Re: [PATCH 4/8] mm: Add __dump_folio()

[Matthew Wilcox <willy@infradead.org>]

On Mon, May 13, 2024 at 09:33:57PM -0700, Kees Cook wrote:
> Hi!
> 
> While working on testing an improved -Warray-bounds in GCC, I encountered
> this, which seems to be reasonable:

Eek.  I think you're right.  This is a bad interaction between the page
dumping code and the fixed fake head code.  I will need to think about
this (and LSFMM is happening right now, so I don't necessarily have a
lot of time to think).  I'll get back to you as soon as I can.

> In file included from ./arch/x86/include/generated/asm/rwonce.h:1,
>                  from ../include/linux/compiler.h:299,
>                  from ../include/linux/array_size.h:5,
>                  from ../include/linux/kernel.h:16,
>                  from ../mm/debug.c:9:
> 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=]
>    44 | #define __READ_ONCE(x)  (*(const volatile __unqual_scalar_typeof(x) *)&(x))
>       |                         ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> ../include/asm-generic/rwonce.h:50:9: note: in expansion of macro '__READ_ONCE'
>    50 |         __READ_ONCE(x);                                                 \
>       |         ^~~~~~~~~~~
> ../include/linux/page-flags.h:226:38: note: in expansion of macro 'READ_ONCE'
>   226 |                 unsigned long head = READ_ONCE(page[1].compound_head);
>       |                                      ^~~~~~~~~
> ../mm/debug.c: In function '__dump_page':
> ../mm/debug.c:116:21: note: at offset 72 into object 'precise' of size 64
>   116 |         struct page precise;
>       |                     ^~~~~~~
> 
> (Not noted in this warning is that the code passes through page_folio()
> _Generic macro.)
> 
> It doesn't like that it can see that "precise" is exactly one page, so
> looking at page[1] later is going to freak out. I suspect this may be
> "impossible" at run-time, but I'm not 100% sure. Regardless, the compiler
> can't tell.
> 
> I suspect just making precise be a 2 page array would make this happy,
> but it wasn't clear to me how such a page should be initialized.
> 
> -Kees
> 
> --
> Kees Cook