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

[Gao Xiang]

On 2023/7/30 22:28, Thomas Weißschuh wrote:
> 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.

I think for libblkid, you could still use:
  EROFS_FEATURE_SB_CSUM is not set, don't check anything;
  EROFS_FEATURE_SB_CSUM only is set, check with crc32c;
  EROFS_FEATURE_SB_CSUM | COMPAT_XATTR_FILTER (or some other bit) is
set, check with a simpler hash?

> 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.

just checking magic for Old versions of libblkid will cause false
positive in which extent?

Thanks,
Gao Xiang