setfsuid — set user identity used for file system checks
#include <unistd.h> /* glibc uses <sys/fsuid.h> */
int
setfsuid( |
uid_t | fsuid) ; |
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(2) 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
.
On success, the previous value of fsuid
is returned. On error,
the current value of fsuid
is returned.
setfsuid
() is Linux-specific
and should not be used in programs intended to be portable.
It is present since Linux 1.1.44 and in libc since libc
4.7.6.
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.
No error messages of any kind are returned to the caller.
At the very least, EPERM
should be returned when the call fails (because the caller
lacks the CAP_SETUID
capability).