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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(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, effec-
       tive 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.

       This system call is present in Linux since version 1.2.

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

       When glibc determines that the argument is not a valid user ID, it will return -1 and set errno to EINVAL with-
       out 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.

       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).

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

       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at

Linux                             2008-12-05                       SETFSUID(2)