SETFSUID(2)                Linux Programmer's Manual               SETFSUID(2)

       setfsuid - set user identity used for filesystem checks

       #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  creden-

       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 secu-
       rity 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 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  in-
       tended 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  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 can change its ef-
       fective  user ID without being vulnerable to receiving signals from un-
       wanted 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),  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)

       This  page  is  part of release 5.05 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

Linux                             2019-05-09                       SETFSUID(2)
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