setfsuid - phpMan

SETFSUID(2) Linux Programmer’s Manual SETFSUID(2)

NAME
setfsuid - set user identity used for file system checks

SYNOPSIS
#include <unistd.h> /* glibc uses <sys/fsuid.h> */

int setfsuid(uid_t fsuid);

DESCRIPTION
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 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.

RETURN VALUE
On success, the previous value of fsuid is returned. On error, the current value
of fsuid is returned.

CONFORMING TO
setfsuid is Linux specific and should not be used in programs intended to be
portable. It is present since Linux 1.1.44 and in libc since libc 4.7.6.

BUGS
No error messages of any kind are returned to the caller. At the very least, EPERM
should be returned when the call fails.

NOTES
When glibc determines that the argument is not a valid uid, 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 sig-
nal to a process with the same effective user ID. Today signal permission handling
is slightly different.