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
       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 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
              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

       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
       requirements 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

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

       This page is part of release 5.06 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)