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       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() 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.


       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).


       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.


       kill(2), setfsgid(2), capabilities(7)