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NAME | DESCRIPTION | STANDARDS | NOTES | SEE ALSO | COLOPHON |
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credentials(7) Miscellaneous Information Manual credentials(7)
credentials - process identifiers
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 jobs 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. Before Linux 2.6.4, a process can
be a member of up to 32 supplementary groups; since Linux
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).
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.
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.
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.9.1 2024-05-02 credentials(7)
Pages that refer to this page: renice(1), access(2), execve(2), fork(2), getgid(2), getgroups(2), getpid(2), getresuid(2), getrlimit(2), getsid(2), getuid(2), intro(2), keyctl(2), kill(2), openat2(2), PR_SET_DUMPABLE(2const), ptrace(2), seteuid(2), setfsgid(2), setfsuid(2), setgid(2), setpgid(2), setresuid(2), setreuid(2), setsid(2), setuid(2), wait(2), euidaccess(3), id_t(3type), initgroups(3), intro(3), killpg(3), lttng-ust(3), sd_bus_creds_get_pid(3), tcgetpgrp(3), proc_sys_fs(5), capabilities(7), cgroup_namespaces(7), landlock(7), namespaces(7), nptl(7), path_resolution(7), pid_namespaces(7), unix(7), user_namespaces(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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