[RFC PATCH v1 4/6] exfat: lock FAT2 buffer while copying mirrored FAT entries

[Chi Zhiling <chizhiling@163.com>]

From: Chi Zhiling <chizhiling@kylinos.cn>

exfat_mirror_bh() updates the FAT2 buffer without serializing the copy
against writeback. Once s_lock is converted to a read-write lock,
multiple read-side operations may run concurrently with writeback.
This can allow writeback to observe a partially updated FAT2 block.

Lock the destination buffer while copying the FAT entry data to
serialize the update with writeback and prevent torn FAT2 writes.

Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
---
 fs/exfat/fatent.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/exfat/fatent.c b/fs/exfat/fatent.c
index 2db959669563..f6ad2825ad19 100644
--- a/fs/exfat/fatent.c
+++ b/fs/exfat/fatent.c
@@ -24,7 +24,10 @@ static int exfat_mirror_bh(struct super_block *sb, struct buffer_head *bh)
 		c_bh = sb_getblk(sb, sec2);
 		if (!c_bh)
 			return -ENOMEM;
+		/* Serialize the copy with writeback to avoid a torn FAT2 write */
+		lock_buffer(c_bh);
 		memcpy(c_bh->b_data, bh->b_data, sb->s_blocksize);
+		unlock_buffer(c_bh);
 		err = exfat_update_bh(c_bh, sb->s_flags & SB_SYNCHRONOUS);
 		brelse(c_bh);
 	}
-- 
2.53.0