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       setfsuid - set user identity used for filesystem checks


       #include <sys/fsuid.h>

       int setfsuid(uid_t fsuid);


       The  system  call setfsuid() changes the value of the caller's filesystem user ID—the user
       ID that the Linux kernel uses to check for all accesses to the filesystem.  Normally,  the
       value  of the filesystem user ID will shadow the value of the effective user ID.  In fact,
       whenever the effective user ID is changed, the filesystem user ID will also be changed  to
       the new value of the effective user ID.

       Explicit calls to setfsuid() and setfsgid(2) are usually used only 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 succeed only if the caller is the superuser or if fsuid matches either the
       caller's real user ID, effective user ID, saved set-user-ID, or  current  filesystem  user


       On  both  success  and  failure,  this call returns the previous filesystem user ID of the


       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.

       At  the  time  when  this  system  call was introduced, one process could send a signal to
       another process with the same effective user ID.  This meant that if a  privilged  process
       changed  its  effective user ID for the purpose of file permission checking, then it could
       become vulnerable to receiving signals sent by another  (unprivileged)  process  with  the
       same  user  ID.   The  filesystem  user  ID attribute was thus added to allow a process to
       change its user ID for the purposes of file permission checking without at the  same  time
       becoming  vulnerable  to  receiving  unwanted signals.  Since Linux 2.0, signal permission
       handling is different (see kill(2)), with the result that a process change can change  its
       effective  user  ID without being vulnerable to receiving signals from unwanted processes.
       Thus, setfsuid() is nowadays unneeded and should be avoided in new applications  (likewise
       for setfsgid(2)).

       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  indications  of  any  kind  are  returned to the caller, and the fact that both
       successful and unsuccessful calls return the same value makes it  impossible  to  directly
       determine  whether  the  call  succeeded  or  failed.   Instead, the caller must resort to
       looking at the return value from a further call such as setfsuid(-1)  (which  will  always
       fail), in order to determine if a preceding call to setfsuid() changed the filesystem user
       ID.  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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