Re: [PATCH] arm: dma-mapping: fix potential endless loop in __dma_page_dev_to_cpu()

[Russell King (Oracle)]

On Mon, Aug 07, 2023 at 11:14:13PM +0100, Matthew Wilcox wrote:
> On Mon, Aug 07, 2023 at 05:26:57PM +0200, Marek Szyprowski wrote:
> > diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> > index 70cb7e63a9a5..02250106e5ed 100644
> > --- a/arch/arm/mm/dma-mapping.c
> > +++ b/arch/arm/mm/dma-mapping.c
> > @@ -718,7 +718,7 @@ static void __dma_page_dev_to_cpu(struct page *page, unsigned long off,
> >  			folio = folio_next(folio);
> >  		}
> >  
> > -		while (left >= (ssize_t)folio_size(folio)) {
> > +		while (left && left >= (ssize_t)folio_size(folio)) {
> >  			set_bit(PG_dcache_clean, &folio->flags);
> >  			left -= folio_size(folio);
> >  			folio = folio_next(folio);
> 
> I've been thinking about this and I think this is the right fix for the
> wrong reason.  I don't understand how it can produce the failure you
> saw, but we shouldn't be calling folio_next() if left is zero, let alone
> calling folio_size() on it.  So I'd rather see this fix:
> 
> 		while (left >= (ssize_t)folio_size(folio)) {
> 			set_bit(PG_dcache_clean, &folio->flags);
> 			left -= folio_size(folio);
> +			if (!left)
> +				break;

Given that set_bit() involves atomics, wouldn't it be better if this
had been written as:

		while (left >= folio_size(folio)) {
			left -= folio_size(folio);
			set_bit(PG_dcache_clean, &folio->flags);
			if (!left)
				break;
> 			folio = folio_next(folio);
> 		}

That likely means that folio_size() will only be evaluated once per
loop rather than twice. I may be wrong though, I didn't check the
generated code.

Also, I'm wondering what that ssize_t cast is doing there - "left"
is a size_t, which is unsigned. folio_size() returns a size_t, so
is also unsigned. Why convert folio_size() to a signed number to
then be compared with an unsigned number?

Or did "left" get converted to ssize_t along with the folio
conversion?

Even if it did, how could "left" be negative (except through casting
a large positive number as "size" that in 2's complement would be
negative after casting to "left") ?

-- 
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