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);