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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 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.
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 5.08 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
https://www.kernel.org/doc/man-pages/.
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)
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