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>

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;
+
+	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);

fs/fat/dir.c

@@ -77,7 +77,7 @@ static int fat__get_entry(struct inode *dir, loff_t *pos,
 
 	*bh = NULL;
 	iblock = *pos >> sb->s_blocksize_bits;
-	err = fat_bmap(dir, iblock, &phys, &mapped_blocks);
+	err = fat_bmap(dir, iblock, &phys, &mapped_blocks, 0);
 	if (err || !phys)
 		return -1;	/* beyond EOF or error */
 

fs/fat/fat.h

@@ -91,7 +91,9 @@ struct msdos_inode_info {
 	/* for avoiding the race between fat_free() and fat_get_cluster() */
 	unsigned int cache_valid_id;
 
-	loff_t mmu_private;
+	/* NOTE: mmu_private is 64bits, so must hold ->i_mutex to access */
+	loff_t mmu_private;	/* physically allocated size */
+
 	int i_start;		/* first cluster or 0 */
 	int i_logstart;		/* logical first cluster */
 	int i_attrs;		/* unused attribute bits */
@@ -222,7 +224,7 @@ extern void fat_cache_inval_inode(struct inode *inode);
 extern int fat_get_cluster(struct inode *inode, int cluster,
 			   int *fclus, int *dclus);
 extern int fat_bmap(struct inode *inode, sector_t sector, sector_t *phys,
-		    unsigned long *mapped_blocks);
+		    unsigned long *mapped_blocks, int create);
 
 /* fat/dir.c */
 extern const struct file_operations fat_dir_operations;

fs/fat/inode.c

@@ -64,7 +64,7 @@ static inline int __fat_get_block(struct inode *inode, sector_t iblock,
 	sector_t phys;
 	int err, offset;
 
-	err = fat_bmap(inode, iblock, &phys, &mapped_blocks);
+	err = fat_bmap(inode, iblock, &phys, &mapped_blocks, create);
 	if (err)
 		return err;
 	if (phys) {
@@ -94,7 +94,7 @@ static inline int __fat_get_block(struct inode *inode, sector_t iblock,
 	*max_blocks = min(mapped_blocks, *max_blocks);
 	MSDOS_I(inode)->mmu_private += *max_blocks << sb->s_blocksize_bits;
 
-	err = fat_bmap(inode, iblock, &phys, &mapped_blocks);
+	err = fat_bmap(inode, iblock, &phys, &mapped_blocks, create);
 	if (err)
 		return err;