Re: [PATCH 04/19] mm: assert that we preserve header vs region alignment

[Daniel Kiper]

On Tue, Oct 12, 2021 at 06:29:53PM +1100, Daniel Axtens wrote:
> grub_mm_region_init() does:
>
>   h = (grub_mm_header_t) (r + 1);
>
> where h is a grub_mm_header_t and r is a grub_mm_region_t.
>
> Cells are supposed to be GRUB_MM_ALIGN aligned, but while grub_mm_dump
> ensures this vs the region header, grub_mm_region_init() does not.
>
> It's better to be explicit than implicit here: rather than changing
> grub_mm_region_init() to ALIGN_UP(), require that the struct is
> explictly a multiple of the header size.
>
> Signed-off-by: Daniel Axtens <dja@axtens.net>
> ---
>  include/grub/mm_private.h | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/include/grub/mm_private.h b/include/grub/mm_private.h
> index e80a059dd4e4..533b47173e18 100644
> --- a/include/grub/mm_private.h
> +++ b/include/grub/mm_private.h
> @@ -20,6 +20,7 @@
>  #define GRUB_MM_PRIVATE_H    1
>
>  #include <grub/mm.h>
> +#include <grub/misc.h>
>
>  /* Magic words.  */
>  #define GRUB_MM_FREE_MAGIC   0x2d3c2808
> @@ -82,4 +83,11 @@ typedef struct grub_mm_region
>  extern grub_mm_region_t EXPORT_VAR (grub_mm_base);
>  #endif
>
> +static inline void grub_mm_size_sanity_check(void) {

static inline void
grub_mm_size_sanity_check (void)
{

> +  /* Ensure we preserve alignment when doing h = (grub_mm_header_t) (r + 1) 
> */
> +  COMPILE_TIME_ASSERT((sizeof(struct grub_mm_region) %

Missing spaces before "(" above and below...

s/struct grub_mm_region/grub_mm_region_t/
s/struct grub_mm_header/grub_mm_header_t/

> +                    sizeof(struct grub_mm_header)) == 0);

I think this check is insufficient. The GRUB_MM_ALIGN should be checked
somehow too. E.g. if sizeof(struct grub_mm_region) == 16,
sizeof(struct grub_mm_header) == 8 and GRUB_MM_ALIGN == 32 then assert
above will pass but the GRUB will blow up later. Of course numbers are not
real but I chose them to show the problem. Hmmm... If we could choose
grub_mm_region_t and grub_mm_header_t sizes properly then probably we
could drop GRUB_MM_ALIGN...

Daniel