linux-ext4 - Re: [PATCH 2/2] ext4: always verify the magic number in xattr blocks

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Date:   Wed, 13 Jun 2018 01:26:11 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Wen Xu <wen.xu@...ech.edu>
Subject: Re: [PATCH 2/2] ext4: always verify the magic number in xattr blocks

On Tue, Jun 12, 2018 at 11:12:02PM -0600, Andreas Dilger wrote:
> 
> This also could be a problem with other incorrectly-shared blocks that
> use the verified flag (e.g. directory leaf blocks), etc.  Should we
> separate these into BH_verified_dir, BH_verified_ibmap, BH_verified_bbmap,
> and BH_verified_xattr to avoid the potential conflicts?

That thought did cross my mind.  It's more bits than that, though,
since we also have extent tree blocks (and do we distinguish between
leaf and interior node blocks, etc.)

On the other hand we can detect some of these conflicts by using the
block_validity test (since allocation bitmaps are fixed metadata), and
a some of these metadata blocks (but not all) have magic numbers at
the beginning of each block which can be used to disambiguate them.

Or we could steal some bits from b_this_page or b_page (which would
never be used in metadata buffer heads) to simply stash a block type.
I'm somewhat more partial to that solution, myself.

					- Ted