Re: [PATCH 1/4] efi/libstub: Allocate headspace in efi_get_memory_map()

[Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>]

Hi,

On Sun, Jul 17, 2016 at 02:45:59PM -0600, Jeffrey Hugo wrote:
> +	if (status == EFI_BUFFER_TOO_SMALL ||
> +				(*buff_size - *map_size) / sizeof(*m) < 8) {
>  		efi_call_early(free_pool, m);
> +		/*
> +		 * Make sure there is 8 entries worth of headroom so that the
> +		 * buffer can be reused for a new map after allocations are
> +		 * no longer permitted.  Its unlikely that the map will grow to
> +		 * exceed this headroom once we are ready to trigger
> +		 * ExitBootServices()
> +		 */
> +		*map_size += sizeof(*m) * 8;

It's probably worth having something like...

  #define EFI_MMAP_NR_SLACK_SLOTS	8

... at the top of the file, and changing the comment here to not refer
to the specific value. That way we avoid duplicating the value, and
making it easier to modify / backport in future.

As a general note, it's a shame that we don't know desc_size first time
through. If descs grow, 8 * sizeof(*m) might be too small. That's an
existing problem though, so I guess it makes sense to be consistent and
use sizeof(*m) here.

Otherwise, this looks fine to me.

Thanks,
Mark.