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

       setuid - set user identity

       #include <sys/types.h>
       #include <unistd.h>

       int setuid(uid_t uid);

       setuid()  sets  the effective user ID of the calling process.  If the effective UID of the
       caller is root, the real UID and saved set-user-ID are also set.

       Under Linux, setuid() is implemented like the POSIX version with the _POSIX_SAVED_IDS fea‐
       ture.   This allows a set-user-ID (other than root) program to drop all of its user privi‐
       leges, do some un-privileged work, and then reengage the original effective user ID  in  a
       secure manner.

       If  the  user is root or the program is set-user-ID-root, special care must be taken.  The
       setuid() function checks the effective user ID of the caller and if it is  the  superuser,
       all  process-related  user ID's are set to uid.  After this has occurred, it is impossible
       for the program to regain root privileges.

       Thus, a set-user-ID-root program wishing to temporarily drop root privileges,  assume  the
       identity  of  an  unprivileged  user, and then regain root privileges afterward cannot use
       setuid().  You can accomplish this with seteuid(2).

       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

       Note: there are cases where setuid() can fail even when the caller is UID 0; it is a grave
       security error to omit checking for a failure return from setuid().

       EAGAIN The  call would change the caller's real UID (i.e., uid does not match the caller's
              real UID), but there was a temporary failure allocating the necessary  kernel  data

       EAGAIN uid  does  not  match  the real user ID of the caller and this call would bring the
              number  of  processes  belonging  to  the  real  user  ID  uid  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 The user ID specified in uid is not valid in this user namespace.

       EPERM  The user is not privileged (Linux: does not have the CAP_SETUID capability) and uid
              does not match the real UID or saved set-user-ID of the calling process.

       SVr4, POSIX.1-2001.  Not quite compatible with the 4.4BSD call,  which  sets  all  of  the
       real, saved, and effective user IDs.

       Linux  has the concept of the filesystem user ID, normally equal to the effective user ID.
       The setuid() call also sets the filesystem user ID of  the  calling  process.   See  setf‐

       If uid is different from the old effective UID, the process will be forbidden from leaving
       core dumps.

       The original Linux setuid() system call supported only  16-bit  user  IDs.   Subsequently,
       Linux  2.4  added  setuid32()  supporting 32-bit IDs.  The glibc setuid() wrapper function
       transparently deals with the variation across kernel versions.

       getuid(2),  seteuid(2),   setfsuid(2),   setreuid(2),   capabilities(7),   credentials(7),

       This  page  is  part of release 3.74 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                                       2014-09-21                                  SETUID(2)

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