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RedHat 9 (Linux i386) - man page for setfsuid (redhat section 2)

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

       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 with-
       out 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.

       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

       kill(2), setfsgid(2)

Linux 1.3.15				    2002-07-23				      SETFSUID(2)

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