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 get-
sid(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 collec-
tion of processes that share the same process group ID; the shell cre-
ates 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 ses-
sion 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 termi-
nal (see termios(3)), then only the foreground job may write to the
terminal; writes from background job cause a SIGTTOU signal to be gen-
erated, 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), set-
priority(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 filesys-
tem 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 corre-
sponding effective IDs that were set when the program was executed
(see execve(2)). A set-user-ID program can assume and drop privi-
leges 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 sup-
plementary 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_setaffin-
ity(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
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), set-
priv(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), set-
groups(2), setpgid(2), setresgid(2), setresuid(2), setsid(2),
setuid(2), waitpid(2), euidaccess(3), initgroups(3), killpg(3), tcgetp-
grp(3), tcsetpgrp(3), group(5), passwd(5), shadow(5), capabilities(7),
namespaces(7), path_resolution(7), pid_namespaces(7), pthreads(7), sig-
nal(7), unix(7), user_namespaces(7), sudo(8)
COLOPHON
This page is part of release 4.15 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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