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

[Chunhai Guo]

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;

> 
>>
>>>>        - Expected result:
>>>>            - cur = 0
>>>>        - Actual result:
>>>>            - cur = 0x370
>>>>
>>>> Signed-off-by: Chunhai Guo <guochunhai at vivo.com>
>>>> ---
>>>>     fs/erofs/zdata.c | 13 ++++++++++---
>>>>     1 file changed, 10 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
>>>> index 5f1890e309c6..6abbd4510076 100644
>>>> --- a/fs/erofs/zdata.c
>>>> +++ b/fs/erofs/zdata.c
>>>> @@ -972,7 +972,8 @@ static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
>>>>         struct erofs_map_blocks *const map = &fe->map;
>>>>         const loff_t offset = page_offset(page);
>>>>         bool tight = true, exclusive;
>>>> -    unsigned int cur, end, spiltted;
>>>> +    erofs_off_t cur, end;
>>>> +    unsigned int spiltted;
>>>>         int err = 0;
>>>>         /* register locked file pages as online pages in pack */
>>>> @@ -1035,7 +1036,7 @@ static int z_erofs_do_read_page(struct z_erofs_decompress_frontend *fe,
>>>>          */
>>>>         tight &= (fe->mode > Z_EROFS_PCLUSTER_FOLLOWED_NOINPLACE);
>>>> -    cur = end - min_t(unsigned int, offset + end - map->m_la, end);
>>>> +    cur = end - min_t(erofs_off_t, offset + end - map->m_la, end);
>>>>         if (!(map->m_flags & EROFS_MAP_MAPPED)) {
>>>>             zero_user_segment(page, cur, end);
>>>>             goto next_part;
>>>> @@ -1841,7 +1842,7 @@ static void z_erofs_pcluster_readmore(struct z_erofs_decompress_frontend *f,
>>>>         }
>>>>         cur = map->m_la + map->m_llen - 1;
>>>> -    while (cur >= end) {
>>>> +    while ((cur >= end) && (cur < i_size_read(inode))) {
>>>>             pgoff_t index = cur >> PAGE_SHIFT;
>>>>             struct page *page;
>>>> @@ -1876,6 +1877,12 @@ static int z_erofs_read_folio(struct file *file, struct folio *folio)
>>>>         trace_erofs_readpage(page, false);
>>>>         f.headoffset = (erofs_off_t)page->index << PAGE_SHIFT;
>>>> +    /* when trying to read beyond EOF, return zero page directly */
>>>> +    if (f.headoffset >= i_size_read(inode)) {
>>>> +        zero_user_segment(page, 0, PAGE_SIZE);
>>>> +        return 0;
>>>> +    }
>>> Do we really need to optimize this rare case?
>>> I guess the follow readmore fix is enough, thoughts? >
>>> Thanks,
>>> Gao Xiang
>>
>> Since the code is constantly being updated and someone may encounter this bug again, I think we had better fix it if possible.
> 
> z_erofs_do_read_page() already covers this case so I'm
> not sure why we need to add another logic here.

You are right. I have removed this logic and sent the patch.

> 
> Thanks,
> Gao Xiang
> 
>>
>> Thanks.
>> Guo Chunhai