setfsuid(2) - Linux manual page (original) (raw)

setfsuid(2) System Calls Manual setfsuid(2)

NAME top

   setfsuid - set user identity used for filesystem checks

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <sys/fsuid.h>**

   **[[deprecated]] 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)](../man7/credentials.7.html)).

   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)](../man2/setfsgid.2.html) 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.

STANDARDS top

   Linux.

HISTORY top

   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)](../man2/kill.2.html)), 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)](../man2/setfsgid.2.html)).

   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** capability).

SEE ALSO top

   [kill(2)](../man2/kill.2.html), [setfsgid(2)](../man2/setfsgid.2.html), [capabilities(7)](../man7/capabilities.7.html), [credentials(7)](../man7/credentials.7.html)

COLOPHON top

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Linux man-pages 6.10 2024-07-23 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)