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


       Standard C library (libc, -lc)


       #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


       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 to indicate the

       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)