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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 (more precisely: if the caller has the CAP_SETUID capability), 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).
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 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.
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)
COLOPHON
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