− set user identity used for filesystem checks
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.
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.)
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
On both success
and failure, this call returns the previous filesystem user
ID of the caller.
call is present in Linux since version 1.2.
is Linux-specific and should not be used in programs
intended to be portable.
determines that the argument is not a valid user ID, it will
return −1 and set errno to EINVAL
without attempting the system call.
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)).
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
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
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