setreuid(2) — Linux manual page


setreuid(2)                System Calls Manual               setreuid(2)

NAME         top

       setreuid, setregid - set real and/or effective user or group ID

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <unistd.h>

       int setreuid(uid_t ruid, uid_t euid);
       int setregid(gid_t rgid, gid_t egid);

   Feature Test Macro Requirements for glibc (see

       setreuid(), setregid():
           _XOPEN_SOURCE >= 500
               || /* Since glibc 2.19: */ _DEFAULT_SOURCE
               || /* glibc <= 2.19: */ _BSD_SOURCE

DESCRIPTION         top

       setreuid() sets real and effective user IDs of the calling

       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".

RETURN VALUE         top

       On success, zero is returned.  On error, -1 is returned, and
       errno is set to indicate the error.

       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().

ERRORS         top

       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

       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.

VERSIONS         top

       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.

STANDARDS         top


HISTORY         top

       POSIX.1-2001, 4.3BSD (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).

       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).

SEE ALSO         top

       getgid(2), getuid(2), seteuid(2), setgid(2), setresuid(2),
       setuid(2), capabilities(7), credentials(7), user_namespaces(7)

Linux man-pages (unreleased)     (date)                      setreuid(2)

Pages that refer to this page: execve(2)getgid(2)getresuid(2)getuid(2)seteuid(2)setgid(2)setresuid(2)setuid(2)syscalls(2)capabilities(7)credentials(7)nptl(7)user_namespaces(7)