linux-ext4 - [PATCH] ext2: correct max file size computing

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Date:   Sat, 12 Feb 2022 13:05:32 +0800
From:   Zhang Yi <yi.zhang@...wei.com>
To:     <linux-ext4@...r.kernel.org>
CC:     <jack@...e.cz>, <yi.zhang@...wei.com>, <yukuai3@...wei.com>
Subject: [PATCH] ext2: correct max file size computing

We need to calculate the max file size accurately if the total blocks
that can address by block tree exceed the upper_limit. But this check is
not correct now, it only compute the total data blocks but missing
metadata blocks are needed. So in the case of "data blocks < upper_limit
&& total blocks > upper_limit", we will get wrong result. Fortunately,
this case could not happen in reality, but it's confused and better to
correct the computing.

  bits   data blocks   metadatablocks   upper_limit
  10        16843020            66051    2147483647
  11       134480396           263171    1073741823
  12      1074791436          1050627     536870911 (*)
  13      8594130956          4198403     268435455 (*)
  14     68736258060         16785411     134217727 (*)
  15    549822930956         67125251      67108863 (*)
  16   4398314962956        268468227      33554431 (*)

  [*] Need to calculate in depth.

Fixes: 1c2d14212b15 ("ext2: Fix underflow in ext2_max_size()")
Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
---
 fs/ext2/super.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/ext2/super.c b/fs/ext2/super.c
index 94f1fbd7d3ac..6d4f5ef74766 100644
--- a/fs/ext2/super.c
+++ b/fs/ext2/super.c
@@ -753,8 +753,12 @@ static loff_t ext2_max_size(int bits)
 	res += 1LL << (bits-2);
 	res += 1LL << (2*(bits-2));
 	res += 1LL << (3*(bits-2));
+	/* 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 < upper_limit)
+	if (res + meta_blocks <= upper_limit)
 		goto check_lfs;
 
 	res = upper_limit;
-- 
2.31.1