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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 ID.


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


       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 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
       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)


       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

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