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 of "user".
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
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
return 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 execve(2).
EINVAL One or more of the target user or group IDs is not valid in this user namespace.
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) was specified.
POSIX.1-2001, POSIX.1-2008, 4.3BSD (setreuid() and setregid() first appeared 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
setreuid32() and setregid32(), supporting 32-bit IDs. The glibc setreuid() 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 requirements 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 nptl(7).
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)