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-.\" Copyright (c) 2007 by Michael Kerrisk <mtk.manpages@gmail.com>
-.\"
-.\" SPDX-License-Identifier: Linux-man-pages-copyleft
-.\"
-.\" 2007-06-13 Creation
-.\"
-.TH credentials 7 2023-11-19 "Linux man-pages 6.7"
-.SH NAME
-credentials \- process identifiers
-.SH DESCRIPTION
-.SS Process ID (PID)
-Each process has a unique nonnegative integer identifier
-that is assigned when the process is created using
-.BR fork (2).
-A process can obtain its PID using
-.BR getpid (2).
-A PID is represented using the type
-.I pid_t
-(defined in
-.IR <sys/types.h> ).
-.P
-PIDs are used in a range of system calls to identify the process
-affected by the call, for example:
-.BR kill (2),
-.BR ptrace (2),
-.BR setpriority (2),
-.\" .BR sched_rr_get_interval (2),
-.\" .BR sched_getaffinity (2),
-.\" .BR sched_setaffinity (2),
-.\" .BR sched_getparam (2),
-.\" .BR sched_setparam (2),
-.\" .BR sched_setscheduler (2),
-.\" .BR sched_getscheduler (2),
-.BR setpgid (2),
-.\" .BR getsid (2),
-.BR setsid (2),
-.BR sigqueue (3),
-and
-.BR waitpid (2).
-.\" .BR waitid (2),
-.\" .BR wait4 (2),
-.P
-A process's PID is preserved across an
-.BR execve (2).
-.SS Parent process ID (PPID)
-A process's parent process ID identifies the process that created
-this process using
-.BR fork (2).
-A process can obtain its PPID using
-.BR getppid (2).
-A PPID is represented using the type
-.IR pid_t .
-.P
-A process's PPID is preserved across an
-.BR execve (2).
-.SS Process group ID and session ID
-Each process has a session ID and a process group ID,
-both represented using the type
-.IR pid_t .
-A process can obtain its session ID using
-.BR getsid (2),
-and its process group ID using
-.BR getpgrp (2).
-.P
-A child created by
-.BR fork (2)
-inherits its parent's session ID and process group ID.
-A process's session ID and process group ID are preserved across an
-.BR execve (2).
-.P
-Sessions and process groups are abstractions devised to support shell
-job control.
-A process group (sometimes called a "job") is a collection of
-processes that share the same process group ID;
-the shell creates a new process group for the process(es) used
-to execute single command or pipeline (e.g., the two processes
-created to execute the command "ls\ |\ wc" are placed in the
-same process group).
-A process's group membership can be set using
-.BR setpgid (2).
-The process whose process ID is the same as its process group ID is the
-\fIprocess group leader\fP for that group.
-.P
-A session is a collection of processes that share the same session ID.
-All of the members of a process group also have the same session ID
-(i.e., all of the members of a process group always belong to the
-same session, so that sessions and process groups form a strict
-two-level hierarchy of processes.)
-A new session is created when a process calls
-.BR setsid (2),
-which creates a new session whose session ID is the same
-as the PID of the process that called
-.BR setsid (2).
-The creator of the session is called the \fIsession leader\fP.
-.P
-All of the processes in a session share a
-.IR "controlling terminal" .
-The controlling terminal is established when the session leader
-first opens a terminal (unless the
-.B O_NOCTTY
-flag is specified when calling
-.BR open (2)).
-A terminal may be the controlling terminal of at most one session.
-.P
-At most one of the jobs in a session may be the
-.IR "foreground job" ;
-other jobs in the session are
-.IR "background jobs" .
-Only the foreground job may read from the terminal;
-when a process in the background attempts to read from the terminal,
-its process group is sent a
-.B SIGTTIN
-signal, which suspends the job.
-If the
-.B TOSTOP
-flag has been set for the terminal (see
-.BR termios (3)),
-then only the foreground job may write to the terminal;
-writes from background jobs cause a
-.B SIGTTOU
-signal to be generated, which suspends the job.
-When terminal keys that generate a signal (such as the
-.I interrupt
-key, normally control-C)
-are pressed, the signal is sent to the processes in the foreground job.
-.P
-Various system calls and library functions
-may operate on all members of a process group,
-including
-.BR kill (2),
-.BR killpg (3),
-.BR getpriority (2),
-.BR setpriority (2),
-.BR ioprio_get (2),
-.BR ioprio_set (2),
-.BR waitid (2),
-and
-.BR waitpid (2).
-See also the discussion of the
-.BR F_GETOWN ,
-.BR F_GETOWN_EX ,
-.BR F_SETOWN ,
-and
-.B F_SETOWN_EX
-operations in
-.BR fcntl (2).
-.SS User and group identifiers
-Each process has various associated user and group IDs.
-These IDs are integers, respectively represented using the types
-.I uid_t
-and
-.I gid_t
-(defined in
-.IR <sys/types.h> ).
-.P
-On Linux, each process has the following user and group identifiers:
-.IP \[bu] 3
-Real user ID and real group ID.
-These IDs determine who owns the process.
-A process can obtain its real user (group) ID using
-.BR getuid (2)
-.RB ( getgid (2)).
-.IP \[bu]
-Effective user ID and effective group ID.
-These IDs are used by the kernel to determine the permissions
-that the process will have when accessing shared resources such
-as message queues, shared memory, and semaphores.
-On most UNIX systems, these IDs also determine the
-permissions when accessing files.
-However, Linux uses the filesystem IDs described below
-for this task.
-A process can obtain its effective user (group) ID using
-.BR geteuid (2)
-.RB ( getegid (2)).
-.IP \[bu]
-Saved set-user-ID and saved set-group-ID.
-These IDs are used in set-user-ID and set-group-ID programs to save
-a copy of the corresponding effective IDs that were set when
-the program was executed (see
-.BR execve (2)).
-A set-user-ID program can assume and drop privileges by
-switching its effective user ID back and forth between the values
-in its real user ID and saved set-user-ID.
-This switching is done via calls to
-.BR seteuid (2),
-.BR setreuid (2),
-or
-.BR setresuid (2).
-A set-group-ID program performs the analogous tasks using
-.BR setegid (2),
-.BR setregid (2),
-or
-.BR setresgid (2).
-A process can obtain its saved set-user-ID (set-group-ID) using
-.BR getresuid (2)
-.RB ( getresgid (2)).
-.IP \[bu]
-Filesystem user ID and filesystem group ID (Linux-specific).
-These IDs, in conjunction with the supplementary group IDs described
-below, are used to determine permissions for accessing files; see
-.BR path_resolution (7)
-for details.
-Whenever a process's effective user (group) ID is changed,
-the kernel also automatically changes the filesystem user (group) ID
-to the same value.
-Consequently, the filesystem IDs normally have the same values
-as the corresponding effective ID, and the semantics for file-permission
-checks are thus the same on Linux as on other UNIX systems.
-The filesystem IDs can be made to differ from the effective IDs
-by calling
-.BR setfsuid (2)
-and
-.BR setfsgid (2).
-.IP \[bu]
-Supplementary group IDs.
-This is a set of additional group IDs that are used for permission
-checks when accessing files and other shared resources.
-Before Linux 2.6.4,
-a process can be a member of up to 32 supplementary groups;
-since Linux 2.6.4,
-a process can be a member of up to 65536 supplementary groups.
-The call
-.I sysconf(_SC_NGROUPS_MAX)
-can be used to determine the number of supplementary groups
-of which a process may be a member.
-.\" Since Linux 2.6.4, the limit is visible via the read-only file
-.\" /proc/sys/kernel/ngroups_max.
-.\" As at 2.6.22-rc2, this file is still read-only.
-A process can obtain its set of supplementary group IDs using
-.BR getgroups (2).
-.P
-A child process created by
-.BR fork (2)
-inherits copies of its parent's user and groups IDs.
-During an
-.BR execve (2),
-a process's real user and group ID and supplementary
-group IDs are preserved;
-the effective and saved set IDs may be changed, as described in
-.BR execve (2).
-.P
-Aside from the purposes noted above,
-a process's user IDs are also employed in a number of other contexts:
-.IP \[bu] 3
-when determining the permissions for sending signals (see
-.BR kill (2));
-.IP \[bu]
-when determining the permissions for setting
-process-scheduling parameters (nice value, real time
-scheduling policy and priority, CPU affinity, I/O priority) using
-.BR setpriority (2),
-.BR sched_setaffinity (2),
-.BR sched_setscheduler (2),
-.BR sched_setparam (2),
-.BR sched_setattr (2),
-and
-.BR ioprio_set (2);
-.IP \[bu]
-when checking resource limits (see
-.BR getrlimit (2));
-.IP \[bu]
-when checking the limit on the number of inotify instances
-that the process may create (see
-.BR inotify (7)).
-.\"
-.SS Modifying process user and group IDs
-Subject to rules described in the relevant manual pages,
-a process can use the following APIs to modify its user and group IDs:
-.TP
-.BR setuid (2)\~(\c
-.BR setgid (2))
-Modify the process's real (and possibly effective and saved-set)
-user (group) IDs.
-.TP
-.BR seteuid (2)\~(\c
-.BR setegid (2))
-Modify the process's effective user (group) ID.
-.TP
-.BR setfsuid (2)\~(\c
-.BR setfsgid (2))
-Modify the process's filesystem user (group) ID.
-.TP
-.BR setreuid (2)\~(\c
-.BR setregid (2))
-Modify the process's real and effective (and possibly saved-set)
-user (group) IDs.
-.TP
-.BR setresuid (2)\~(\c
-.BR setresgid (2))
-Modify the process's real, effective, and saved-set user (group) IDs.
-.TP
-.BR setgroups (2)
-Modify the process's supplementary group list.
-.P
-Any changes to a process's effective user (group) ID
-are automatically carried over to the process's
-filesystem user (group) ID.
-Changes to a process's effective user or group ID can also affect the
-process "dumpable" attribute, as described in
-.BR prctl (2).
-.P
-Changes to process user and group IDs can affect the capabilities
-of the process, as described in
-.BR capabilities (7).
-.SH STANDARDS
-Process IDs, parent process IDs, process group IDs, and session IDs
-are specified in POSIX.1.
-The real, effective, and saved set user and groups IDs,
-and the supplementary group IDs, are specified in POSIX.1.
-.P
-The filesystem user and group IDs are a Linux extension.
-.SH NOTES
-Various fields in the
-.IR /proc/ pid /status
-file show the process credentials described above.
-See
-.BR proc (5)
-for further information.
-.P
-The POSIX threads specification requires that
-credentials are shared by all of the threads in a process.
-However, at the kernel level, Linux maintains separate user and group
-credentials for each thread.
-The NPTL threading implementation does some work to ensure
-that any change to user or group credentials
-(e.g., calls to
-.BR setuid (2),
-.BR setresuid (2))
-is carried through to all of the POSIX threads in a process.
-See
-.BR nptl (7)
-for further details.
-.SH SEE ALSO
-.BR bash (1),
-.BR csh (1),
-.BR groups (1),
-.BR id (1),
-.BR newgrp (1),
-.BR ps (1),
-.BR runuser (1),
-.BR setpriv (1),
-.BR sg (1),
-.BR su (1),
-.BR access (2),
-.BR execve (2),
-.BR faccessat (2),
-.BR fork (2),
-.BR getgroups (2),
-.BR getpgrp (2),
-.BR getpid (2),
-.BR getppid (2),
-.BR getsid (2),
-.BR kill (2),
-.BR setegid (2),
-.BR seteuid (2),
-.BR setfsgid (2),
-.BR setfsuid (2),
-.BR setgid (2),
-.BR setgroups (2),
-.BR setpgid (2),
-.BR setresgid (2),
-.BR setresuid (2),
-.BR setsid (2),
-.BR setuid (2),
-.BR waitpid (2),
-.BR euidaccess (3),
-.BR initgroups (3),
-.BR killpg (3),
-.BR tcgetpgrp (3),
-.BR tcgetsid (3),
-.BR tcsetpgrp (3),
-.BR group (5),
-.BR passwd (5),
-.BR shadow (5),
-.BR capabilities (7),
-.BR namespaces (7),
-.BR path_resolution (7),
-.BR pid_namespaces (7),
-.BR pthreads (7),
-.BR signal (7),
-.BR system_data_types (7),
-.BR unix (7),
-.BR user_namespaces (7),
-.BR sudo (8)