linux-ext4 - Re: [PATCH v4 14/16] ext4: add basic fs-verity support

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Message-ID: <20190615153112.GO6142@mit.edu>
Date:   Sat, 15 Jun 2019 11:31:12 -0400
From:   "Theodore Ts'o" <tytso@....edu>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     linux-fscrypt@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-integrity@...r.kernel.org, Jaegeuk Kim <jaegeuk@...nel.org>,
        Victor Hsieh <victorhsieh@...gle.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v4 14/16] ext4: add basic fs-verity support

On Thu, Jun 06, 2019 at 08:52:03AM -0700, Eric Biggers wrote:
> +/*
> + * Format of ext4 verity xattr.  This points to the location of the verity
> + * descriptor within the file data rather than containing it directly because
> + * the verity descriptor *must* be encrypted when ext4 encryption is used.  But,
> + * ext4 encryption does not encrypt xattrs.
> + */
> +struct fsverity_descriptor_location {
> +	__le32 version;
> +	__le32 size;
> +	__le64 pos;
> +};

What's the benefit of storing the location in an xattr as opposed to
just keying it off the end of i_size, rounded up to next page size (or
64k) as I had suggested earlier?

Using an xattr burns xattr space, which is a limited resource, and it
adds some additional code complexity.  Does the benefits outweigh the
added complexity?

						- Ted