setfsuid(2) — Linux manual page

setfsuid(2)                System Calls Manual               setfsuid(2)

NAME

       setfsuid - set user identity used for filesystem checks

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/fsuid.h>

       [[deprecated]] 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.

STANDARDS

       Linux.

HISTORY

       Linux 1.2.

       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

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Linux man-pages 6.9.1          2024-05-02                    setfsuid(2)

Pages that refer to this page: setfsgid(2), setresuid(2), setuid(2), syscalls(2), capabilities(7), credentials(7), path_resolution(7), user_namespaces(7)

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