credentials

CREDENTIALS(7)             Linux Programmer's Manual            CREDENTIALS(7)



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 represented 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 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).  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), 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 process 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 discussion 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) (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 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), 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 supplementary 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.  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) 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 supplementary group IDs using getgroups(2),
          and can modify the set using setgroups(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));

       *  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)).

CONFORMING TO
       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
       Various fields in the /proc/[pid]/status file show the process
       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 implementation 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), unix(7), user_namespaces(7),
       sudo(8)

COLOPHON
       This page is part of release 5.04 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/.



Linux                             2019-08-02                    CREDENTIALS(7)