linux-kernel - Re: [PATCH] udf: fix the problem that the disc content is not displayed

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Message-ID: <20210114104157.GB25790@quack2.suse.cz>
Date:   Thu, 14 Jan 2021 11:41:57 +0100
From:   Jan Kara <jack@...e.cz>
To:     lianzhi chang <changlianzhi@...ontech.com>
Cc:     magnani@...e.org, jack@...e.com, linux-kernel@...r.kernel.org,
        282827961@...com
Subject: Re: [PATCH] udf: fix the problem that the disc content is not
 displayed

On Thu 14-01-21 15:57:41, lianzhi chang wrote:
> When the capacity of the disc is too large (assuming the 4.7G
> specification), the disc (UDF file system) will be burned
> multiple times in the windows (Multisession Usage). When the
> remaining capacity of the CD is less than 300M (estimated
> value, for reference only), open the CD in the Linux system,
> the content of the CD is displayed as blank (the kernel will
> say "No VRS found"). Windows can display the contents of the
> CD normally.
> Through analysis, in the "fs/udf/super.c": udf_check_vsd
> function, the actual value of VSD_MAX_SECTOR_OFFSET may
> be much larger than 0x800000. According to the current code
> logic, it is found that the type of sbi->s_session is "__s32",
>  when the remaining capacity of the disc is less than 300M
> (take a set of test values: sector=3154903040,
> sbi->s_session=1540464, sb->s_blocksize_bits=11 ), the
> calculation result of "sbi->s_session << sb->s_blocksize_bits"
>  will overflow. Therefore, it is necessary to convert the
> type of s_session to "loff_t" (when udf_check_vsd starts,
> assign a value to _sector, which is also converted in this
> way), so that the result will not overflow, and then the
> content of the disc can be displayed normally.
> 
> Signed-off-by: lianzhi chang <changlianzhi@...ontech.com>
> ---
>  fs/udf/super.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/udf/super.c b/fs/udf/super.c
> index 5bef3a68395d..f2ff98f393fb 100644
> --- a/fs/udf/super.c
> +++ b/fs/udf/super.c
> @@ -705,6 +705,7 @@ static int udf_check_vsd(struct super_block *sb)
>  	struct buffer_head *bh = NULL;
>  	int nsr = 0;
>  	struct udf_sb_info *sbi;
> +	loff_t sector_offset;
>  
>  	sbi = UDF_SB(sb);
>  	if (sb->s_blocksize < sizeof(struct volStructDesc))
> @@ -712,7 +713,8 @@ static int udf_check_vsd(struct super_block *sb)
>  	else
>  		sectorsize = sb->s_blocksize;
>  
> -	sector += (((loff_t)sbi->s_session) << sb->s_blocksize_bits);
> +	sector_offset = (loff_t)sbi->s_session << sb->s_blocksize_bits);

There's imbalanced parentheses here, I'll fix it up on commit. Otherwise
the fix looks good to me. Thanks!

								Honza

> +	sector += sector_offset;
>  
>  	udf_debug("Starting at sector %u (%lu byte sectors)\n",
>  		  (unsigned int)(sector >> sb->s_blocksize_bits),
> @@ -757,8 +759,7 @@ static int udf_check_vsd(struct super_block *sb)
>  
>  	if (nsr > 0)
>  		return 1;
> -	else if (!bh && sector - (sbi->s_session << sb->s_blocksize_bits) ==
> -			VSD_FIRST_SECTOR_OFFSET)
> +	else if (!bh && sector - sector_offset == VSD_FIRST_SECTOR_OFFSET)
>  		return -1;
>  	else
>  		return 0;
> -- 
> 2.20.1
> 
> 
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR