[patch] unprivileged mounts update

[Serge E. Hallyn]

Quoting Miklos Szeredi (miklos at szeredi.hu):
> > Right, I figure if the normal action is to always do
> > mnt->user = current->fsuid, then for the special case we
> > pass a uid in someplace.  Of course...  do we not have a
> > place to do that?  Would it be a no-no to use 'data' for
> > a non-fs-specific arg?
> 
> I guess it would be OK for bind, but not for new- and remounts, where
> 'data' is already used.
> 
> Maybe it's best to stay with fsuid after all, and live with having to
> restore capabilities.  It's not so bad after all, this seems to do the
> trick:
> 
> 	cap_t cap = cap_get_proc();
> 	setfsuid(uid);
> 	cap_set_proc(cap);
> 
> Unfortunately these functions are not in libc, but in a separate
> "libcap" library.  Ugh.

Ok, are you still planning to nix the MS_SETUSER flag, though, as Eric
suggested?  I think it's cleanest - always set the mnt->user field to
current->fsuid, and require CAP_SYS_ADMIN if the mountpoint->mnt->user !=
current->fsuid.

-serge