credentials(7) — Linux manual page


CREDENTIALS(7)            Linux Programmer's 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
       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

       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)

       *  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)

       *  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

       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

       *  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.

              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 capabilities of
       the process, as described in capabilities(7).

CONFORMING TO         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) 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         top

       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         top

       This page is part of release 5.09 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

Linux                            2020-11-01                   CREDENTIALS(7)

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