linux-kernel - Re: [PATCH v2 01/12] fs-verity: add a documentation file

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Date:   Tue, 18 Dec 2018 23:14:20 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fscrypt@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Victor Hsieh <victorhsieh@...gle.com>,
        Chandan Rajendra <chandan@...ux.vnet.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 01/12] fs-verity: add a documentation file

On Tue, Dec 18, 2018 at 07:16:03PM -0500, Theodore Y. Ts'o wrote:
> Sure, but what would be the benefit of doing different things on the
> back end?  I think this is a really more of a philophical objection
> than anything else.  With both fsverity and fscrypt, well over 95% of
> the implementation is shared between ext4 and f2fs.  And from a
> cryptographic design, that's something I consider a feature, not a
> bug.  Cryptographic code is subtle in very different ways compared to
> file system code.  So it's a good thing to having it done once and
> audited by crypto specialists, as opposed to having each file system
> doing it differently / independently.

Where the data is located on disk should not matter for the crypto
details.  If it does you have severe implementation issues.

> Right, the current interface makes it somewhat more awkward to do
> these other things --- but the question is *why* would you want to in
> the first place?  Why add the extra complexity?  I'm a big believer of
> the KISS principle, and if there was a reason why a file system would
> want to store the Merkle tree somewhere else, we could talk about it,
> but I see only downside, and no upside.

Filesystems already use blocks beyond EOF for preallocation, either
speculative by the file system itself, or explicitly by the user with
fallocate.  I bet you will run into bugs with your creative abuse
sooner or later.  Indepnd of that the interface simply is gross, which
is enough of a reason not to merge it.