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

[Chunhai Guo]

On 2023/7/10 13:02, Gao Xiang wrote:
> 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

Got it. I have sent the patch. Please have a check.

Thanks,
Guo Chunhai