[PATCH 5/6] f2fs: switch to using fscrypt_match_name()

[Richard Weinberger]

Eric,

Am 25.04.2017 um 19:46 schrieb Eric Biggers:
>> Sorry if this is a stupid question, but why do you have to compare hashes _and_
>> the last few bytes of the bigname?
>> A lookup via bigname gives you two 32bits hash values, and there I'd assume that
>> this is sufficient for a collisions free lookup. Especially since an
>> resumed readdir()
>> with a 64bits cookie has to work too on your filesystem.
>>
> 
> Well, the problem is that hashes may not be sufficient to uniquely identify a
> name in all cases.  f2fs uses only a 32-bit hash so it's trivial to create
> collisions on it, as I demonstrated.  Even collisions of two 32-bit hashes, as
> used by ext4 and ubifs, are possible.  And ext4 currently doesn't even compare
> the hashes during directory searches, beyond using them to find the correct
> directory block, since the hashes aren't stored in the directory entries.

I agree that finding a collision in a 32bits hash is easy, but for 64bits it
is *much* harder.

> Could this mean that telldir()/seekdir() is unreliable too, probably.  But for
> lookups of the "digested" names we aren't limited to just the 64-bit readdir
> position, so we can avoid duplicating the bug.  Also, collisions in the digested
> names are very problematic since they result in undeletable files, rather than
> just poor performance and broken telldir()/seekdir().

True.
Let me think whether we can add such a check to UBIFS.

Thanks,
//richard