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


       #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 ||


       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

       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


       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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