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

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Message-ID: <CAHk-=wjZmzuHP10Trg_7EBnix4mFLfODPM+FsZz0Jjj+YAFDeg@mail.gmail.com>
Date:   Tue, 24 Jan 2023 08:50:34 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Sasha Levin <sashal@...nel.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        kernel test robot <oliver.sang@...el.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Andreas Dilger <adilger@...ger.ca>,
        "Theodore Ts'o" <tytso@....edu>, Jason Donenfeld <Jason@...c4.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 6.1 35/35] ext4: deal with legacy signed xattr
 name hash values

On Tue, Jan 24, 2023 at 5:42 AM Sasha Levin <sashal@...nel.org> wrote:
>
> From: Linus Torvalds <torvalds@...ux-foundation.org>
>
> [ 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