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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(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.


       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 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

       The  original  Linux setfsuid() system call supported only 16-bit user IDs.  Subsequently,
       Linux 2.4 added setfsuid32() supporting 32-bit IDs.  The glibc setfsuid() wrapper function
       transparently deals with the variation across kernel versions.


       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)


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