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

[Gao Xiang <hsiangkao@linux.alibaba.com>]

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?

> 
>>>       - Expected result:
>>>           - cur = 0
>>>       - Actual result:
>>>           - cur = 0x370
>>>
>>> Signed-off-by: Chunhai Guo <guochunhai@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.

Thanks,
Gao Xiang

> 
> Thanks.
> Guo Chunhai