Re: [PATCH] VFS: fix recent breakage of FS_REVAL_DOT

[Al Viro <viro@ZenIV.linux.org.uk>]

On Tue, May 25, 2010 at 12:01:09AM +0100, Al Viro wrote:
> Client question: what stops you from stack overflows in that area?  Call
> chains you've got are *deep*, and I really wonder what happens if you
> hit a referral point while traversing nested symlink, get pathname
> resolution (already several levels into recursion) call ->follow_link(),
> bounce down through nfs_do_refmount/nfs_follow_referral/try_location/
> vfs_kern_mount/nfs4_referral_get_sb/nfs_follow_remote_path into
> vfs_path_lookup, which will cheerfully add a few more loops like that.
> 
> Sure, the *total* nesting depth through symlinks is still limited by 8, but
> that pile of stack frames is _MUCH_ fatter than what we normally have in
> pathname resolution.  You've suddenly added ~60 extra stack frames to the
> worst-case stack footprint of the pathname resolution.  Don't try that
> on sparc64, boys and girls, it won't be happy with attempt to carve ~12Kb
> extra out of its kernel stack...  In fact, it's worse than just ~60 stack
> frames - several will contain (on-stack) struct nameidata in them, which
> very definitely will _not_ fit into the minimal stack frame.  It's about
> 160 bytes extra, for each of those (up to 7).

Actually, just what will happen if you have a referral that would eventually
resolve to a directory you have no permissions to access?  AFAICS, you'll
end up trying it on all alternates, since nfs_follow_referral() will cheerfully
keep trying one variant after another, getting -EACCES from each.  Worse,
if there are nested referrals in it, you'll get all sequences of alternates
tried before you give up.

..o*O(at least it's merely exponential; Ackermann would be even more fun)