Re: [PATCH v2] erofs: deprecate superblock checksum feature

[Thomas Weißschuh]

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.