Re: [PATCH AUTOSEL 6.1 35/35] ext4: deal with legacy signed xattr name hash values

[Linus Torvalds]

On Tue, Jan 24, 2023 at 5:42 AM Sasha Levin <sashal@xxxxxxxxxx> wrote:
>
> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>
> [ Upstream commit f3bbac32475b27f49be201f896d98d4009de1562 ]
>
> We potentially have old hashes of the xattr names generated on systems
> with signed 'char' types.  Now that everybody uses '-funsigned-char',
> those hashes will no longer match.

This patch does not work correctly without '-funsigned-char', and I
don't think that has been back-ported to stable kernels.

That said, the patch *almost* works. You'd just have to add something
like this to it:

  --- a/fs/ext4/xattr.c
  +++ b/fs/ext4/xattr.c
  @@ -3096,7 +3096,7 @@ static __le32 ext4_xattr_hash_entry(char *name,
        while (name_len--) {
                hash = (hash << NAME_HASH_SHIFT) ^
                       (hash >> (8*sizeof(hash) - NAME_HASH_SHIFT)) ^
  -                    *name++;
  +                    (unsigned char)*name++;
        }
        while (value_count--) {
                hash = (hash << VALUE_HASH_SHIFT) ^

to make it work right (ie just make sure that the proper xattr name
hashing actually uses unsigned chars for its hash).

                    Linus