setfsuid

SETFSUID(2)		  Linux Programmer’s Manual		  SETFSUID(2)



NAME
       setfsuid - set user identity used for file system checks

SYNOPSIS
       #include <unistd.h> /* glibc uses <sys/fsuid.h> */

       int setfsuid(uid_t fsuid);

DESCRIPTION
       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, when-
       ever 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 pro-
       grams 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.

RETURN VALUE
       On success, the previous value of fsuid is returned.   On  error,  the
       current value of fsuid is returned.

CONFORMING TO
       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.

BUGS
       No  error messages of any kind are returned to the caller. At the very
       least, EPERM should be returned when the call fails.

NOTES
       When glibc determines that the argument is not a valid  uid,  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.

SEE ALSO
       kill(2), setfsgid(2)



Linux 1.3.15			  2002-07-23			  SETFSUID(2)