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

NAME
       setuid - set user identity

SYNOPSIS
       #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 supe‐
       ruser, 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 after‐
       ward 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 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
       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)

Linux                                                                          2021-03-22                                                                      SETUID(2)