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     NAME
          credentials - process identifiers

     DESCRIPTION
        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 rep-
          resented using the type pid_t (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),
          ptrace(2), setpriority(2) setpgid(2), setsid(2),
          sigqueue(3), and waitpid(2).

          A process's PID is preserved across an execve(2).

        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 pid_t.

          A process's PPID is preserved across an execve(2).

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

          A child created by 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 execve(2).

          Sessions and process groups are abstractions devised to sup-
          port shell job control.  A process group (sometimes called a
          "job") is a collection of processes that share the same pro-
          cess 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).  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

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          and process groups form a strict two-level hierarchy of pro-
          cesses.)  A new session is created when a process calls
          setsid(2), which creates a new session whose session ID is
          the same as the PID of the process that called setsid(2).
          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)).  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 pro-
          cess group is sent a SIGTTIN signal, which suspends the job.
          If the TOSTOP flag has been set for the terminal (see
          termios(3)), then only the foreground job may write to the
          terminal; writes from background job 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),
          killpg(3), getpriority(2), setpriority(2), ioprio_get(2),
          ioprio_set(2), waitid(2), and waitpid(2).  See also the dis-
          cussion of the F_GETOWN, F_GETOWN_EX, F_SETOWN, and
          F_SETOWN_EX operations in fcntl(2).

        User and group identifiers
          Each process has various associated user and group IDs.
          These IDs are integers, respectively represented using the
          types uid_t and gid_t (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) (getgid(2)).

          *  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)

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             (getegid(2)).

          *  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)).  A set-
             user-ID program can assume and drop privileges by switch-
             ing its effective user ID back and forth between the val-
             ues in its real user ID and saved set-user-ID.  This
             switching is done via calls to seteuid(2), setreuid(2),
             or setresuid(2).  A set-group-ID program performs the
             analogous tasks using setegid(2), setregid(2), or
             setresgid(2).  A process can obtain its saved set-user-ID
             (set-group-ID) using getresuid(2) (getresgid(2)).

          *  Filesystem user ID and filesystem group ID (Linux-
             specific).  These IDs, in conjunction with the supplemen-
             tary group IDs described below, are used to determine
             permissions for accessing files; see 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.  Conse-
             quently, 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) and
             setfsgid(2).

          *  Supplementary group IDs.  This is a set of additional
             group IDs that are used for permission checks when
             accessing files and other shared resources.  On Linux
             kernels before 2.6.4, a process can be a member of up to
             32 supplementary groups; since kernel 2.6.4, a process
             can be a member of up to 65536 supplementary groups.  The
             call sysconf(_SC_NGROUPS_MAX) 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 supplemen-
             tary group IDs using getgroups(2).

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

          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));

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          *  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), sched_setaffinity(2),
             sched_setscheduler(2), sched_setparam(2),
             sched_setattr(2), and ioprio_set(2);

          *  when checking resource limits (see getrlimit(2));

          *  when checking the limit on the number of inotify
             instances that the process may create (see inotify(7)).

        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) (setgid(2))
               Modify the process's real (and possibly effective and
               saved-set) user (group) IDs.

          seteuid(2) (setegid(2))
               Modify the process's effective user (group) ID.

          setfsuid(2) (setfsgid(2))
               Modify the process's filesystem user (group) ID.

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

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

          setgroups(2)
               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).

          Changes to process user and group IDs can affect the capa-
          bilities of the process, as described in capabilities(7).

     CONFORMING TO
          Process IDs, parent process IDs, process group IDs, and ses-
          sion 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

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          group IDs are a Linux extension.

     NOTES
          Various fields in the /proc/[pid]/status file show the pro-
          cess credentials described above.  See proc(5) 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 implementa-
          tion does some work to ensure that any change to user or
          group credentials (e.g., calls to setuid(2), setresuid(2))
          is carried through to all of the POSIX threads in a process.
          See nptl(7) for further details.

     SEE ALSO
          bash(1), csh(1), groups(1), id(1), newgrp(1), ps(1),
          runuser(1), setpriv(1), sg(1), su(1), access(2), execve(2),
          faccessat(2), fork(2), getgroups(2), getpgrp(2), getpid(2),
          getppid(2), getsid(2), kill(2), setegid(2), seteuid(2),
          setfsgid(2), setfsuid(2), setgid(2), setgroups(2),
          setpgid(2), setresgid(2), setresuid(2), setsid(2),
          setuid(2), waitpid(2), euidaccess(3), initgroups(3),
          killpg(3), tcgetpgrp(3), tcgetsid(3), tcsetpgrp(3),
          group(5), passwd(5), shadow(5), capabilities(7),
          namespaces(7), path_resolution(7), pid_namespaces(7),
          pthreads(7), signal(7), system_data_types(7), unix(7),
          user_namespaces(7), sudo(8)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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