ext2: Fix underflow in ext2_max_size()

When ext2 filesystem is created with 64k block size, ext2_max_size()
will return value less than 0. Also, we cannot write any file in this fs
since the sb->maxbytes is less than 0. The core of the problem is that
the size of block index tree for such large block size is more than
i_blocks can carry. So fix the computation to count with this
possibility.

File size limits computed with the new function for the full range of
possible block sizes look like:

bits file_size
10     17247252480
11    275415851008
12   2196873666560
13   2197948973056
14   2198486220800
15   2198754754560
16   2198888906752

CC: stable@vger.kernel.org
Reported-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext2/super.c

@@ -757,7 +757,8 @@ static loff_t ext2_max_size(int bits)
 {
 	loff_t res = EXT2_NDIR_BLOCKS;
 	int meta_blocks;
-	loff_t upper_limit;
+	unsigned int upper_limit;
+	unsigned int ppb = 1 << (bits-2);
 
 	/* This is calculated to be the largest file size for a
 	 * dense, file such that the total number of
@@ -771,24 +772,34 @@ static loff_t ext2_max_size(int bits)
 	/* total blocks in file system block size */
 	upper_limit >>= (bits - 9);
 
-
-	/* indirect blocks */
-	meta_blocks = 1;
-	/* double indirect blocks */
-	meta_blocks += 1 + (1LL << (bits-2));
-	/* tripple indirect blocks */
-	meta_blocks += 1 + (1LL << (bits-2)) + (1LL << (2*(bits-2)));
-
-	upper_limit -= meta_blocks;
-	upper_limit <<= bits;
-
+	/* Compute how many blocks we can address by block tree */
 	res += 1LL << (bits-2);
 	res += 1LL << (2*(bits-2));
 	res += 1LL << (3*(bits-2));
-	res <<= bits;
-	if (res > upper_limit)
-		res = upper_limit;
+	/* Does block tree limit file size? */
+	if (res < upper_limit)
+		goto check_lfs;
 
+	res = upper_limit;
+	/* How many metadata blocks are needed for addressing upper_limit? */
+	upper_limit -= EXT2_NDIR_BLOCKS;
+	/* indirect blocks */
+	meta_blocks = 1;
+	upper_limit -= ppb;
+	/* double indirect blocks */
+	if (upper_limit < ppb * ppb) {
+		meta_blocks += 1 + DIV_ROUND_UP(upper_limit, ppb);
+		res -= meta_blocks;
+		goto check_lfs;
+	}
+	meta_blocks += 1 + ppb;
+	upper_limit -= ppb * ppb;
+	/* tripple indirect blocks for the rest */
+	meta_blocks += 1 + DIV_ROUND_UP(upper_limit, ppb) +
+		DIV_ROUND_UP(upper_limit, ppb*ppb);
+	res -= meta_blocks;
+check_lfs:
+	res <<= bits;
 	if (res > MAX_LFS_FILESIZE)
 		res = MAX_LFS_FILESIZE;