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

NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

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

DESCRIPTION

       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

       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

       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.

STANDARDS

       POSIX.1-2001, POSIX.1-2008, 4.3BSD (setreuid() and setregid() first appeared in 4.2BSD).

NOTES

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).