setuid(2) -- Linux man page
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 current process.
If the effective userid of the caller is root,
the real and saved user ID's are also set.
Under Linux,
setuid
is implemented like the POSIX version with the _POSIX_SAVED_IDS feature.
This allows a setuid (other than root) program to drop all of its user
privileges, do some un-privileged work, and then re-engage the original
effective user ID in a secure manner.
If the user is root or the program is setuid root, special care must be
taken. The
setuid
function checks the effective uid 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 setuid-root program wishing to temporarily drop root
privileges, assume the identity of a non-root user, and then regain
root privileges afterwards cannot use
setuid.
You can accomplish this with the (non-POSIX, BSD) call
seteuid.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and
errno
is set appropriately.
ERRORS
- EPERM
-
The user is not the super-user, and
uid
does not match the real or saved user ID of the calling process.
- EAGAIN
-
The
uid
does not match the current uid and
uid
brings process over it's NPROC rlimit.
CONFORMING TO
SVr4, SVID, POSIX.1. Not quite compatible with the 4.4BSD call, which
sets all of the real, saved, and effective user IDs. SVr4 documents an
additional EINVAL error condition.
LINUX-SPECIFIC REMARKS
Linux has the concept of filesystem user ID, normally equal to the
effective user ID. The
setuid
call also sets the filesystem user ID of the current process.
See
setfsuid(2).
If
uid
is different from the old effective uid, the process will
be forbidden from leaving core dumps.
SEE ALSO
getuid(2),
setreuid(2),
seteuid(2),
setfsuid(2)
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