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).

       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)).

   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 capabilities of the
       process, as described in capabilities(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), system_data_types(7), unix(7),
       user_namespaces(7), sudo(8)

COLOPHON
       This page is part of release 5.13 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                              2020-11-01                     CREDENTIALS(7)