bits requests the maximum supported vector length.
.IP
In addition, the other bits of
.I arg2
must be set to one of the following combinations of flags:
.RS
.TP
.B 0
Perform the change immediately.
At the next
.BR execve (2)
in the thread,
the vector length will be reset to the value configured in
.IR /proc/sys/abi/sve_default_vector_length .
.TP
.B PR_SVE_VL_INHERIT
Perform the change immediately.
Subsequent
.BR execve (2)
calls will preserve the new vector length.
.TP
.B PR_SVE_SET_VL_ONEXEC
Defer the change, so that it is performed at the next
.BR execve (2)
in the thread.
Further
.BR execve (2)
calls will reset the vector length to the value configured in
.IR /proc/sys/abi/sve_default_vector_length .
.TP
.B "PR_SVE_SET_VL_ONEXEC | PR_SVE_VL_INHERIT"
Defer the change, so that it is performed at the next
.BR execve (2)
in the thread.
Further
.BR execve (2)
calls will preserve the new vector length.
.RE
.IP
In all cases,
any previously pending deferred change is canceled.
.IP
The call fails with error
.B EINVAL
if SVE is not supported on the platform, if
.I arg2
is unrecognized or invalid, or the value in the bits of
.I arg2
corresponding to
.B PR_SVE_VL_LEN_MASK
is outside the range
.BR SVE_VL_MIN .. SVE_VL_MAX
or is not a multiple of 16.
.IP
On success,
a nonnegative value is returned that describes the
.I selected
configuration.
If
.B PR_SVE_SET_VL_ONEXEC
was included in
.IR arg2 ,
then the configuration described by the return value
will take effect at the next
.BR execve (2).
Otherwise, the configuration is already in effect when the
.B PR_SVE_SET_VL
call returns.
In either case, the value is encoded in the same way as the return value of
.BR PR_SVE_GET_VL .
Note that there is no explicit flag in the return value
corresponding to
.BR PR_SVE_SET_VL_ONEXEC .
.IP
The configuration (including any pending deferred change)
is inherited across
.BR fork (2)
and
.BR clone (2).
.IP
For more information, see the kernel source file
.I Documentation/arm64/sve.rst
.\"commit b693d0b372afb39432e1c49ad7b3454855bc6bed
(or
.I Documentation/arm64/sve.txt
before Linux 5.3).
.IP
.B Warning:
Because the compiler or run-time environment
may be using SVE, using this call without the
.B PR_SVE_SET_VL_ONEXEC
flag may crash the calling process.
The conditions for using it safely are complex and system-dependent.
Don't use it unless you really know what you are doing.
.\" prctl PR_SVE_GET_VL
.TP
.BR PR_SVE_GET_VL " (since Linux 4.15, only on arm64)"
Get the thread's current SVE vector length configuration.
.IP
Arguments
.IR arg2 ", " arg3 ", " arg4 ", and " arg5
are ignored.
.IP
Provided that the kernel and platform support SVE,
this operation always succeeds,
returning a nonnegative value that describes the
.I current
configuration.
The bits corresponding to
.B PR_SVE_VL_LEN_MASK
contain the currently configured vector length in bytes.
The bit corresponding to
.B PR_SVE_VL_INHERIT
indicates whether the vector length will be inherited
across
.BR execve (2).
.IP
Note that there is no way to determine whether there is
a pending vector length change that has not yet taken effect.
.IP
For more information, see the kernel source file
.I Documentation/arm64/sve.rst
.\"commit b693d0b372afb39432e1c49ad7b3454855bc6bed
(or
.I Documentation/arm64/sve.txt
before Linux 5.3).
.TP
.\" prctl PR_SET_SYSCALL_USER_DISPATCH
.\" commit 1446e1df9eb183fdf81c3f0715402f1d7595d4
.BR PR_SET_SYSCALL_USER_DISPATCH " (since Linux 5.11, x86 only)"
Configure the Syscall User Dispatch mechanism
for the calling thread.
This mechanism allows an application
to selectively intercept system calls
so that they can be handled within the application itself.
Interception takes the form of a thread-directed
.B SIGSYS
signal that is delivered to the thread
when it makes a system call.
If intercepted,
the system call is not executed by the kernel.
.IP
To enable this mechanism,
.I arg2
should be set to
.BR PR_SYS_DISPATCH_ON .
Once enabled, further system calls will be selectively intercepted,
depending on a control variable provided by user space.
In this case,
.I arg3
and
.I arg4
respectively identify the
.I offset
and
.I length
of a single contiguous memory region in the process address space
from where system calls are always allowed to be executed,
regardless of the control variable.
(Typically, this area would include the area of memory
containing the C library.)
.IP
.I arg5
points to a char-sized variable
that is a fast switch to allow/block system call execution
without the overhead of doing another system call
to re-configure Syscall User Dispatch.
This control variable can either be set to
.B SYSCALL_DISPATCH_FILTER_BLOCK
to block system calls from executing
or to
.B SYSCALL_DISPATCH_FILTER_ALLOW
to temporarily allow them to be executed.
This value is checked by the kernel
on every system call entry,
and any unexpected value will raise
an uncatchable
.B SIGSYS
at that time,
killing the application.
.IP
When a system call is intercepted,
the kernel sends a thread-directed
.B SIGSYS
signal to the triggering thread.
Various fields will be set in the
.I siginfo_t
structure (see
.BR sigaction (2))
associated with the signal:
.RS
.IP * 3
.I si_signo
will contain
.BR SIGSYS .
.IP *
.I si_call_addr
will show the address of the system call instruction.
.IP *
.I si_syscall
and
.I si_arch
will indicate which system call was attempted.
.IP *
.I si_code
will contain
.BR SYS_USER_DISPATCH .
.IP *
.I si_errno
will be set to 0.
.RE
.IP
The program counter will be as though the system call happened
(i.e., the program counter will not point to the system call instruction).
.IP
When the signal handler returns to the kernel,
the system call completes immediately
and returns to the calling thread,
without actually being executed.
If necessary
(i.e., when emulating the system call on user space.),
the signal handler should set the system call return value
to a sane value,
by modifying the register context stored in the
.I ucontext
argument of the signal handler.
See
.BR sigaction (2),
.BR sigreturn (2),
and
.BR getcontext (3)
for more information.
.IP
If
.I arg2
is set to
.BR PR_SYS_DISPATCH_OFF ,
Syscall User Dispatch is disabled for that thread.
the remaining arguments must be set to 0.
.IP
The setting is not preserved across
.BR fork (2),
.BR clone (2),
or
.BR execve (2).
.IP
For more information,
see the kernel source file
.I Documentation/admin-guide/syscall-user-dispatch.rst
.\" prctl PR_SET_TAGGED_ADDR_CTRL
.\" commit 63f0c60379650d82250f22e4cf4137ef3dc4f43d
.TP
.BR PR_SET_TAGGED_ADDR_CTRL " (since Linux 5.4, only on arm64)"
Controls support for passing tagged user-space addresses to the kernel
(i.e., addresses where bits 56\(em63 are not all zero).
.IP
The level of support is selected by
.IR "arg2" ,
which can be one of the following:
.RS
.TP
.B 0
Addresses that are passed
for the purpose of being dereferenced by the kernel
must be untagged.
.TP
.B PR_TAGGED_ADDR_ENABLE
Addresses that are passed
for the purpose of being dereferenced by the kernel
may be tagged, with the exceptions summarized below.
.RE
.IP
The remaining arguments
.IR arg3 ", " arg4 ", and " arg5
must all be zero.
.\" Enforcement added in
.\" commit 3e91ec89f527b9870fe42dcbdb74fd389d123a95
.IP
On success, the mode specified in
.I arg2
is set for the calling thread and the return value is 0.
If the arguments are invalid,
the mode specified in
.I arg2
is unrecognized,
or if this feature is unsupported by the kernel
or disabled via
.IR /proc/sys/abi/tagged_addr_disabled ,
the call fails with the error
.BR EINVAL .
.IP
In particular, if
.BR prctl ( PR_SET_TAGGED_ADDR_CTRL ,
0, 0, 0, 0)
fails with
.BR EINVAL ,
then all addresses passed to the kernel must be untagged.
.IP
Irrespective of which mode is set,
addresses passed to certain interfaces
must always be untagged:
.RS
.IP \(bu 2
.BR brk (2),
.BR mmap (2),
.BR shmat (2),
.BR shmdt (2),
and the
.I new_address
argument of
.BR mremap (2).
.IP
(Prior to Linux 5.6 these accepted tagged addresses,
but the behaviour may not be what you expect.
Don't rely on it.)
.IP \(bu
\(oqpolymorphic\(cq interfaces
that accept pointers to arbitrary types cast to a
.I void *
or other generic type, specifically
.BR prctl (),
.BR ioctl (2),
and in general
.BR setsockopt (2)
(only certain specific
.BR setsockopt (2)
options allow tagged addresses).
.RE
.IP
This list of exclusions may shrink
when moving from one kernel version to a later kernel version.
While the kernel may make some guarantees
for backwards compatibility reasons,
for the purposes of new software
the effect of passing tagged addresses to these interfaces
is unspecified.
.IP
The mode set by this call is inherited across
.BR fork (2)
and
.BR clone (2).
The mode is reset by
.BR execve (2)
to 0
(i.e., tagged addresses not permitted in the user/kernel ABI).
.IP
For more information, see the kernel source file
.IR Documentation/arm64/tagged\-address\-abi.rst .
.IP
.B Warning:
This call is primarily intended for use by the run-time environment.
A successful
.B PR_SET_TAGGED_ADDR_CTRL
call elsewhere may crash the calling process.
The conditions for using it safely are complex and system-dependent.
Don't use it unless you know what you are doing.
.\" prctl PR_GET_TAGGED_ADDR_CTRL
.\" commit 63f0c60379650d82250f22e4cf4137ef3dc4f43d
.TP
.BR PR_GET_TAGGED_ADDR_CTRL " (since Linux 5.4, only on arm64)"
Returns the current tagged address mode
for the calling thread.
.IP
Arguments
.IR arg2 ", " arg3 ", " arg4 ", and " arg5
must all be zero.
.IP
If the arguments are invalid
or this feature is disabled or unsupported by the kernel,
the call fails with
.BR EINVAL .
In particular, if
.BR prctl ( PR_GET_TAGGED_ADDR_CTRL ,
0, 0, 0, 0)
fails with
.BR EINVAL ,
then this feature is definitely either unsupported,
or disabled via
.IR /proc/sys/abi/tagged_addr_disabled .
In this case,
all addresses passed to the kernel must be untagged.
.IP
Otherwise, the call returns a nonnegative value
describing the current tagged address mode,
encoded in the same way as the
.I arg2
argument of
.BR PR_SET_TAGGED_ADDR_CTRL .
.IP
For more information, see the kernel source file
.IR Documentation/arm64/tagged\-address\-abi.rst .
.\"
.\" prctl PR_TASK_PERF_EVENTS_DISABLE
.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.
.\"
.\" prctl PR_TASK_PERF_EVENTS_ENABLE
.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.
.\"
.\" prctl PR_SET_THP_DISABLE
.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).
.\" prctl PR_GET_THP_DISABLE
.TP
.BR PR_GET_THP_DISABLE " (since Linux 3.15)"
Return (as 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.
.\" prctl PR_GET_TID_ADDRESS
.TP
.BR PR_GET_TID_ADDRESS " (since Linux 3.5)"
.\" commit 300f786b2683f8bb1ec0afb6e1851183a479c86d
Return 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
.B 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.
.\" prctl PR_SET_TIMERSLACK
.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.
.I arg2
is an unsigned long value, then maximum "current" value is ULONG_MAX and
the minimum "current" value is 1.
If the nanosecond value supplied in
.I arg2
is greater than zero, then the "current" value is set to this value.
If
.I arg2
is equal to zero,
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
.I init
(PID 1), the ancestor of all processes,
are 50,000 nanoseconds (50 microseconds).
The timer slack value is inherited by a child created via
.BR fork (2),
and is 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).
.\" prctl PR_GET_TIMERSLACK
.TP
.BR PR_GET_TIMERSLACK " (since Linux 2.6.28)"
Return (as the function result)
the "current" timer slack value of the calling thread.
.\" prctl PR_SET_TIMING
.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.
.\" prctl PR_GET_TIMING
.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.
.\" prctl PR_SET_TSC
.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.
.\" prctl PR_GET_TSC
.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" .
.\" prctl PR_SET_UNALIGN
.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
.B UAC_NOFIX
flag in
.B SSI_NVPAIRS
operation of the
.BR setsysinfo ()
system call on Tru64).
.\" prctl PR_GET_UNALIGN
.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_CAP_AMBIENT + PR_CAP_AMBIENT_IS_SET ,
.BR PR_CAPBSET_READ ,
.BR PR_GET_DUMPABLE ,
.BR PR_GET_FP_MODE ,
.BR PR_GET_IO_FLUSHER ,
.BR PR_GET_KEEPCAPS ,
.BR PR_MCE_KILL_GET ,
.BR PR_GET_NO_NEW_PRIVS ,
.BR PR_GET_SECUREBITS ,
.BR PR_GET_SPECULATION_CTRL ,
.BR PR_SVE_GET_VL ,
.BR PR_SVE_SET_VL ,
.BR PR_GET_TAGGED_ADDR_CTRL ,
.BR PR_GET_THP_DISABLE ,
.BR PR_GET_TIMING ,
.BR PR_GET_TIMERSLACK ,
and (if it returns)
.B 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 to indicate the error.
.SH ERRORS
.TP
.B EACCES
.I option
is
.B PR_SET_SECCOMP
and
.I arg2
is
.BR SECCOMP_MODE_FILTER ,
but the process does not have the
.B CAP_SYS_ADMIN
capability or has not set the
.I no_new_privs
attribute (see the discussion of
.B 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
.IR /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 EFAULT
.I option
is
.B PR_SET_SYSCALL_USER_DISPATCH
and
.I arg5
has an invalid address.
.TP
.B EINVAL
The value of
.I option
is not recognized,
or not supported on this system.