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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():
           _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

       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

       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  (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

       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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