credentials(7) - Linux manual page (original) (raw)

credentials(7) Miscellaneous Information Manual credentials(7)

NAME top

   credentials - process identifiers

DESCRIPTION top

Process ID (PID) Each process has a unique nonnegative integer identifier that is assigned when the process is created using fork(2). A process can obtain its PID using getpid(2). A PID is represented using the type pidt (defined in <sys/types.h>).

   PIDs are used in a range of system calls to identify the process
   affected by the call, for example: [kill(2)](../man2/kill.2.html), [ptrace(2)](../man2/ptrace.2.html),
   [setpriority(2)](../man2/setpriority.2.html), [setpgid(2)](../man2/setpgid.2.html), [setsid(2)](../man2/setsid.2.html), [sigqueue(3)](../man3/sigqueue.3.html), and
   [waitpid(2)](../man2/waitpid.2.html).

   A process's PID is preserved across an [execve(2)](../man2/execve.2.html).

Parent process ID (PPID) A process's parent process ID identifies the process that created this process using fork(2). A process can obtain its PPID using getppid(2). A PPID is represented using the type pidt.

   A process's PPID is preserved across an [execve(2)](../man2/execve.2.html).

Process group ID and session ID Each process has a session ID and a process group ID, both represented using the type pidt. A process can obtain its session ID using getsid(2), and its process group ID using getpgrp(2).

   A child created by [fork(2)](../man2/fork.2.html) inherits its parent's session ID and
   process group ID.  A process's session ID and process group ID are
   preserved across an [execve(2)](../man2/execve.2.html).

   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
   [setpgid(2)](../man2/setpgid.2.html).  The process whose process ID is the same as its
   process group ID is the _process group leader_ for that group.

   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 [setsid(2)](../man2/setsid.2.html), which creates a new
   session whose session ID is the same as the PID of the process
   that called [setsid(2)](../man2/setsid.2.html).  The creator of the session is called the
   _session leader_.

   All of the processes in a session share a _controlling terminal_.
   The controlling terminal is established when the session leader
   first opens a terminal (unless the **O_NOCTTY** flag is specified when
   calling [open(2)](../man2/open.2.html)).  A terminal may be the controlling terminal of
   at most one session.

   At most one of the jobs in a session may be the _foreground job_;
   other jobs in the session are _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 **SIGTTIN** signal, which suspends the job.  If the **TOSTOP**
   flag has been set for the terminal (see [termios(3)](../man3/termios.3.html)), then only the
   foreground job may write to the terminal; writes from background
   jobs cause a **SIGTTOU** signal to be generated, which suspends the
   job.  When terminal keys that generate a signal (such as the
   _interrupt_ key, normally control-C) are pressed, the signal is sent
   to the processes in the foreground job.

   Various system calls and library functions may operate on all
   members of a process group, including [kill(2)](../man2/kill.2.html), [killpg(3)](../man3/killpg.3.html),
   [getpriority(2)](../man2/getpriority.2.html), [setpriority(2)](../man2/setpriority.2.html), [ioprio_get(2)](../man2/ioprio%5Fget.2.html), [ioprio_set(2)](../man2/ioprio%5Fset.2.html),
   [waitid(2)](../man2/waitid.2.html), and [waitpid(2)](../man2/waitpid.2.html).  See also the discussion of the
   **F_GETOWN**, **F_GETOWN_EX**, **F_SETOWN**, and **F_SETOWN_EX** operations in
   [fcntl(2)](../man2/fcntl.2.html).

User and group identifiers Each process has various associated user and group IDs. These IDs are integers, respectively represented using the types uidt and gidt (defined in <sys/types.h>).

   On Linux, each process has the following user and group
   identifiers:

   •  Real user ID and real group ID.  These IDs determine who owns
      the process.  A process can obtain its real user (group) ID
      using [getuid(2)](../man2/getuid.2.html) ([getgid(2)](../man2/getgid.2.html)).

   •  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
      [geteuid(2)](../man2/geteuid.2.html) ([getegid(2)](../man2/getegid.2.html)).

   •  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 [execve(2)](../man2/execve.2.html)).  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 [seteuid(2)](../man2/seteuid.2.html),
      [setreuid(2)](../man2/setreuid.2.html), or [setresuid(2)](../man2/setresuid.2.html).  A set-group-ID program performs
      the analogous tasks using [setegid(2)](../man2/setegid.2.html), [setregid(2)](../man2/setregid.2.html), or
      [setresgid(2)](../man2/setresgid.2.html).  A process can obtain its saved set-user-ID (set-
      group-ID) using [getresuid(2)](../man2/getresuid.2.html) ([getresgid(2)](../man2/getresgid.2.html)).

   •  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 [path_resolution(7)](../man7/path%5Fresolution.7.html) 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 [setfsuid(2)](../man2/setfsuid.2.html) and
      [setfsgid(2)](../man2/setfsgid.2.html).

   •  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 _sysconf(SCNGROUPSMAX)_ can be used to determine the
      number of supplementary groups of which a process may be a
      member.  A process can obtain its set of supplementary group
      IDs using [getgroups(2)](../man2/getgroups.2.html).

   A child process created by [fork(2)](../man2/fork.2.html) inherits copies of its parent's
   user and groups IDs.  During an [execve(2)](../man2/execve.2.html), 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
   [execve(2)](../man2/execve.2.html).

   Aside from the purposes noted above, a process's user IDs are also
   employed in a number of other contexts:

   •  when determining the permissions for sending signals (see
      [kill(2)](../man2/kill.2.html));

   •  when determining the permissions for setting process-scheduling
      parameters (nice value, real time scheduling policy and
      priority, CPU affinity, I/O priority) using [setpriority(2)](../man2/setpriority.2.html),
      [sched_setaffinity(2)](../man2/sched%5Fsetaffinity.2.html), [sched_setscheduler(2)](../man2/sched%5Fsetscheduler.2.html), [sched_setparam(2)](../man2/sched%5Fsetparam.2.html),
      [sched_setattr(2)](../man2/sched%5Fsetattr.2.html), and [ioprio_set(2)](../man2/ioprio%5Fset.2.html);

   •  when checking resource limits (see [getrlimit(2)](../man2/getrlimit.2.html));

   •  when checking the limit on the number of inotify instances that
      the process may create (see [inotify(7)](../man7/inotify.7.html)).

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:

   [setuid(2)](../man2/setuid.2.html) ([setgid(2)](../man2/setgid.2.html))
          Modify the process's real (and possibly effective and
          saved-set) user (group) IDs.

   [seteuid(2)](../man2/seteuid.2.html) ([setegid(2)](../man2/setegid.2.html))
          Modify the process's effective user (group) ID.

   [setfsuid(2)](../man2/setfsuid.2.html) ([setfsgid(2)](../man2/setfsgid.2.html))
          Modify the process's filesystem user (group) ID.

   [setreuid(2)](../man2/setreuid.2.html) ([setregid(2)](../man2/setregid.2.html))
          Modify the process's real and effective (and possibly
          saved-set) user (group) IDs.

   [setresuid(2)](../man2/setresuid.2.html) ([setresgid(2)](../man2/setresgid.2.html))
          Modify the process's real, effective, and saved-set user
          (group) IDs.

   [setgroups(2)](../man2/setgroups.2.html)
          Modify the process's supplementary group list.

   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
   [prctl(2)](../man2/prctl.2.html).

   Changes to process user and group IDs can affect the capabilities
   of the process, as described in [capabilities(7)](../man7/capabilities.7.html).

STANDARDS top

   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.

   The filesystem user and group IDs are a Linux extension.

NOTES top

   Various fields in the _/proc/_pid_/status_ file show the process
   credentials described above.  See [proc(5)](../man5/proc.5.html) for further information.

   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 [setuid(2)](../man2/setuid.2.html), [setresuid(2)](../man2/setresuid.2.html)) is carried through to all of the POSIX
   threads in a process.  See [nptl(7)](../man7/nptl.7.html) for further details.

COLOPHON top

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Linux man-pages 6.10 2024-05-02 credentials(7)