setfsuid - set user identity used for file system checks
#include <unistd.h> /* glibc uses <sys/fsuid.h> */
int setfsuid(uid_t fsuid);
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 new value of effective
An explict call to setfsuid is usually only used by programs such as
the Linux NFS server that need to change what user ID is used for file
access without a corresponding change in the real and effective user
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 from
other user IDs.
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.
setfsuid is Linux specific and should not be used in programs intended
to be portable.
No error messages of any kind are returned to the caller. At the very
least, EPERM should be returned when the call fails.
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.
Linux 1.3.15 1995-08-06 SETFSUID(2)
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