setfsuid - set user identity used for filesystem checks
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.
This system call is present in Linux since version 1.2.
setfsuid() is Linux-specific and should not be used in programs intended
to be portable.
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.
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).
This page is part of release 5.13 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/.