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setfsuid(2) - setfsuid, setfsuid - set user identity used for file system checks - man 2 setfsuid

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

       setfsuid - set(7,n,1 builtins) user identity used for file(1,n) 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(1,n) system. Normally, the value of fsuid
       will  shadow(3,5)  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 pro-
       grams such as the Linux NFS server that need to change  what  user  and
       group  ID is used for file(1,n) access(2,5) without a corresponding change in(1,8) the
       real and effective user and group IDs.  A change in(1,8) 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(3,n) the caller is the superuser or  if(3,n)  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(8,n),  the
       current value of fsuid is returned.

       setfsuid  is Linux specific and should not be used in(1,8) programs intended
       to be portable.  It is present since Linux 1.1.44  and  in(1,8)  libc  since
       libc 4.7.6.

       No  error(8,n)  messages of any kind are returned to the caller. At the very
       lacks the CAP_SETUID capability).

       When glibc determines that the argument is not a valid user ID, it will
       return -1 and set(7,n,1 builtins) errno to EINVAL without attempting the system call.

       Note that at the time(1,2,n) this system call was introduced, a process  could
       send(2,n) a signal(2,7) to a process with the same effective user ID.  Today sig-
       nal(2,7) permission handling is slightly different.

       kill(1,2,1 builtins)(2), setfsgid(2), capabilities(7)

Linux 2.6.6                       2004-05-27                       SETFSUID(2)

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