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, 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.

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 signal permission handling is slightly different.

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 3.25 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                                                      2008-12-05                                                SETFSUID(2)