setuid32(2) — Linux manual page


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

NAME         top

       setuid - set user identity

SYNOPSIS         top

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

       int setuid(uid_t uid);

DESCRIPTION         top

       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         top

       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         top

       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

CONFORMING TO         top

       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

NOTES         top

       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         top

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

COLOPHON         top

       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

Linux                            2019-03-06                        SETUID(2)

Pages that refer to this page: syscalls(2)