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Date: Tue, 5 Mar 2024 21:01:18 -0800
From: Eric Biggers <ebiggers@kernel.org>
To: Andrey Albershteyn <aalbersh@redhat.com>
Cc: fsverity@lists.linux.dev, linux-xfs@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, chandan.babu@oracle.com,
	djwong@kernel.org
Subject: Re: [PATCH v5 21/24] xfs: add fs-verity support
Message-ID: <20240306050118.GD68962@sol.localdomain>
References: <20240304191046.157464-2-aalbersh@redhat.com>
 <20240304191046.157464-23-aalbersh@redhat.com>
 <20240306045543.GC68962@sol.localdomain>
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In-Reply-To: <20240306045543.GC68962@sol.localdomain>

On Tue, Mar 05, 2024 at 08:55:43PM -0800, Eric Biggers wrote:
> On Mon, Mar 04, 2024 at 08:10:44PM +0100, Andrey Albershteyn wrote:
> > +static void
> > +xfs_verity_put_listent(
> > +	struct xfs_attr_list_context	*context,
> > +	int				flags,
> > +	unsigned char			*name,
> > +	int				namelen,
> > +	int				valuelen)
> > +{
> > +	struct fsverity_blockbuf	block = {
> > +		.offset = xfs_fsverity_name_to_block_offset(name),
> > +		.size = valuelen,
> > +	};
> > +	/*
> > +	 * Verity descriptor is smaller than 1024; verity block min size is
> > +	 * 1024. Exclude verity descriptor
> > +	 */
> > +	if (valuelen < 1024)
> > +		return;
> > +
> 
> Is there no way to directly check whether it's the verity descriptor?  The
> 'valuelen < 1024' check is fragile because it will break if support for smaller
> Merkle tree block sizes is ever added.  (Silently, because this is doing
> invalidation which is hard to test and we need to be super careful with.)
> 
> If you really must introduce the assumption that the Merkle tree block size is
> at least 1024, this needs to be documented in the comment in
> fsverity_init_merkle_tree_params() that explains the reasoning behind the
> current restrictions on the Merkle tree block size.

Also, the verity descriptor can be >= 1024 bytes if there is a large builtin
signature attached to it.

- Eric