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SETFSUID(2) Linux Programmer's Manual SETFSUID(2)
setfsuid - set user identity used for file system checks
#include <unistd.h> /* glibc uses <sys/fsuid.h> */
int setfsuid(uid_t fsuid);
The system call setfsuid() sets the user ID that the Linux kernel uses to
check for all accesses to the file system. Normally, the value of fsuid will
shadow the value of the effective user ID. In fact, whenever the effective
user ID is changed, fsuid will also be changed to the new value of the
effective user ID.
Explicit calls to setfsuid() and setfsgid(2) are usually only used 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 only succeed if the caller is the superuser or if fsuid
matches either the real user ID, effective user ID, saved set-user-ID, or the
current value of fsuid.
On success, the previous value of fsuid is returned. On error, the current
value of fsuid is returned.
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.
When glibc determines that the argument is not a valid user ID, it will return
-1 and set errno to EINVAL without attempting the system call.
Note that at the time this system call was introduced, a process could send a
signal to a process with the same effective user ID. Today signal permission
handling is slightly different.
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.
No error messages of any kind are returned to the caller. 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 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-11-22 SETFSUID(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface