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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
effective UID of the caller is root, 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 pro-
gram to regain root privileges.
Thus, a set-user-ID-root program wishing to temporarily drop root priv-
ileges, 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 applica-
tions should check for this error); see the description of
EAGAIN in execve(2).
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.
CONFORMING TO
SVr4, POSIX.1-2001. 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.
SEE ALSO
getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7), cre-
dentials(7)
COLOPHON
This page is part of release 3.69 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
http://www.kernel.org/doc/man-pages/.
Linux 2014-06-13 SETUID(2)
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