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
       calling process is privileged (more precisely: if the process  has  the
       CAP_SETUID  capability  in  its user namespace), 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  feature.  This allows a set-user-ID (other than root)
       program to drop all of its user privileges, 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: setuid() 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 priv-
       ileges, 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  re-
       turn 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 structures.

       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 applica-
              tions  should  check for this error); see the description of EA-
              GAIN in execve(2).

       EINVAL The user ID specified in uid is not valid  in  this  user  name-

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

       POSIX.1-2001, POSIX.1-2008, SVr4.  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 setfsuid(2).

       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.

   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  re-
       quirements  by providing wrapper functions for the various system calls
       that change process UIDs and GIDs.  These wrapper functions  (including
       the  one  for  setuid()) 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).

       getuid(2),  seteuid(2), setfsuid(2), setreuid(2), capabilities(7), cre-
       dentials(7), user_namespaces(7)

       This page is part of release 5.05 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

Linux                             2019-03-06                         SETUID(2)
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