Re: [PATCH] efi: get_memory_map: add sufficient slack for memory descriptors

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

On Thu, Feb 12, 2015 at 05:24:19AM +0000, Ard Biesheuvel wrote:
> As it turns out, when allocating room for the UEFI memory map using
> UEFI's AllocatePool (), it may result in two new memory map entries
> being created, for instance, when using Tianocore's preallocated region
> feature. For example, the following region
> 
> 0x00005ead5000-0x00005ebfffff [Conventional Memory|   |  |  |  |   |WB|WT|WC|UC]
> 
> may be split like this
> 
> 0x00005ead5000-0x00005eae2fff [Conventional Memory|   |  |  |  |   |WB|WT|WC|UC]
> 0x00005eae3000-0x00005eae4fff [Loader Data        |   |  |  |  |   |WB|WT|WC|UC]
> 0x00005eae5000-0x00005ebfffff [Conventional Memory|   |  |  |  |   |WB|WT|WC|UC]
> 
> if the preallocated Loader Data region was chosen to be right in the
> middle of the original free space.
> 
> After patch d1a8d66b9177 ("efi/libstub: Call get_memory_map() to
> obtain map and desc sizes"), this is not being dealt with correctly
> anymore, as the existing logic to allocate room for a single additional
> entry has become insufficient.
> 
> So instead, add room for two additional entries instead.
> 
> Fixes: d1a8d66b9177 ("efi/libstub: Call get_memory_map() to obtain map and desc sizes")
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> ---
>  drivers/firmware/efi/libstub/efi-stub-helper.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
> index af5d63c7cc53..ca0b07ed3b14 100644
> --- a/drivers/firmware/efi/libstub/efi-stub-helper.c
> +++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
> @@ -84,10 +84,10 @@ efi_status_t efi_get_memory_map(efi_system_table_t *sys_table_arg,
>  		return EFI_LOAD_ERROR;
>  
>  	/*
> -	 * Add an additional efi_memory_desc_t because we're doing an
> -	 * allocation which may be in a new descriptor region.
> +	 * Add room for two additional efi_memory_desc_t entries because we're
> +	 * doing an allocation which may be in a new descriptor region.

It might be worth noting that a existing regions can be
split/reorganised here, otherwise it's a little difficult to deduce from
the comment why to regions are needed.

>  	 */
> -	*map_size += *desc_size;
> +	*map_size += *desc_size * 2;

Can we forsee any cases where we might need more than two additional
descs? Is it perhaps adding a little more slack now?

Otherwise this looks fine to me.

Thanks,
Mark.

>  	status = efi_call_early(allocate_pool, EFI_LOADER_DATA,
>  				*map_size, (void **)&m);
>  	if (status != EFI_SUCCESS)
> -- 
> 1.8.3.2
> 
>