SETREUID(2) Linux Programmer's Manual SETREUID(2)
setreuid, setregid - set real and/or effective user or group ID
int setreuid(uid_t ruid, uid_t euid);
int setregid(gid_t rgid, gid_t egid);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE
setreuid() sets real and effective user IDs of the calling process.
Supplying a value of -1 for either the real or effective user ID forces
the system to leave that ID unchanged.
Unprivileged processes may only set the effective user ID to the real
user ID, the effective user ID, or the saved set-user-ID.
Unprivileged users may only set the real user ID to the real user ID or
the effective user ID.
If the real user ID is set (i.e., ruid is not -1) or the effective user
ID is set to a value not equal to the previous real user ID, the saved
set-user-ID will be set to the new effective user ID.
Completely analogously, setregid() sets real and effective group ID's
of the calling process, and all of the above holds with "group" instead
On success, zero is returned. On error, -1 is returned, and errno is
Note: there are cases where setreuid() can fail even when the caller is
UID 0; it is a grave security error to omit checking for a failure re-
turn from setreuid().
EAGAIN The call would change the caller's real UID (i.e., ruid does not
match the caller's real UID), but there was a temporary failure
allocating the necessary kernel data structures.
EAGAIN ruid does not match the caller's real UID and this call would
bring the number of processes belonging to the real user ID ruid
over the caller's RLIMIT_NPROC resource limit. Since Linux 3.1,
this error case no longer occurs (but robust applications should
check for this error); see the description of EAGAIN in ex-
EINVAL One or more of the target user or group IDs is not valid in this
EPERM The calling process is not privileged (on Linux, does not have
the necessary capability in its user namespace: CAP_SETUID in
the case of setreuid(), or CAP_SETGID in the case of setregid())
and a change other than (i) swapping the effective user (group)
ID with the real user (group) ID, or (ii) setting one to the
value of the other or (iii) setting the effective user (group)
ID to the value of the saved set-user-ID (saved set-group-ID)
POSIX.1-2001, POSIX.1-2008, 4.3BSD (setreuid() and setregid() first ap-
peared in 4.2BSD).
Setting the effective user (group) ID to the saved set-user-ID (saved
set-group-ID) is possible since Linux 1.1.37 (1.1.38).
POSIX.1 does not specify all of the UID changes that Linux permits for
an unprivileged process. For setreuid(), the effective user ID can be
made the same as the real user ID or the saved set-user-ID, and it is
unspecified whether unprivileged processes may set the real user ID to
the real user ID, the effective user ID, or the saved set-user-ID. For
setregid(), the real group ID can be changed to the value of the saved
set-group-ID, and the effective group ID can be changed to the value of
the real group ID or the saved set-group-ID. The precise details of
what ID changes are permitted vary across implementations.
POSIX.1 makes no specification about the effect of these calls on the
saved set-user-ID and saved set-group-ID.
The original Linux setreuid() and setregid() system calls supported
only 16-bit user and group IDs. Subsequently, Linux 2.4 added se-
treuid32() and setregid32(), supporting 32-bit IDs. The glibc se-
treuid() and setregid() wrapper functions transparently deal with the
variations across kernel versions.
C library/kernel differences
At the kernel level, user IDs and group IDs are a per-thread attribute.
However, POSIX requires that all threads in a process share the same
credentials. The NPTL threading implementation handles the POSIX re-
quirements by providing wrapper functions for the various system calls
that change process UIDs and GIDs. These wrapper functions (including
those for setreuid() and setregid()) employ a signal-based technique to
ensure that when one thread changes credentials, all of the other
threads in the process also change their credentials. For details, see
getgid(2), getuid(2), seteuid(2), setgid(2), setresuid(2), setuid(2),
capabilities(7), credentials(7), user_namespaces(7)
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Linux 2017-09-15 SETREUID(2)
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