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

From: Daniel Axtens
Subject: Re: [PATCH 04/19] mm: assert that we preserve header vs region alignment
Date: Wed, 23 Mar 2022 16:47:51 +1100

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

The problem is that grub_mm_{region,header}_t is a pointer type, not a
struct type. So sizeof (grub_mm_region_t) == sizeof(void *). You also
can't do sizeof (*grub_mm_region_t), because you can't dereference a
type. If there's a better way to express this I am open to it, but for
now I will stick with the struct type...

>> +                   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...

I have added that sizeof(struct header) == GRUB_MM_ALIGN. GRUB_MM_ALIGN
is supposed to align you to a cell, and a header is supposed to be 1

We probably _could_ do away with the constant but I think that requires
a bit more close thought.

Kind regards,

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