Merging upstream version 6.7.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 691 insertions, 0 deletions

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+.\" Copyright (C) 1994, 1995, Daniel Quinlan <quinlan@yggdrasil.com>
+.\" Copyright (C) 2002-2008, 2017, Michael Kerrisk <mtk.manpages@gmail.com>
+.\" Copyright (C) , Andries Brouwer <aeb@cwi.nl>
+.\" Copyright (C) 2023, Alejandro Colomar <alx@kernel.org>
+.\"
+.\" SPDX-License-Identifier: GPL-3.0-or-later
+.\"
+.TH proc_sys_kernel 5 2023-09-30 "Linux man-pages 6.7"
+.SH NAME
+/proc/sys/kernel/ \- control a range of kernel parameters
+.SH DESCRIPTION
+.TP
+.I /proc/sys/kernel/
+This directory contains files controlling a range of kernel parameters,
+as described below.
+.TP
+.I /proc/sys/kernel/acct
+This file
+contains three numbers:
+.IR highwater ,
+.IR lowwater ,
+and
+.IR frequency .
+If BSD-style process accounting is enabled, these values control
+its behavior.
+If free space on filesystem where the log lives goes below
+.I lowwater
+percent, accounting suspends.
+If free space gets above
+.I highwater
+percent, accounting resumes.
+.I frequency
+determines
+how often the kernel checks the amount of free space (value is in
+seconds).
+Default values are 4, 2, and 30.
+That is, suspend accounting if 2% or less space is free; resume it
+if 4% or more space is free; consider information about amount of free space
+valid for 30 seconds.
+.TP
+.IR /proc/sys/kernel/auto_msgmni " (Linux 2.6.27 to Linux 3.18)"
+.\" commit 9eefe520c814f6f62c5d36a2ddcd3fb99dfdb30e (introduces feature)
+.\" commit 0050ee059f7fc86b1df2527aaa14ed5dc72f9973 (rendered redundant)
+From Linux 2.6.27 to Linux 3.18,
+this file was used to control recomputing of the value in
+.I /proc/sys/kernel/msgmni
+upon the addition or removal of memory or upon IPC namespace creation/removal.
+Echoing "1" into this file enabled
+.I msgmni
+automatic recomputing (and triggered a recomputation of
+.I msgmni
+based on the current amount of available memory and number of IPC namespaces).
+Echoing "0" disabled automatic recomputing.
+(Automatic recomputing was also disabled if a value was explicitly assigned to
+.IR /proc/sys/kernel/msgmni .)
+The default value in
+.I auto_msgmni
+was 1.
+.IP
+Since Linux 3.19, the content of this file has no effect (because
+.I msgmni
+.\" FIXME Must document the 3.19 'msgmni' changes.
+defaults to near the maximum value possible),
+and reads from this file always return the value "0".
+.TP
+.IR /proc/sys/kernel/cap_last_cap " (since Linux 3.2)"
+See
+.BR capabilities (7).
+.TP
+.IR /proc/sys/kernel/cap\-bound " (from Linux 2.2 to Linux 2.6.24)"
+This file holds the value of the kernel
+.I "capability bounding set"
+(expressed as a signed decimal number).
+This set is ANDed against the capabilities permitted to a process
+during
+.BR execve (2).
+Starting with Linux 2.6.25,
+the system-wide capability bounding set disappeared,
+and was replaced by a per-thread bounding set; see
+.BR capabilities (7).
+.TP
+.I /proc/sys/kernel/core_pattern
+See
+.BR core (5).
+.TP
+.I /proc/sys/kernel/core_pipe_limit
+See
+.BR core (5).
+.TP
+.I /proc/sys/kernel/core_uses_pid
+See
+.BR core (5).
+.TP
+.I /proc/sys/kernel/ctrl\-alt\-del
+This file
+controls the handling of Ctrl-Alt-Del from the keyboard.
+When the value in this file is 0, Ctrl-Alt-Del is trapped and
+sent to the
+.BR init (1)
+program to handle a graceful restart.
+When the value is greater than zero, Linux's reaction to a Vulcan
+Nerve Pinch (tm) will be an immediate reboot, without even
+syncing its dirty buffers.
+Note: when a program (like dosemu) has the keyboard in "raw"
+mode, the Ctrl-Alt-Del is intercepted by the program before it
+ever reaches the kernel tty layer, and it's up to the program
+to decide what to do with it.
+.TP
+.IR /proc/sys/kernel/dmesg_restrict " (since Linux 2.6.37)"
+The value in this file determines who can see kernel syslog contents.
+A value of 0 in this file imposes no restrictions.
+If the value is 1, only privileged users can read the kernel syslog.
+(See
+.BR syslog (2)
+for more details.)
+Since Linux 3.4,
+.\" commit 620f6e8e855d6d447688a5f67a4e176944a084e8
+only users with the
+.B CAP_SYS_ADMIN
+capability may change the value in this file.
+.TP
+.IR /proc/sys/kernel/domainname " and " /proc/sys/kernel/hostname
+can be used to set the NIS/YP domainname and the
+hostname of your box in exactly the same way as the commands
+.BR domainname (1)
+and
+.BR hostname (1),
+that is:
+.IP
+.in +4n
+.EX
+.RB "#" " echo \[aq]darkstar\[aq] > /proc/sys/kernel/hostname"
+.RB "#" " echo \[aq]mydomain\[aq] > /proc/sys/kernel/domainname"
+.EE
+.in
+.IP
+has the same effect as
+.IP
+.in +4n
+.EX
+.RB "#" " hostname \[aq]darkstar\[aq]"
+.RB "#" " domainname \[aq]mydomain\[aq]"
+.EE
+.in
+.IP
+Note, however, that the classic darkstar.frop.org has the
+hostname "darkstar" and DNS (Internet Domain Name Server)
+domainname "frop.org", not to be confused with the NIS (Network
+Information Service) or YP (Yellow Pages) domainname.
+These two
+domain names are in general different.
+For a detailed discussion
+see the
+.BR hostname (1)
+man page.
+.TP
+.I /proc/sys/kernel/hotplug
+This file
+contains the pathname for the hotplug policy agent.
+The default value in this file is
+.IR /sbin/hotplug .
+.TP
+.\" Removed in commit 87f504e5c78b910b0c1d6ffb89bc95e492322c84 (tglx/history.git)
+.IR /proc/sys/kernel/htab\-reclaim " (before Linux 2.4.9.2)"
+(PowerPC only) If this file is set to a nonzero value,
+the PowerPC htab
+.\" removed in commit 1b483a6a7b2998e9c98ad985d7494b9b725bd228, before Linux 2.6.28
+(see kernel file
+.IR Documentation/powerpc/ppc_htab.txt )
+is pruned
+each time the system hits the idle loop.
+.TP
+.I /proc/sys/kernel/keys/
+This directory contains various files that define parameters and limits
+for the key-management facility.
+These files are described in
+.BR keyrings (7).
+.TP
+.IR /proc/sys/kernel/kptr_restrict " (since Linux 2.6.38)"
+.\" 455cd5ab305c90ffc422dd2e0fb634730942b257
+The value in this file determines whether kernel addresses are exposed via
+.I /proc
+files and other interfaces.
+A value of 0 in this file imposes no restrictions.
+If the value is 1, kernel pointers printed using the
+.I %pK
+format specifier will be replaced with zeros unless the user has the
+.B CAP_SYSLOG
+capability.
+If the value is 2, kernel pointers printed using the
+.I %pK
+format specifier will be replaced with zeros regardless
+of the user's capabilities.
+The initial default value for this file was 1,
+but the default was changed
+.\" commit 411f05f123cbd7f8aa1edcae86970755a6e2a9d9
+to 0 in Linux 2.6.39.
+Since Linux 3.4,
+.\" commit 620f6e8e855d6d447688a5f67a4e176944a084e8
+only users with the
+.B CAP_SYS_ADMIN
+capability can change the value in this file.
+.TP
+.I /proc/sys/kernel/l2cr
+(PowerPC only) This file
+contains a flag that controls the L2 cache of G3 processor
+boards.
+If 0, the cache is disabled.
+Enabled if nonzero.
+.TP
+.I /proc/sys/kernel/modprobe
+This file contains the pathname for the kernel module loader.
+The default value is
+.IR /sbin/modprobe .
+The file is present only if the kernel is built with the
+.B CONFIG_MODULES
+.RB ( CONFIG_KMOD
+in Linux 2.6.26 and earlier)
+option enabled.
+It is described by the Linux kernel source file
+.I Documentation/kmod.txt
+(present only in Linux 2.4 and earlier).
+.TP
+.IR /proc/sys/kernel/modules_disabled " (since Linux 2.6.31)"
+.\" 3d43321b7015387cfebbe26436d0e9d299162ea1
+.\" From Documentation/sysctl/kernel.txt
+A toggle value indicating if modules are allowed to be loaded
+in an otherwise modular kernel.
+This toggle defaults to off (0), but can be set true (1).
+Once true, modules can be neither loaded nor unloaded,
+and the toggle cannot be set back to false.
+The file is present only if the kernel is built with the
+.B CONFIG_MODULES
+option enabled.
+.TP
+.IR /proc/sys/kernel/msgmax " (since Linux 2.2)"
+This file defines
+a system-wide limit specifying the maximum number of bytes in
+a single message written on a System V message queue.
+.TP
+.IR /proc/sys/kernel/msgmni " (since Linux 2.4)"
+This file defines the system-wide limit on the number of
+message queue identifiers.
+See also
+.IR /proc/sys/kernel/auto_msgmni .
+.TP
+.IR /proc/sys/kernel/msgmnb " (since Linux 2.2)"
+This file defines a system-wide parameter used to initialize the
+.I msg_qbytes
+setting for subsequently created message queues.
+The
+.I msg_qbytes
+setting specifies the maximum number of bytes that may be written to the
+message queue.
+.TP
+.IR /proc/sys/kernel/ngroups_max " (since Linux 2.6.4)"
+This is a read-only file that displays the upper limit on the
+number of a process's group memberships.
+.TP
+.IR /proc/sys/kernel/ns_last_pid " (since Linux 3.3)"
+See
+.BR pid_namespaces (7).
+.TP
+.IR /proc/sys/kernel/ostype " and " /proc/sys/kernel/osrelease
+These files
+give substrings of
+.IR /proc/version .
+.TP
+.IR /proc/sys/kernel/overflowgid " and " /proc/sys/kernel/overflowuid
+These files duplicate the files
+.I /proc/sys/fs/overflowgid
+and
+.IR /proc/sys/fs/overflowuid .
+.TP
+.I /proc/sys/kernel/panic
+This file gives read/write access to the kernel variable
+.IR panic_timeout .
+If this is zero, the kernel will loop on a panic; if nonzero,
+it indicates that the kernel should autoreboot after this number
+of seconds.
+When you use the
+software watchdog device driver, the recommended setting is 60.
+.TP
+.IR /proc/sys/kernel/panic_on_oops " (since Linux 2.5.68)"
+This file controls the kernel's behavior when an oops
+or BUG is encountered.
+If this file contains 0, then the system
+tries to continue operation.
+If it contains 1, then the system
+delays a few seconds (to give klogd time to record the oops output)
+and then panics.
+If the
+.I /proc/sys/kernel/panic
+file is also nonzero, then the machine will be rebooted.
+.TP
+.IR /proc/sys/kernel/pid_max " (since Linux 2.5.34)"
+This file specifies the value at which PIDs wrap around
+(i.e., the value in this file is one greater than the maximum PID).
+PIDs greater than this value are not allocated;
+thus, the value in this file also acts as a system-wide limit
+on the total number of processes and threads.
+The default value for this file, 32768,
+results in the same range of PIDs as on earlier kernels.
+On 32-bit platforms, 32768 is the maximum value for
+.IR pid_max .
+On 64-bit systems,
+.I pid_max
+can be set to any value up to 2\[ha]22
+.RB ( PID_MAX_LIMIT ,
+approximately 4 million).
+.\" Prior to Linux 2.6.10, pid_max could also be raised above 32768 on 32-bit
+.\" platforms, but this broke /proc/[pid]
+.\" See http://marc.theaimsgroup.com/?l=linux-kernel&m=109513010926152&w=2
+.TP
+.IR /proc/sys/kernel/powersave\-nap " (PowerPC only)"
+This file contains a flag.
+If set, Linux-PPC will use the "nap" mode of
+powersaving,
+otherwise the "doze" mode will be used.
+.TP
+.I /proc/sys/kernel/printk
+See
+.BR syslog (2).
+.TP
+.IR /proc/sys/kernel/pty " (since Linux 2.6.4)"
+This directory contains two files relating to the number of UNIX 98
+pseudoterminals (see
+.BR pts (4))
+on the system.
+.TP
+.I /proc/sys/kernel/pty/max
+This file defines the maximum number of pseudoterminals.
+.\" FIXME Document /proc/sys/kernel/pty/reserve
+.\"     New in Linux 3.3
+.\"     commit e9aba5158a80098447ff207a452a3418ae7ee386
+.TP
+.I /proc/sys/kernel/pty/nr
+This read-only file
+indicates how many pseudoterminals are currently in use.
+.TP
+.I /proc/sys/kernel/random/
+This directory
+contains various parameters controlling the operation of the file
+.IR /dev/random .
+See
+.BR random (4)
+for further information.
+.TP
+.IR /proc/sys/kernel/random/uuid " (since Linux 2.4)"
+Each read from this read-only file returns a randomly generated 128-bit UUID,
+as a string in the standard UUID format.
+.TP
+.IR /proc/sys/kernel/randomize_va_space " (since Linux 2.6.12)"
+.\" Some further details can be found in Documentation/sysctl/kernel.txt
+Select the address space layout randomization (ASLR) policy for the system
+(on architectures that support ASLR).
+Three values are supported for this file:
+.RS
+.TP
+.B 0
+Turn ASLR off.
+This is the default for architectures that don't support ASLR,
+and when the kernel is booted with the
+.I norandmaps
+parameter.
+.TP
+.B 1
+Make the addresses of
+.BR mmap (2)
+allocations, the stack, and the VDSO page randomized.
+Among other things, this means that shared libraries will be
+loaded at randomized addresses.
+The text segment of PIE-linked binaries will also be loaded
+at a randomized address.
+This value is the default if the kernel was configured with
+.BR CONFIG_COMPAT_BRK .
+.TP
+.B 2
+(Since Linux 2.6.25)
+.\" commit c1d171a002942ea2d93b4fbd0c9583c56fce0772
+Also support heap randomization.
+This value is the default if the kernel was not configured with
+.BR CONFIG_COMPAT_BRK .
+.RE
+.TP
+.I /proc/sys/kernel/real\-root\-dev
+This file is documented in the Linux kernel source file
+.I Documentation/admin\-guide/initrd.rst
+.\" commit 9d85025b0418163fae079c9ba8f8445212de8568
+(or
+.I Documentation/initrd.txt
+before Linux 4.10).
+.TP
+.IR /proc/sys/kernel/reboot\-cmd " (Sparc only)"
+This file seems to be a way to give an argument to the SPARC
+ROM/Flash boot loader.
+Maybe to tell it what to do after
+rebooting?
+.TP
+.I /proc/sys/kernel/rtsig\-max
+(Up to and including Linux 2.6.7; see
+.BR setrlimit (2))
+This file can be used to tune the maximum number
+of POSIX real-time (queued) signals that can be outstanding
+in the system.
+.TP
+.I /proc/sys/kernel/rtsig\-nr
+(Up to and including Linux 2.6.7.)
+This file shows the number of POSIX real-time signals currently queued.
+.TP
+.IR /proc/ pid /sched_autogroup_enabled " (since Linux 2.6.38)"
+.\" commit 5091faa449ee0b7d73bc296a93bca9540fc51d0a
+See
+.BR sched (7).
+.TP
+.IR /proc/sys/kernel/sched_child_runs_first " (since Linux 2.6.23)"
+If this file contains the value zero, then, after a
+.BR fork (2),
+the parent is first scheduled on the CPU.
+If the file contains a nonzero value,
+then the child is scheduled first on the CPU.
+(Of course, on a multiprocessor system,
+the parent and the child might both immediately be scheduled on a CPU.)
+.TP
+.IR /proc/sys/kernel/sched_rr_timeslice_ms " (since Linux 3.9)"
+See
+.BR sched_rr_get_interval (2).
+.TP
+.IR /proc/sys/kernel/sched_rt_period_us " (since Linux 2.6.25)"
+See
+.BR sched (7).
+.TP
+.IR /proc/sys/kernel/sched_rt_runtime_us " (since Linux 2.6.25)"
+See
+.BR sched (7).
+.TP
+.IR /proc/sys/kernel/seccomp/ " (since Linux 4.14)"
+.\" commit 8e5f1ad116df6b0de65eac458d5e7c318d1c05af
+This directory provides additional seccomp information and
+configuration.
+See
+.BR seccomp (2)
+for further details.
+.TP
+.IR /proc/sys/kernel/sem " (since Linux 2.4)"
+This file contains 4 numbers defining limits for System V IPC semaphores.
+These fields are, in order:
+.RS
+.TP
+SEMMSL
+The maximum semaphores per semaphore set.
+.TP
+SEMMNS
+A system-wide limit on the number of semaphores in all semaphore sets.
+.TP
+SEMOPM
+The maximum number of operations that may be specified in a
+.BR semop (2)
+call.
+.TP
+SEMMNI
+A system-wide limit on the maximum number of semaphore identifiers.
+.RE
+.TP
+.I /proc/sys/kernel/sg\-big\-buff
+This file
+shows the size of the generic SCSI device (sg) buffer.
+You can't tune it just yet, but you could change it at
+compile time by editing
+.I include/scsi/sg.h
+and changing
+the value of
+.BR SG_BIG_BUFF .
+However, there shouldn't be any reason to change this value.
+.TP
+.IR /proc/sys/kernel/shm_rmid_forced " (since Linux 3.1)"
+.\" commit b34a6b1da371ed8af1221459a18c67970f7e3d53
+.\" See also Documentation/sysctl/kernel.txt
+If this file is set to 1, all System V shared memory segments will
+be marked for destruction as soon as the number of attached processes
+falls to zero;
+in other words, it is no longer possible to create shared memory segments
+that exist independently of any attached process.
+.IP
+The effect is as though a
+.BR shmctl (2)
+.B IPC_RMID
+is performed on all existing segments as well as all segments
+created in the future (until this file is reset to 0).
+Note that existing segments that are attached to no process will be
+immediately destroyed when this file is set to 1.
+Setting this option will also destroy segments that were created,
+but never attached,
+upon termination of the process that created the segment with
+.BR shmget (2).
+.IP
+Setting this file to 1 provides a way of ensuring that
+all System V shared memory segments are counted against the
+resource usage and resource limits (see the description of
+.B RLIMIT_AS
+in
+.BR getrlimit (2))
+of at least one process.
+.IP
+Because setting this file to 1 produces behavior that is nonstandard
+and could also break existing applications,
+the default value in this file is 0.
+Set this file to 1 only if you have a good understanding
+of the semantics of the applications using
+System V shared memory on your system.
+.TP
+.IR /proc/sys/kernel/shmall " (since Linux 2.2)"
+This file
+contains the system-wide limit on the total number of pages of
+System V shared memory.
+.TP
+.IR /proc/sys/kernel/shmmax " (since Linux 2.2)"
+This file
+can be used to query and set the run-time limit
+on the maximum (System V IPC) shared memory segment size that can be
+created.
+Shared memory segments up to 1 GB are now supported in the
+kernel.
+This value defaults to
+.BR SHMMAX .
+.TP
+.IR /proc/sys/kernel/shmmni " (since Linux 2.4)"
+This file
+specifies the system-wide maximum number of System V shared memory
+segments that can be created.
+.TP
+.IR /proc/sys/kernel/sysctl_writes_strict " (since Linux 3.16)"
+.\" commit f88083005ab319abba5d0b2e4e997558245493c8
+.\" commit 2ca9bb456ada8bcbdc8f77f8fc78207653bbaa92
+.\" commit f4aacea2f5d1a5f7e3154e967d70cf3f711bcd61
+.\" commit 24fe831c17ab8149413874f2fd4e5c8a41fcd294
+The value in this file determines how the file offset affects
+the behavior of updating entries in files under
+.IR /proc/sys .
+The file has three possible values:
+.RS
+.TP 4
+\-1
+This provides legacy handling, with no printk warnings.
+Each
+.BR write (2)
+must fully contain the value to be written,
+and multiple writes on the same file descriptor
+will overwrite the entire value, regardless of the file position.
+.TP
+0
+(default) This provides the same behavior as for \-1,
+but printk warnings are written for processes that
+perform writes when the file offset is not 0.
+.TP
+1
+Respect the file offset when writing strings into
+.I /proc/sys
+files.
+Multiple writes will
+.I append
+to the value buffer.
+Anything written beyond the maximum length
+of the value buffer will be ignored.
+Writes to numeric
+.I /proc/sys
+entries must always be at file offset 0 and the value must be
+fully contained in the buffer provided to
+.BR write (2).
+.\" FIXME .
+.\"     With /proc/sys/kernel/sysctl_writes_strict==1, writes at an
+.\"     offset other than 0 do not generate an error. Instead, the
+.\"     write() succeeds, but the file is left unmodified.
+.\"     This is surprising. The behavior may change in the future.
+.\"     See thread.gmane.org/gmane.linux.man/9197
+.\"		From: Michael Kerrisk (man-pages <mtk.manpages@...>
+.\"		Subject: sysctl_writes_strict documentation + an oddity?
+.\"		Newsgroups: gmane.linux.man, gmane.linux.kernel
+.\"		Date: 2015-05-09 08:54:11 GMT
+.RE
+.TP
+.I /proc/sys/kernel/sysrq
+This file controls the functions allowed to be invoked by the SysRq key.
+By default,
+the file contains 1 meaning that every possible SysRq request is allowed
+(in older kernel versions, SysRq was disabled by default,
+and you were required to specifically enable it at run-time,
+but this is not the case any more).
+Possible values in this file are:
+.RS
+.TP 5
+0
+Disable sysrq completely
+.TP
+1
+Enable all functions of sysrq
+.TP
+> 1
+Bit mask of allowed sysrq functions, as follows:
+.PD 0
+.RS
+.TP 5
+\ \ 2
+Enable control of console logging level
+.TP
+\ \ 4
+Enable control of keyboard (SAK, unraw)
+.TP
+\ \ 8
+Enable debugging dumps of processes etc.
+.TP
+\ 16
+Enable sync command
+.TP
+\ 32
+Enable remount read-only
+.TP
+\ 64
+Enable signaling of processes (term, kill, oom-kill)
+.TP
+128
+Allow reboot/poweroff
+.TP
+256
+Allow nicing of all real-time tasks
+.RE
+.PD
+.RE
+.IP
+This file is present only if the
+.B CONFIG_MAGIC_SYSRQ
+kernel configuration option is enabled.
+For further details see the Linux kernel source file
+.I Documentation/admin\-guide/sysrq.rst
+.\" commit 9d85025b0418163fae079c9ba8f8445212de8568
+(or
+.I Documentation/sysrq.txt
+before Linux 4.10).
+.TP
+.I /proc/sys/kernel/version
+This file contains a string such as:
+.IP
+.in +4n
+.EX
+#5 Wed Feb 25 21:49:24 MET 1998
+.EE
+.in
+.IP
+The "#5" means that
+this is the fifth kernel built from this source base and the
+date following it indicates the time the kernel was built.
+.TP
+.IR /proc/sys/kernel/threads\-max " (since Linux 2.3.11)"
+.\" The following is based on Documentation/sysctl/kernel.txt
+This file specifies the system-wide limit on the number of
+threads (tasks) that can be created on the system.
+.IP
+Since Linux 4.1,
+.\" commit 230633d109e35b0a24277498e773edeb79b4a331
+the value that can be written to
+.I threads\-max
+is bounded.
+The minimum value that can be written is 20.
+The maximum value that can be written is given by the
+constant
+.B FUTEX_TID_MASK
+(0x3fffffff).
+If a value outside of this range is written to
+.IR threads\-max ,
+the error
+.B EINVAL
+occurs.
+.IP
+The value written is checked against the available RAM pages.
+If the thread structures would occupy too much (more than 1/8th)
+of the available RAM pages,
+.I threads\-max
+is reduced accordingly.
+.TP
+.IR /proc/sys/kernel/yama/ptrace_scope " (since Linux 3.5)"
+See
+.BR ptrace (2).
+.TP
+.IR /proc/sys/kernel/zero\-paged " (PowerPC only)"
+This file
+contains a flag.
+When enabled (nonzero), Linux-PPC will pre-zero pages in
+the idle loop, possibly speeding up get_free_pages.
+.SH SEE ALSO
+.BR proc (5),
+.BR proc_sys (5)