fat: fix possible overflow for fat_clusters

Intermediate value of fat_clusters can be overflowed on 32bits arch. Reported-by: Krzysztof Strasburger <[email protected]> Signed-off-by: OGAWA Hirofumi <[email protected]> Cc: <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
LeeHayun · May 24, 2013 · 7b92d03 · 7b92d03
1 parent 5eeb929
commit 7b92d03
Showing 1 changed file with 14 additions and 1 deletion.
diff --git a/fs/fat/inode.c b/fs/fat/inode.c
@@ -1229,6 +1229,19 @@ static int fat_read_root(struct inode *inode)
 	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.
  */
@@ -1434,7 +1447,7 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
 		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)