linux-kernel - Re: [PATCH] reiserfs: Add comment to explain endianness issue in xattr

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Date:   Tue, 23 Apr 2019 21:50:45 +0530
From:   Bharath Vedartham <linux.bhar@...il.com>
To:     akpm@...ux-foundation.org, viro@...iv.linux.org.uk
Cc:     linux-kernel@...r.kernel.org, reiserfs-devel@...r.kernel.org
Subject: Re: [PATCH] reiserfs: Add comment to explain endianness issue in
 xattr_hash

On Tue, Apr 23, 2019 at 09:48:31PM +0530, Bharath Vedartham wrote:
> csum_partial() gives different results for little-endian and big-endian
> hosts. This causes images created on little-endian hosts and mounted on
> big endian hosts to see csum mismatches. This causes an endianness bug.
> Sparse gives a warning as csum_partial returns a restricted integer type
> __wsum_t and xattr_hash expects __u32. This warning acts as a reminder
> for this bug and should not be suppressed.
> 
> This comment aims to convey these endianness issues.
> 
> Signed-off-by: Bharath Vedartham <linux.bhar@...il.com>
> ---
>  fs/reiserfs/xattr.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/fs/reiserfs/xattr.c b/fs/reiserfs/xattr.c
> index 32d8986..0ea6654 100644
> --- a/fs/reiserfs/xattr.c
> +++ b/fs/reiserfs/xattr.c
> @@ -450,6 +450,16 @@ static struct page *reiserfs_get_page(struct inode *dir, size_t n)
>  
>  static inline __u32 xattr_hash(const char *msg, int len)
>  {
> +	/*
> +	 * csum_partial() gives different results for little-endian and
> +	 * big endian hosts. Images created on little-endian hosts and
> +	 * mounted on big-endian hosts(and vice versa) will see csum mismatches
> +	 * when trying to fetch xattrs. Treating the hash as __wsum_t would
> +	 * lower the frequency of mismatch. This is an endianness bug in reiserfs.
> +	 * The return statement would result in a sparse warning. Do not fix the sparse
> +	 * warning so as to not hide the reminder of the bug.
> +	 */
> +
>  	return csum_partial(msg, len, 0);
>  }
>  
> -- 
> 2.7.4
>

Is this good or is it lacking any explanation?