setfsuid

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
       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
       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 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
       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 5.09 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/.



Linux                              2019-05-09                        SETFSUID(2)