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From:   Eric Biggers <ebiggers@kernel.org>
To:     Chris Mason <clm@fb.com>
Cc:     linux-fscrypt@vger.kernel.org
Subject: Re: max fsverity descriptor size?
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+linux-fscrypt

On Wed, Dec 02, 2020 at 09:01:52AM -0500, Chris Mason wrote:
> Hi Eric,
> 
> I’m working on fsverity support in btrfs and wanted to check on the max size
> of the descriptor.  I can go up to any size, just wanted to make sure I had
> things correct in the disk format.
> 
> -chris

The implementations of fs-verity in ext4 and f2fs store the built-in signature
(if there is one) appended to the 'struct fsverity_descriptor', and limit the
total size of those two things combined to 16384 bytes.  See
FS_VERITY_MAX_DESCRIPTOR_SIZE in fs/verity/fsverity_private.h.

Note that there's nothing special about this particular number; it's just an
implementation limit to prevent userspace doing weird things with megabytes
"signatures".

If btrfs will be storing built-in signatures in the same way, it probably should
use the same limit.  Preferably it would be done in a way such that it's
possible to increase the limit later if it's ever needed.

- Eric