Re: oopsably broken case-insensitive support in ext4 and f2fs (Re: vfat: Broken case-insensitive support for UTF-8)

[Al Viro]

On Mon, Jan 20, 2020 at 08:07:21AM +0000, Al Viro wrote:

> > > I hadn't checked ->d_compare() instances for a while; somebody needs to
> > > do that again, by the look of it.  The above definitely is broken;
> > > no idea how many other instaces had grown such bugs...
> > 
> > f2fs one also has the same bug.  Anyway, I'm going down right now, will
> > check the rest tomorrow morning...
> 
> We _probably_ can get away with just checking that inode for NULL and
> buggering off if it is (->d_seq mismatch is guaranteed in that case),
> but I suspect that we might need READ_ONCE() on both dereferences.
> I hate memory barriers...

FWIW, other instances seem to be OK; HFS+ one might or might not be
OK in the face of concurrent rename (wrong result in that case is
no problem; oops would be), but it doesn't play silly buggers with
pointer-chasing.

ext4 and f2fs do, and ->d_compare() is broken in both of them.