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

       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 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.

       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)

       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)