setuid(2) - Linux manual page (original) (raw)

setuid(2) System Calls Manual setuid(2)

NAME top

   setuid - set user identity

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#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)](../man2/seteuid.2.html).

RETURN VALUE top

   On success, zero is returned.  On error, -1 is returned, and _[errno](../man3/errno.3.html)_
   is set to indicate the error.

   _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)](../man2/execve.2.html).

   **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.

VERSIONS top

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).

STANDARDS top

   POSIX.1-2008.

HISTORY top

   POSIX.1-2001, SVr4.

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

   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.

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)](../man2/setfsuid.2.html).

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

SEE ALSO top

   [getuid(2)](../man2/getuid.2.html), [seteuid(2)](../man2/seteuid.2.html), [setfsuid(2)](../man2/setfsuid.2.html), [setreuid(2)](../man2/setreuid.2.html), [capabilities(7)](../man7/capabilities.7.html),
   [credentials(7)](../man7/credentials.7.html), [user_namespaces(7)](../man7/user%5Fnamespaces.7.html)

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Linux man-pages 6.10 2024-07-23 setuid(2)

Pages that refer to this page:capsh(1), access(2), execve(2), getresuid(2), getuid(2), seccomp(2), seteuid(2), setresuid(2), setreuid(2), syscalls(2), vfork(2), cap_get_proc(3), euidaccess(3), posix_spawn(3), systemd.exec(5), capabilities(7), credentials(7), nptl(7), signal-safety(7), user_namespaces(7)