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diff --git a/man7/credentials.7 b/man7/credentials.7 new file mode 100644 index 0000000..653e7a3 --- /dev/null +++ b/man7/credentials.7 @@ -0,0 +1,379 @@ +.\" Copyright (c) 2007 by Michael Kerrisk <mtk.manpages@gmail.com> +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.\" 2007-06-13 Creation +.\" +.TH credentials 7 2023-03-30 "Linux man-pages 6.05.01" +.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> ). +.PP +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), +.PP +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 . +.PP +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). +.PP +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). +.PP +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. +.PP +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. +.PP +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. +.PP +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. +.PP +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> ). +.PP +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). +.PP +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). +.PP +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) (" setgid (2)) +Modify the process's real (and possibly effective and saved-set) +user (group) IDs. +.TP +.BR seteuid "(2) (" setegid (2)) +Modify the process's effective user (group) ID. +.TP +.BR setfsuid "(2) (" setfsgid (2)) +Modify the process's filesystem user (group) ID. +.TP +.BR setreuid "(2) (" setregid (2)) +Modify the process's real and effective (and possibly saved-set) +user (group) IDs. +.TP +.BR setresuid "(2) (" 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. +.PP +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). +.PP +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. +.PP +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. +.PP +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) |