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setfsuid(2)


NAME

   setfsuid - set user identity used for filesystem checks

SYNOPSIS

   #include <sys/fsuid.h>

   int setfsuid(uid_t fsuid);

DESCRIPTION

   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.

RETURN VALUE

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

VERSIONS

   This system call is present in Linux since version 1.2.

CONFORMING TO

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

NOTES

   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.

   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), they will return -1 and set errno to  EINVAL  without
   attempting the system call.

BUGS

   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).

SEE ALSO

   kill(2), setfsgid(2), capabilities(7), credentials(7)

COLOPHON

   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at
   https://www.kernel.org/doc/man-pages/.



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