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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
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. The setuid() function 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
namespace.
EPERM The user is not privileged (Linux: does not have the
CAP_SETUID capability) 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 nptl(7).
getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7),
credentials(7), user_namespaces(7)
This page is part of release 4.12 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 2016-10-08 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), euidaccess(3), posix_spawn(3), capabilities(7), credentials(7), nptl(7), signal-safety(7), user_namespaces(7)