setuid

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



NAME
       setuid - set user identity

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

       int setuid(uid_t uid);

DESCRIPTION
       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).

RETURN VALUE
       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().

ERRORS
       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 namespace.

       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.

CONFORMING TO
       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.

NOTES
       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 nptl(7).

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

COLOPHON
       This page is part of release 5.09 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
       https://www.kernel.org/doc/man-pages/.



Linux                              2019-03-06                          SETUID(2)