linux-ext4 - Re: [PATCH 5/6] f2fs: switch to using fscrypt_match

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Date:   Tue, 25 Apr 2017 21:22:16 +0200
From:   Richard Weinberger <richard@....at>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     linux-fscrypt@...r.kernel.org,
        Ryo Hashimoto <hashimoto@...omium.org>,
        Gwendal Grignou <gwendal@...omium.org>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Eric Biggers <ebiggers@...gle.com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        "linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
        Jaegeuk Kim <jaegeuk@...nel.org>, linux-ext4@...r.kernel.org,
        Kazuhiro Inaba <kinaba@...omium.org>,
        David Gstir <david@...ma-star.at>
Subject: Re: [PATCH 5/6] f2fs: switch to using fscrypt_match_name()

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