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Date:   Tue, 23 Apr 2019 21:48:31 +0530
From:   Bharath Vedartham <linux.bhar@gmail.com>
To:     akpm@linux-foundation.org, viro@zeniv.linux.org.uk
Cc:     linux-kernel@vger.kernel.org, reiserfs-devel@vger.kernel.org,
        linux.bhar@gmail.com
Subject: [PATCH] reiserfs: Add comment to explain endianness issue in
 xattr_hash
Message-ID: <20190423161831.GA15387@bharath12345-Inspiron-5559>
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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@gmail.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