SETFSUID

setfsuid ? set user identity used for file system checks

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 user ID.

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.

setfsgid(2)