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

[Chunhai Guo <guochunhai@vivo.com>]

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?

>>       - 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.

Thanks.
Guo Chunhai