path: root/man7/credentials.7

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.TH CREDENTIALS 7 2008-06-03 "Linux" "Linux Programmer's Manual"
.SH NAME
credentials \- process identifiers
.SH DESCRIPTION
.SS Process ID (PID)
Each process has a unique nonnegative integer identifier
that is assigned when the process is created using
.BR fork (2).
A process can obtain its PID using
.BR getpid (2).
A PID is represented using the type
.I pid_t
(defined in
.IR <sys/types.h> ).

PIDs are used in a range of system calls to identify the process
affected by the call, for example:
.BR kill (2),
.BR ptrace (2),
.BR setpriority (2)
.\" .BR sched_rr_get_interval (2),
.\" .BR sched_getaffinity (2),
.\" .BR sched_setaffinity (2),
.\" .BR sched_getparam (2),
.\" .BR sched_setparam (2),
.\" .BR sched_setscheduler (2),
.\" .BR sched_getscheduler (2),
.BR setpgid (2),
.\" .BR getsid (2),
.BR setsid (2),
.BR sigqueue (2),
and
.BR waitpid (2).
.\" .BR waitid (2),
.\" .BR wait4 (2),

A process's PID is preserved across an
.BR execve (2).
.SS Parent Process ID (PPID)
A process's parent process ID identifies the process that created
this process using
.BR fork (2).
A process can obtain its PPID using
.BR getppid (2).
A PPID is represented using the type
.IR pid_t .

A process's PPID is preserved across an
.BR execve (2).
.SS Process Group ID and Session ID
Each process has a session ID and a process group ID,
both represented using the type
.IR pid_t .
A process can obtain its session ID using
.BR getsid (2),
and its process group ID using
.BR getpgrp (2).

A child created by
.BR 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
.BR 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
.BR setpgid (2).
The process whose process ID is the same as its process group ID is the
\fIprocess group leader\fP 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
.BR setsid (2),
which creates a new session whose session ID is the same
as the PID of the process that called
.BR setsid (2).
The creator of the session is called the \fIsession leader\fP.
.SS User and Group Identifiers
Each process has various associated user and groups IDs.
These IDs are integers, respectively represented using the types
.I uid_t
and
.I gid_t
(defined in
.IR <sys/types.h> ).

On Linux, each process has the following user and group identifiers:
.IP * 3
Real user ID and real group ID.
These IDs determine who owns the process.
A process can obtain its real user (group) ID using
.BR getuid (2)
.RB ( getgid (2)).
.IP *
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 file system IDs described below
for this task.
A process can obtain its effective user (group) ID using
.BR geteuid (2)
.RB ( getegid (2)).
.IP *
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
.BR 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
.BR seteuid (2),
.BR setreuid (2),
or
.BR setresuid (2).
A set-group-ID program performs the analogous tasks using
.BR setegid (2),
.BR setregid (2),
or
.BR setresgid (2).
A process can obtain its saved set-user-ID (set-group-ID) using
.BR getresuid (2)
.RB ( getresgid (2)).
.IP *
File system user ID and file system group ID (Linux-specific).
These IDs, in conjunction with the supplementary group IDs described
below, are used to determine permissions for accessing files; see
.BR path_resolution (7)
for details.
Whenever a process's effective user (group) ID is changed,
the kernel also automatically changes the file system user (group) ID
to the same value.
Consequently, the file system 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 file system IDs can be made to differ from the effective IDs
by calling
.BR setfsuid (2)
and
.BR setfsgid (2).
.IP *
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
.I sysconf(_SC_NGROUPS_MAX)
can be used to determine the number of supplementary groups
of which a process may be a member.
.\" Since kernel 2.6.4, the limit is visible via the read-only file
.\" /proc/sys/kernel/ngroups_max.
.\" As at 2.6.22-rc2, this file is still read-only.
A process can obtain its set of supplementary group IDs using
.BR getgroups (2),
and can modify the set using
.BR setgroups (2).
.PP
A child process created by
.BR fork (2)
inherits copies of its parent's user and groups IDs.
During an
.BR 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
.BR execve (2).

Aside from the purposes noted above,
a process's user IDs are also employed in a number of other contexts:
.IP * 3
when determining the permissions for sending signals \(em see
.BR kill (2);
.IP *
when determining the permissions for setting
process-scheduling parameters (nice value, real time
scheduling policy and priority, CPU affinity, I/O priority) using
.BR setpriority (2),
.BR sched_setaffinity (2),
.BR sched_setscheduler (2),
.BR sched_setparam (2),
and
.BR ioprio_set (2);
.IP *
when checking resource limits; see
.BR getrlimit (2);
.IP *
when checking the limit on the number of inotify instances
that the process may create; see
.BR inotify (7).
.SH "CONFORMING TO"
Process IDs, parent process IDs, process group IDs, and session IDs
are specified in POSIX.1-2001.
The real, effective, and saved set user and groups IDs,
and the supplementary group IDs, are specified in POSIX.1-2001.
The file system user and group IDs are a Linux extension.
.SH 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
.BR setuid (2),
.BR setresuid (2),
etc.)
is carried through to all of the POSIX threads in a process.
.SH "SEE ALSO"
.BR bash (1),
.BR csh (1),
.BR ps (1),
.BR access (2),
.BR execve (2),
.BR faccessat (2),
.BR fork (2),
.BR getpgrp (2),
.BR getpid (2),
.BR getppid (2),
.BR getsid (2),
.BR kill (2),
.BR killpg (2),
.BR setegid (2),
.BR seteuid (2),
.BR setfsgid (2),
.BR setfsuid (2),
.BR setgid (2),
.BR setgroups (2),
.BR setresgid (2),
.BR setresuid (2),
.BR setuid (2),
.BR waitpid (2),
.BR euidaccess (3),
.BR initgroups (3),
.BR tcgetpgrp (3),
.BR tcsetpgrp (3),
.BR capabilities (7),
.BR path_resolution (7),
.BR unix (7)