ext4: fix underflow in ext4_max_bitmap_size()

when ext4 filesystem is created with 64k block size, ^extent and
^huge_file features. the upper_limit would underflow during the
computations in ext4_max_bitmap_size(). The problem is 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. After this fix,
the 'res' cannot overflow loff_t on the extreme case of filesystem with
huge_files and 64K block size, so this patch also revert commit
75ca6ad4 ("ext4: fix loff_t overflow in ext4_max_bitmap_size()").
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220301111704.2153829-1-yi.zhang@huawei.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -3481,8 +3481,9 @@ static loff_t ext4_max_size(int blkbits, int has_huge_files)
  */
 static loff_t ext4_max_bitmap_size(int bits, int has_huge_files)
 {
-	unsigned long long upper_limit, res = EXT4_NDIR_BLOCKS;
+	loff_t upper_limit, res = EXT4_NDIR_BLOCKS;
 	int meta_blocks;
+	unsigned int ppb = 1 << (bits - 2);
 
 	/*
 	 * This is calculated to be the largest file size for a dense, block
@@ -3514,27 +3515,42 @@ static loff_t ext4_max_bitmap_size(int bits, int has_huge_files)
 
 	}
 
+	/* Compute how many blocks we can address by block tree */
+	res += ppb;
+	res += ppb * ppb;
+	res += ((loff_t)ppb) * ppb * ppb;
+	/* Compute how many metadata blocks are needed */
+	meta_blocks = 1;
+	meta_blocks += 1 + ppb;
+	meta_blocks += 1 + ppb + ppb * ppb;
+	/* Does block tree limit file size? */
+	if (res + meta_blocks <= upper_limit)
+		goto check_lfs;
+
+	res = upper_limit;
+	/* How many metadata blocks are needed for addressing upper_limit? */
+	upper_limit -= EXT4_NDIR_BLOCKS;
 	/* indirect blocks */
 	meta_blocks = 1;
+	upper_limit -= ppb;
 	/* 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;
-
-	res += 1LL << (bits-2);
-	res += 1LL << (2*(bits-2));
-	res += 1LL << (3*(bits-2));
+	if (upper_limit < ppb * ppb) {
+		meta_blocks += 1 + DIV_ROUND_UP_ULL(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_ULL(upper_limit, ppb) +
+		DIV_ROUND_UP_ULL(upper_limit, ppb*ppb);
+	res -= meta_blocks;
+check_lfs:
 	res <<= bits;
-	if (res > upper_limit)
-		res = upper_limit;
-
 	if (res > MAX_LFS_FILESIZE)
 		res = MAX_LFS_FILESIZE;
 
-	return (loff_t)res;
+	return res;
 }
 
 static ext4_fsblk_t descriptor_loc(struct super_block *sb,