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SETFSUID(2)                Linux Programmer's Manual               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  vulnera-
       ble  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 permis-
       sion checking without at the same time becoming vulnerable to receiving
       unwanted  signals.  Since Linux 2.0, signal permission handling is dif-
       ferent (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 deter-
       mines  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 4.15 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/.
Linux                             2017-09-15                       SETFSUID(2)