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

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Message-ID: <202101140625555106736@uniontech.com>
Date:   Thu, 14 Jan 2021 06:25:57 +0800
From:   "changlianzhi@...ontech.com" <changlianzhi@...ontech.com>
To:     magnani <magnani@...e.org>
Cc:     jack <jack@...e.com>, linux-kernel <linux-kernel@...r.kernel.org>,
        282827961 <282827961@...com>
Subject: Re: [PATCH] udf: fix the problem that the disc content is not displayed

On 2021-01-13 20:51, 常廉志 wrote:

>> On 2021-01-11 23:53, 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
>>> l>ogic, 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 | 2 +-
>>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/udf/super.c b/fs/udf/super.c
>>>> index 5bef3a68395d..6c3069cd1321 100644
>>>> --- a/fs/udf/super.c
>>>> +++ b/fs/udf/super.c
>>>> @@ -757,7 +757,7 @@ 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)
>>>> ==
>>>> + else if (!bh && sector - ((loff_t)sbi->s_session <<
>>>> sb->s_blocksize_bits) ==
>>>> VSD_FIRST_SECTOR_OFFSET)
>>>> return -1;
>>>> else
>>>
>>>
>>> Looks good. Perhaps consider factoring out the conversion (which also
>>> occurs
>>> earlier in the function) so that the complexity of this "else if" can
>>> be
>>> reduced?
>>>
>>
>>> Reviewed-by: Steven J. Magnani <magnani@...xxxxx>
>>
>> Thank you very much! So, which one of the following methods do you
>> think is better:
>>
>> (1) Change the type of s_session in struct udf_sb_info to __s64. If you
>> modify this way, it may involve some memory copy problems of the
>> structure, and there are more modifications.
>>
>> (2) Definition: loff_t sector_offset=sbi->s_session <<
>> sb->s_blocksize_bits, and then put sector_offset into the "else if"
>> statement.
>>
>> (3) Or is there any other better way?

>I had #2 in mind.
>------------------------------------------------------------------------
>  Steven J. Magnani               "I claim this network for MARS!

Thank you very much for your suggestion, I will submit a new patch