linux-ext4 - [Bug 211971] Incorrect fix by e2fsck for blocks

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Date:   Wed, 03 Mar 2021 17:14:27 +0000
From:   bugzilla-daemon@...zilla.kernel.org
To:     linux-ext4@...r.kernel.org
Subject: [Bug 211971] Incorrect fix by e2fsck for blocks_count corruption

https://bugzilla.kernel.org/show_bug.cgi?id=211971

--- Comment #3 from tmahmud@...tate.edu ---
Hello Ted,

Thank you very much for the detailed clarification! It mostly makes sense to
me. But I still have two questions regarding the debugfs/e2fsck behavior.

(1)
> > > debugfs -w image
> > > debugfs:  ssv blocks_count 4000
> > > debugfs:  q
> 
> This will update the blocks_count in the primary and all secondary
> backups.  

This is different from what I observed. In my experiment, “debugfs: ssv
blocks_count 4000” only updated the blocks_count (and the checksum) in the
primary superblock. All secondary backups were not updated (neither the
blocks_count nor the checksum). Does this imply that there is a potential bug
in debugfs (because it didn’t update all backups as you suggested)?  I’m
attaching two images before and after “debugfs: ssv blocks_count 4000” for
reference (“image1_before”, “image1_after”). I have verified backups are not
updated by dumping the backup superblocks information with dumpe2fs.

(2)
> The problem is that e2fsck can't really determine that the blocks
> count field has been corrupted.  

In my experiment, I observed that e2fsck was able to fix the debugfs-modified
primary superblock using secondary superblocks when the secondary superblocks
are located in default locations (ex. 8193rd block). However, in an image where
secondary superblocks are not in their default locations (ex:513rd block), I
found that e2fsck cannot fix the primary superblock using secondary
superblocks. So e2fsck’s behavior is inconsistent depending on the location of
the secondary superblocks. Could you please comment on this?

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