NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

SETREUID(2)               Linux Programmer's Manual              SETREUID(2)

NAME         top

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

SYNOPSIS         top

       #include <sys/types.h>
       #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 feature_test_macros(7)):

       setreuid(), setregid():
           _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
           _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED

DESCRIPTION         top

       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".

RETURN VALUE         top

       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().

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 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 (Linux: does not have
              the CAP_SETUID capability in the case of setreuid(), or the
              CAP_SETGID capability 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.

CONFORMING TO         top

       POSIX.1-2001, 4.3BSD (the setreuid() and setregid() function calls
       first appeared in 4.2BSD).

NOTES         top

       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).

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 4.01 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

Linux                            2015-07-23                      SETREUID(2)