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

[Chunhai Guo]

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@xxxxxxxx>
> > ---
> >    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