On Wed, Jan 19, 2022 at 06:43:10PM +0000, Robin Murphy wrote:
> Indeed, my impression is that the only legitimate way to get hold of a page
> pointer without assumed provenance is via pfn_to_page(), which is where
> pfn_valid() comes in. Thus pfn_valid(page_to_pfn()) really *should* be a
> tautology.
That can only be true if pfn == page_to_pfn(pfn_to_page(pfn)) for all
values of pfn.
Given how pfn_to_page() is defined in the sparsemem case:
#define __pfn_to_page(pfn) \
({ unsigned long __pfn = (pfn); \
struct mem_section *__sec = __pfn_to_section(__pfn); \
__section_mem_map_addr(__sec) + __pfn; \
})
#define page_to_pfn __page_to_pfn
that isn't the case, especially when looking at page_to_pfn():
#define __page_to_pfn(pg) \
({ const struct page *__pg = (pg); \
int __sec = page_to_section(__pg); \
(unsigned long)(__pg - __section_mem_map_addr(__nr_to_section(__sec))); \
})
Where:
static inline unsigned long page_to_section(const struct page *page)
{
return (page->flags >> SECTIONS_PGSHIFT) & SECTIONS_MASK;
}
So if page_to_section() returns something that is, e.g. zero for an
invalid page in a non-zero section, you're not going to end up with
the right pfn from page_to_pfn().
As I've said now a couple of times, trying to determine of a struct
page pointer is valid is the wrong question to be asking.
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