NAME
setfsuid - set user identity used for file system checks SYNOPSIS
#include
/* glibc uses */ int setfsuid(uidt 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, when‐ ever 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 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 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. VERSIONS This system call is present in Linux since version 1.2. CONFORMING TO
setfsuid() is Linux-specific and should not be used in programs intended to be portable. NOTES 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 sig‐ nal 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. BUGS 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 CAPSETUID capability). SEE ALSO kill(2), setfsgid(2), capabilities(7), credentials(7) COLOPHON
This page is part of release 3.53 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)