Section: Linux Programmer's Manual (2)
setuid - set user identity
int setuid(uid_t uid);
sets the effective user ID of the calling process.
If the calling process is privileged
(more precisely: if the process has the
capability in its user namespace),
the real UID and saved set-user-ID are also set.
is implemented like the POSIX version with the
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
checks the effective user ID of the caller and if it is
the superuser, all process-related user ID's are set to
After this has occurred, it is impossible for the program to regain root
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
You can accomplish this with
On success, zero is returned.
On error, -1 is returned, and
is set appropriately.
there are cases where
can fail even when the caller is UID 0;
it is a grave security error to omit checking for a failure return from
The call would change the caller's real UID (i.e.,
does not match the caller's real UID),
but there was a temporary failure allocating the
necessary kernel data structures.
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
over the caller's
Since Linux 3.1, this error case no longer occurs
(but robust applications should check for this error);
see the description of
The user ID specified in
is not valid in this user namespace.
The user is not privileged (Linux: does not have the
capability in its user namespace) and
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.
call also sets the filesystem user ID of the calling process.
is different from the old effective UID, the process will
be forbidden from leaving core dumps.
The original Linux
system call supported only 16-bit user IDs.
Subsequently, Linux 2.4 added
supporting 32-bit IDs.
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
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
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