2022-02-12 08:42:02

by Zhang Yi

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 <[email protected]>
---
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

2022-02-25 15:00:21

by Jan Kara

Subject: Re: [PATCH] ext2: correct max file size computing

On Sat 12-02-22 13:05:32, Zhang Yi wrote:
> 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 <[email protected]>

Thanks for the cleanup! I've merged the patch to my tree.

Honza

> ---
> 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
>
--
Jan Kara <[email protected]>
SUSE Labs, CR