fat: mmu_private race fix

mmu_private is 64bits value, hence it's not atomic to update.

So, the access rule for mmu_private is we must hold ->i_mutex.  But,
fat_get_block() path doesn't follow the rule on non-allocation path.

This fixes by using i_size instead if non-allocation path.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 18 insertions, 5 deletions

diff --git a/fs/fat/cache.c b/fs/fat/cache.c
index 589edde9053c..b42602298087 100644
--- a/fs/fat/cache.c
+++ b/fs/fat/cache.c
@@ -293,10 +293,12 @@ static int fat_bmap_cluster(struct inode *inode, int cluster)
 }
 
 int fat_bmap(struct inode *inode, sector_t sector, sector_t *phys,
-	     unsigned long *mapped_blocks)
+	     unsigned long *mapped_blocks, int create)
 {
 	struct super_block *sb = inode->i_sb;
 	struct msdos_sb_info *sbi = MSDOS_SB(sb);
+	const unsigned long blocksize = sb->s_blocksize;
+	const unsigned char blocksize_bits = sb->s_blocksize_bits;
 	sector_t last_block;
 	int cluster, offset;
 
@@ -309,10 +311,21 @@ int fat_bmap(struct inode *inode, sector_t sector, sector_t *phys,
 		}
 		return 0;
 	}
-	last_block = (MSDOS_I(inode)->mmu_private + (sb->s_blocksize - 1))
-		>> sb->s_blocksize_bits;
-	if (sector >= last_block)
-		return 0;
+
+	last_block = (i_size_read(inode) + (blocksize - 1)) >> blocksize_bits;
+	if (sector >= last_block) {
+		if (!create)
+			return 0;
+
+		/*
+		 * ->mmu_private can access on only allocation path.
+		 * (caller must hold ->i_mutex)
+		 */
+		last_block = (MSDOS_I(inode)->mmu_private + (blocksize - 1))
+			>> blocksize_bits;
+		if (sector >= last_block)
+			return 0;
+	}
 
 	cluster = sector >> (sbi->cluster_bits - sb->s_blocksize_bits);
 	offset  = sector & (sbi->sec_per_clus - 1);