credentials(7) Miscellaneous Information Manual credentials(7)
NAME
credentials - process identifiers
DESCRIPTION
Process ID (PID)
Each process has a unique nonnegative integer identifier that is as-
signed 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 (de-
fined in <sys/types.h>).
PIDs are used in a range of system calls to identify the process af-
fected 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 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 mem-
bership 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 hi-
erarchy 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 ter-
minal; writes from background jobs cause a SIGTTOU signal to be gener-
ated, 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), setprior-
ity(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_SE-
TOWN_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 (de-
fined 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 ge-
tuid(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 per-
missions when accessing files. However, Linux uses the filesystem
IDs described below for this task. A process can obtain its effec-
tive 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 ex-
ecve(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_reso-
lution(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 supple-
mentary 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 em-
ployed in a number of other contexts:
• when determining the permissions for sending signals (see kill(2));
• when determining the permissions for setting process-scheduling para-
meters (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 io-
prio_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).
STANDARDS
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), set-
priv(1), sg(1), su(1), access(2), execve(2), faccessat(2), fork(2), get-
groups(2), getpgrp(2), getpid(2), getppid(2), getsid(2), kill(2), sete-
gid(2), seteuid(2), setfsgid(2), setfsuid(2), setgid(2), setgroups(2),
setpgid(2), setresgid(2), setresuid(2), setsid(2), setuid(2), wait-
pid(2), euidaccess(3), initgroups(3), killpg(3), tcgetpgrp(3), tcget-
sid(3), tcsetpgrp(3), group(5), passwd(5), shadow(5), capabilities(7),
namespaces(7), path_resolution(7), pid_namespaces(7), pthreads(7), sig-
nal(7), system_data_types(7), unix(7), user_namespaces(7), sudo(8)
Linux man-pages 6.9.1 2024-05-02 credentials(7)
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