2022-03-01 16:00:42

by Zhang Yi

Subject: [PATCH v3] 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
75ca6ad408f4 ("ext4: fix loff_t overflow in ext4_max_bitmap_size()").

Signed-off-by: Zhang Yi <[email protected]>
---
v3->v2: rewrite change log and use ppb to compute 'res' blocks.
v2->v1: use DIV_ROUND_UP_ULL instead of DIV_ROUND_UP.

fs/ext4/super.c | 46 +++++++++++++++++++++++++++++++---------------
1 file changed, 31 insertions(+), 15 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index c5021ca0a28a..bfba62206a14 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3468,8 +3468,9 @@ 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
@@ -3501,27 +3502,42 @@ 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,
--
2.31.1

2022-03-03 15:54:07

by Theodore Ts'o

Subject: Re: [PATCH v3] ext4: fix underflow in ext4_max_bitmap_size()

On Tue, 1 Mar 2022 19:17:04 +0800, Zhang Yi wrote:
> 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
> 75ca6ad408f4 ("ext4: fix loff_t overflow in ext4_max_bitmap_size()").
>
> [...]

Applied, thanks!

[1/1] ext4: fix underflow in ext4_max_bitmap_size()
commit: 5c93e8ecd5bd3bfdee013b6da0850357eb6ca4d8

Best regards,
--
Theodore Ts'o <[email protected]>