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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
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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);
The system call setfsuid() changes the value of the caller's
filesystem user ID—the user ID that the Linux kernel uses to check
for all accesses to the filesystem. Normally, the value of the
filesystem user ID will shadow the value of the effective user ID.
In fact, whenever the effective user ID is changed, the filesystem
user ID will also be changed to the new value of the effective user
ID.
Explicit calls to setfsuid() and setfsgid(2) are 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 a security hole that
can expose it to unwanted signals. (But 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 change 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), they 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 4.15 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 2017-09-15 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)
Copyright and license for this manual page
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