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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | STANDARDS | HISTORY | BUGS | SEE ALSO | COLOPHON |
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setfsuid(2) System Calls Manual setfsuid(2)
setfsuid - set user identity used for filesystem checks
Standard C library (libc, -lc)
#include <sys/fsuid.h>
[[deprecated]] int setfsuid(uid_t fsuid);
On Linux, a process has both a filesystem user ID and an effective
user ID. The (Linux-specific) filesystem user ID is used for
permissions checking when accessing filesystem objects, while the
effective user ID is used for various other kinds of permissions
checks (see credentials(7)).
Normally, the value of the process's filesystem user ID is the
same as the value of its effective user ID. This is so, because
whenever a process's effective user ID is changed, the kernel also
changes the filesystem user ID to be the same as the new value of
the effective user ID. A process can cause the value of its
filesystem user ID to diverge from its effective user ID by using
setfsuid() to change its filesystem user ID to the value given in
fsuid.
Explicit calls to setfsuid() and setfsgid(2) are (were) 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 (was) a security hole that can expose it to unwanted
signals. (However, this issue is historical; 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.
On both success and failure, this call returns the previous
filesystem user ID of the caller.
Linux.
Linux 1.2.
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 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), it will return -1 and set errno to EINVAL
without attempting the system call.
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).
kill(2), setfsgid(2), capabilities(7), credentials(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 setfsuid(2)
Pages that refer to this page: setfsgid(2), setresuid(2), setuid(2), syscalls(2), capabilities(7), credentials(7), path_resolution(7), user_namespaces(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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