setfsuid(2) — Linux manual page


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

NAME         top

       setfsuid - set user identity used for filesystem checks

SYNOPSIS         top

       #include <sys/fsuid.h>

       int setfsuid(uid_t fsuid);

DESCRIPTION         top

       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 credentials(7)).

       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.

RETURN VALUE         top

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

VERSIONS         top

       This system call is present in Linux since version 1.2.

CONFORMING TO         top

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

NOTES         top

       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.

BUGS         top

       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

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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)

Pages that refer to this page: setfsgid(2)setfsgid32(2)setresgid(2)setresgid32(2)setresuid(2)setresuid32(2)setuid(2)setuid32(2)syscalls(2)capabilities(7)credentials(7)path_resolution(7)user_namespaces(7)