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

From: Gao Xiang
Date: Mon Jul 10 2023 - 01:03:07 EST




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