[PATCH] fat: Fix possible overflow for fat_clusters

[OGAWA Hirofumi]

Intermediate value of fat_clusters can be overflowed on 32bits arch.

Reported-by: Krzysztof Strasburger <strasbur@xxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
---

 fs/fat/inode.c |   15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff -puN fs/fat/inode.c~fat-fat_clusters-overflow-fix fs/fat/inode.c
--- linux/fs/fat/inode.c~fat-fat_clusters-overflow-fix	2013-05-01 09:37:27.000000000 +0900
+++ linux-hirofumi/fs/fat/inode.c	2013-05-01 09:43:13.000000000 +0900
@@ -1223,6 +1223,19 @@ static int fat_read_root(struct inode *i
 	return 0;
 }
 
+static unsigned long calc_fat_clusters(struct super_block *sb)
+{
+	struct msdos_sb_info *sbi = MSDOS_SB(sb);
+
+	/* Divide first to avoid overflow */
+	if (sbi->fat_bits != 12) {
+		unsigned long ent_per_sec = sb->s_blocksize * 8 / sbi->fat_bits;
+		return ent_per_sec * sbi->fat_length;
+	}
+
+	return sbi->fat_length * sb->s_blocksize * 8 / sbi->fat_bits;
+}
+
 /*
  * Read the super block of an MS-DOS FS.
  */
@@ -1427,7 +1440,7 @@ int fat_fill_super(struct super_block *s
 		sbi->dirty = b->fat16.state & FAT_STATE_DIRTY;
 
 	/* check that FAT table does not overflow */
-	fat_clusters = sbi->fat_length * sb->s_blocksize * 8 / sbi->fat_bits;
+	fat_clusters = calc_fat_clusters(sb);
 	total_clusters = min(total_clusters, fat_clusters - FAT_START_ENT);
 	if (total_clusters > MAX_FAT(sb)) {
 		if (!silent)
_
-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
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