linux-kernel - Re: [PATCH] erofs: fix two loop issues when read page beyond EOF

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Date:   Mon, 10 Jul 2023 13:02:58 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     Chunhai Guo <guochunhai@...o.com>,
        "xiang@...nel.org" <xiang@...nel.org>,
        "chao@...nel.org" <chao@...nel.org>
Cc:     "huyue2@...lpad.com" <huyue2@...lpad.com>,
        "jefflexu@...ux.alibaba.com" <jefflexu@...ux.alibaba.com>,
        "linux-erofs@...ts.ozlabs.org" <linux-erofs@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] erofs: fix two loop issues when read page beyond EOF



On 2023/7/10 12:35, Chunhai Guo wrote:
> 
> 
> On 2023/7/10 11:37, Gao Xiang wrote:
>>
>>
>> On 2023/7/10 11:32, Chunhai Guo wrote:
>>> Hi Xiang,
>>>
>>> On 2023/7/8 17:00, Gao Xiang wrote:
>>>> Hi Chunhai,
>>>>
>>>> On 2023/7/8 14:24, Chunhai Guo wrote:
>>>>> When z_erofs_read_folio() reads a page with an offset far beyond EOF, two
>>>>> issues may occur:
>>>>> - z_erofs_pcluster_readmore() may take a long time to loop when the offset
>>>>>      is big enough, which is unnecessary.
>>>>>        - For example, it will loop 4691368 times and take about 27 seconds
>>>>>          with following case.
>>>>>            - offset = 19217289215
>>>>>            - inode_size = 1442672
>>>>> - z_erofs_do_read_page() may loop infinitely due to the inappropriate
>>>>>      truncation in the below statement. Since the offset is 64 bits and
>>>>> min_t() truncates the result to 32 bits. The solution is to replace
>>>>> unsigned int with another 64-bit type, such as erofs_off_t.
>>>>>        cur = end - min_t(unsigned int, offset + end - map->m_la, end);
>>>>>        - For example:
>>>>>            - offset = 0x400160000
>>>>>            - end = 0x370
>>>>>            - map->m_la = 0x160370
>>>>>            - offset + end - map->m_la = 0x400000000
>>>>>            - offset + end - map->m_la = 0x00000000 (truncated as unsigned int)
>>>>
>>>> Thanks for the catch!
>>>>
>>>> Could you split these two into two patches?
>>>>
>>>> how about using:
>>>> cur = end - min_t(erofs_off_t, offend + end - map->m_la, end)
>>>> for this?
>>>>
>>>> since cur and end are all [0, PAGE_SIZE - 1] for now, and
>>>> folio_size() later.
>>>
>>> OK. I will split the patch.
>>>
>>> Sorry that I can not understand what is 'offend' refer to and what do you mean. Could you please describe it more clearly?
>>
>> Sorry, there is a typo here, I meant 'offset'.
>>
>> `cur` and `end` both are not exceed 4096 if your page_size
>> is 4096.
>>
>> Does
>> cur = end - min_t(erofs_off_t, offset + end - map->m_la, end)
>>
>> fix your issue?
> 
> Yes. I think this will fix this issue. Do you mean the below change is unncessary?
>  >>>> -    unsigned int cur, end, spiltted;
>  >>>> +    erofs_off_t cur, end;
>  >>>> +    unsigned int spiltted;

Yes, please help send a fix for this!

Thanks,
Gao Xiang