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       setfsuid - set user identity used for filesystem checks


       Standard C library (libc, -lc)


       #include <sys/fsuid.h>

       int setfsuid(uid_t fsuid);


       On  Linux,  a process has both a filesystem user ID and an effective user ID.  The (Linux-
       specific) filesystem user ID is used for permissions checking  when  accessing  filesystem
       objects, while the effective user ID is used for various other kinds of permissions checks
       (see credentials(7)).

       Normally, the value of the process's filesystem user ID is the same as the  value  of  its
       effective user ID.  This is so, because whenever a process's effective user ID is changed,
       the kernel also changes the filesystem user ID to be the same as  the  new  value  of  the
       effective  user  ID.   A  process can cause the value of its filesystem user ID to diverge
       from its effective user ID by using setfsuid() to change its filesystem  user  ID  to  the
       value given in fsuid.

       Explicit calls to setfsuid() and setfsgid(2) are (were) 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 (was) a security hole that can
       expose it to unwanted signals.  (However, this issue is historical; 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 since Linux 1.2.


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


       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 privileged 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 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.

   C library/kernel differences
       In glibc 2.15 and earlier, when the wrapper for  this  system  call  determines  that  the
       argument  can't  be passed to the kernel without integer truncation (because the kernel is
       old and does not support 32-bit user IDs), it will return  -1  and  set  errno  to  EINVAL
       without attempting the system call.


       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)