The system call setfsuid sets the user ID that the Linux kernel uses to
check for all accesses to the file system. Normally, the value of fsuid
will shadow the value of the effective user ID. In fact, whenever the
effective user ID is changed, fsuid will also be changed to the new
value of the effective user ID.
Explicit calls to setfsuid and setfsgid are usually only used 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 only succeed if the caller is the superuser or if
fsuid matches either the real user ID, effective user ID, saved
set-user-ID, or the current value of fsuid.
When glibc determines that the argument is not a valid user ID, it will return
-1 and set errno to EINVAL without attempting the system call.
Note that at the time this system call was introduced, a process could send a
signal to a process with the same effective user ID. Today signal permission
handling is slightly different.