linux-kernel - Re: [PATCH] ext4: Fix rec

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Date:   Sat, 29 Jul 2023 13:09:38 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     zhangshida <starzhangzsd@...il.com>
Cc:     tytso@....edu, adilger.kernel@...ger.ca,
        linux-kernel@...r.kernel.org, zhangshida@...inos.cn
Subject: Re: [PATCH] ext4: Fix rec_len verify error

On Jul 29, 2023, at 00:14, zhangshida <starzhangzsd@...il.com> wrote:
> 
> From: Shida Zhang <zhangshida@...inos.cn>
> 
> with the configuration PAGE_SIZE 64k and filesystem blocksize 64k,
> a problem occurred when more than 13 millon files were directly created
> under a directory:
> 
> EXT4-fs error (device xx): ext4_dx_csum_set:492: inode #xxxx: comm xxxxx: dir seems corrupt?  Run e2fsck -D.
> EXT4-fs error (device xx): ext4_dx_csum_verify:463: inode #xxxx: comm xxxxx: dir seems corrupt?  Run e2fsck -D.
> EXT4-fs error (device xx): dx_probe:856: inode #xxxx: block 8188: comm xxxxx: Directory index failed checksum
> 
> when enough files are created, the fake_dirent->reclen will be 0xffff.
> it doesn't equal to blocksize 65536, i.e. 0x10000.
> 
> But it is not the same condition when blocksize equals to 4k.
> when enough files are created, the fake_dirent->reclen will be 0x1000.
> it equals to blocksize 4k, i.e. 0x1000.
> 
> The problem seems to be related to the limitation of the 16-bit field
> when the blocksize is set to 64k. To address this, a special condition
> was introduced to handle it properly.
> 
> Signed-off-by: Shida Zhang <zhangshida@...inos.cn>
> ---
> fs/ext4/namei.c | 3 +++
> 1 file changed, 3 insertions(+)
> 
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index 0caf6c730ce3..a422cff25216 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -458,6 +458,9 @@ static struct dx_countlimit *get_dx_countlimit(struct inode *inode,
>            root->info_length != sizeof(struct dx_root_info))
>            return NULL;
>        count_offset = 32;
> +    } else if ((EXT4_BLOCK_SIZE(inode->i_sb) == 65536)
> +        && (le16_to_cpu(dirent->rec_len) == 65535)) {
> +        count_offset = 8;

This should be moved up to the first if-block that is checking the block size: 

........        if (le16_to_cpu(dirent->rec_len) == EXT4_BLOCK_SIZE(inode->i_sb) ||
             (le16_to_cpu(dirent->rec_len) == 65535 &&
              EXT4_BLOCK_SIZE(inode->i_sb) >= 65536))
................                count_offset = 8;

since this is really the same case. 

Ecen better would be to use ext4_rec_len_from_disk() to check the
length so that it keeps this large PAGE_SIZE logic in one place, and
does not add overhead on systems with smaller PAGE_SIZE:

        int blocksize = EXT4_BLOCK_SIZE(inode->i_sb);

        if (ext4_rec_len_from_disk(dirent->rec_len, blocksize) == blocksize)
                count_offset = 8;

Cheers, Andreas

>    } else
>        return NULL;
> 
> -- 
> 2.27.0
>