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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(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.)

       setfsuid() will succeed only 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.

VERSIONS
       This system call is present in Linux since version 1.2.

CONFORMING TO
       setfsuid()  is  Linux-specific  and  should  not	 be  used  in programs
       intended to be portable.

NOTES
       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 sig‐
       nal permission handling is slightly different.

       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.

BUGS
       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).

SEE ALSO
       kill(2), setfsgid(2), capabilities(7), credentials(7)

COLOPHON
       This page is part of release 3.53 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2010-11-22			   SETFSUID(2)

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