setfsuid32(2)
NAME
setfsuid - set user identity used for filesystem checks
SYNOPSIS
#include <sys/fsuid.h>
int setfsuid(uid_t fsuid);
DESCRIPTION
The system call setfsuid() changes the value of the caller's filesystem
user ID—the user ID that the Linux kernel uses to check for all
accesses to the filesystem. Normally, the value of the filesystem user
ID will shadow the value of the effective user ID. In fact, whenever
the effective user ID is changed, the filesystem user ID will also be
changed to the new value of the effective user ID.
Explicit calls to setfsuid() and setfsgid(2) are usually used only by
programs such as the Linux NFS server that need to change what user and
group ID is used for file access without a corresponding change in the
real and effective user and group IDs. A change in the normal user IDs
for a program such as the NFS server is a security hole that can expose
it to unwanted signals. (But see below.)
setfsuid() will succeed only if the caller is the superuser or if fsuid
matches either the caller's real user ID, effective user ID, saved set-
user-ID, or current filesystem user ID.
RETURN VALUE
On both success and failure, this call returns the previous filesystem
user ID of the caller.
VERSIONS
This system call is present in Linux since version 1.2.
CONFORMING TO
setfsuid() is Linux-specific and should not be used in programs
intended to be portable.
NOTES
At the time when this system call was introduced, one process could
send a signal to another process with the same effective user ID. This
meant that if a privileged process changed its effective user ID for
the purpose of file permission checking, then it could become
vulnerable to receiving signals sent by another (unprivileged) process
with the same user ID. The filesystem user ID attribute was thus added
to allow a process to change its user ID for the purposes of file
permission checking without at the same time becoming vulnerable to
receiving unwanted signals. Since Linux 2.0, signal permission
handling is different (see kill(2)), with the result that a process
change can change its effective user ID without being vulnerable to
receiving signals from unwanted processes. Thus, setfsuid() is
nowadays unneeded and should be avoided in new applications (likewise
for setfsgid(2)).
The original Linux setfsuid() system call supported only 16-bit user
IDs. Subsequently, Linux 2.4 added setfsuid32() supporting 32-bit IDs.
The glibc setfsuid() wrapper function transparently deals with the
variation across kernel versions.
C library/kernel differences
In glibc 2.15 and earlier, when the wrapper for this system call
determines that the argument can't be passed to the kernel without
integer truncation (because the kernel is old and does not support
32-bit user IDs), they will return -1 and set errno to EINVAL without
attempting the system call.
BUGS
No error indications of any kind are returned to the caller, and the
fact that both successful and unsuccessful calls return the same value
makes it impossible to directly determine whether the call succeeded or
failed. Instead, the caller must resort to looking at the return value
from a further call such as setfsuid(-1) (which will always fail), in
order to determine if a preceding call to setfsuid() changed the
filesystem user ID. At the very least, EPERM should be returned when
the call fails (because the caller lacks the CAP_SETUID capability).
SEE ALSO
kill(2), setfsgid(2), capabilities(7), credentials(7)
COLOPHON
This page is part of release 4.09 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/.
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