Re: [PATCH] mm: Handle compound pages better in __dump_page()

[Matthew Wilcox <willy@infradead.org>]

On Sat, Nov 16, 2024 at 09:52:44PM -0800, Kees Cook wrote:
> GCC 15's -Warray-bounds reports:
> 
> In function 'page_fixed_fake_head',
>     inlined from '_compound_head' at ../include/linux/page-flags.h:251:24,
>     inlined from '__dump_page' at ../mm/debug.c:123:11:
> ../include/asm-generic/rwonce.h:44:26: warning: array subscript 9 is outside array bounds of 'struct page[1]' [-Warray-bounds=]

Thanks for bringing this back up.  I have a somewhat orphaned patch in
my tree that has a terrible commit message which was no help.

That said, this patch is definitely wrong because it's unsafe to
call page_fixed_fake_head().

> (Not noted in this warning is that the code passes through page_folio()
> _Generic macro.)
> 
> It may not be that "precise" is always 1 page, so accessing "page[1]"
> in either page_folio() or folio_test_large() may cause problems.

folio_test_large() does not touch page[1].  Look:

static inline bool folio_test_large(const struct folio *folio)
{
        return folio_test_head(folio);

static __always_inline bool folio_test_head(const struct folio *folio)
{
        return test_bit(PG_head, const_folio_flags(folio, FOLIO_PF_ANY));

#define FOLIO_PF_ANY            0

static const unsigned long *const_folio_flags(const struct folio *folio,
                unsigned n)
{
        const struct page *page = &folio->page;

        VM_BUG_ON_PGFLAGS(PageTail(page), page);
        VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags), page);
        return &page[n].flags;

so we only look at page[0].

> Instead, explicitly make precise 2 pages. Just open-coding page_folio()
> isn't sufficient to avoid the warning[1].

Why not?  What goes wrong?  I'm trying to get gcc-15 installed here now
...