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Date:   Sun, 30 Jul 2023 16:28:21 +0200
From:   Thomas =?utf-8?Q?Wei=C3=9Fschuh?= <thomas@t-8ch.de>
To:     Gao Xiang <hsiangkao@linux.alibaba.com>
Cc:     Jingbo Xu <jefflexu@linux.alibaba.com>, chao@kernel.org,
        huyue2@coolpad.com, linux-erofs@lists.ozlabs.org,
        linux-kernel@vger.kernel.org, Karel Zak <kzak@redhat.com>
Subject: Re: [PATCH v2] erofs: deprecate superblock checksum feature
Message-ID: <9299dd4c-c2da-4ed1-8979-87fa44c68f77@t-8ch.de>
References: <20230717112703.60130-1-jefflexu@linux.alibaba.com>
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 <bdd94a7c-7364-c262-ed01-d7e6fcb26007@linux.alibaba.com>
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Hi Gao!

On 2023-07-30 22:01:11+0800, Gao Xiang wrote:
> On 2023/7/30 21:31, Thomas Weißschuh wrote:
> > On 2023-07-17 19:27:03+0800, Jingbo Xu wrote:
> > > Later we're going to try the self-contained image verification.
> > > The current superblock checksum feature has quite limited
> > > functionality, instead, merkle trees can provide better protection
> > > for image integrity.
> > 
> > The crc32c checksum is also used by libblkid to gain more confidence
> > in its filesystem detection.
> > I guess a merkle tree would be much harder to implement.
> > 
> > This is for example used by the mount(8) cli program to allow mounting
> > of devices without explicitly needing to specify a filesystem.
> > 
> > Note: libblkid tests for EROFS_FEATURE_SB_CSUM so at least it won't
> > break when the checksum is removed.

> I'm not sure if we could switch EROFS_FEATURE_SB_CSUM to a simpler
> checksum instead (e.g. just sum each byte up if both
> EROFS_FEATURE_SB_CSUM and COMPAT_XATTR_FILTER bits are set, or
> ignore checksums completely at least in the kernel) if the better
> filesystem detection by using sb chksum is needed (not sure if other
> filesystems have sb chksum or just do magic comparsion)?

Overloading EROFS_FEATURE_SB_CSUM in combination with
COMPAT_XATTR_FILTER would break all existing deployments of libblkid, so
it's not an option.

All other serious and halfway modern filesystems do have superblock
checksums which are also checked by libblkid.

> The main problem here is after xattr name filter feature is added
> (xxhash is generally faster than crc32c), there could be two
> hard-depended hashing algorithms, this increases more dependency
> especially for embededed devices.

From libblkid side nothing really speaks against a simpler checksum.
XOR is easy to implement and xxhash is already part of libblkid for
other filesystems.

The drawbacks are:
* It would need a completely new feature bit in erofs.
* Old versions of libblkid could not validate checksums on newer
  filesystems.