Re: [PATCH] efi: stub: call get_memory_map() to obtain map and desc sizes

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

Hi Ard,

On Thu, Jan 08, 2015 at 05:51:47PM +0000, Ard Biesheuvel wrote:
> This fixes two minor issues in the implementation of get_memory_map():
> - Currently, it assumes that sizeof(efi_memory_desc_t) == desc_size,
>   which is usually true, but not mandated by the spec. (This was added
>   intentionally to allow future additions to the definition of
>   efi_memory_desc_t). The way the loop is implemented currently, the
>   added slack space may be insufficient if desc_size is larger, which in
>   some corner cases could result in the loop never terminating.
> - It allocates 32 efi_memory_desc_t entries first (again, using the size
>   of the struct instead of desc_size), and frees and reallocates if it
>   turns out to be insufficient. Few implementations of UEFI have such small
>   memory maps, which results in a unnecessary allocate/free pair on each
>   invocation.
> 
> Fix this by calling the get_memory_map() boot service first with a '0'
> input value for map size to retrieve the map size and desc size from the
> firmware and only then perform the allocation, using desc_size rather
> than sizeof(efi_memory_desc_t).

Is the desc_size guaranteed to be set up correctly if the size is too
small?

As far as I can see, for that case the spec only mandates that
MemoryMapSize is updated nd EFI_BUFFER_TOO_SMALL is returned.

It's not clear to me whether DescriptorSize or DescriptorVersion are
initialised in cases other than success.

Thanks,
Mark.

> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> ---
>  drivers/firmware/efi/libstub/efi-stub-helper.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
> index e766df60fbfb..caf91eab0bfc 100644
> --- a/drivers/firmware/efi/libstub/efi-stub-helper.c
> +++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
> @@ -75,25 +75,29 @@ efi_status_t efi_get_memory_map(efi_system_table_t *sys_table_arg,
>  	unsigned long key;
>  	u32 desc_version;
>  
> -	*map_size = sizeof(*m) * 32;
> -again:
> +	*map_size = 0;
> +	*desc_size = 0;
> +	key = 0;
> +	status = efi_call_early(get_memory_map, map_size, NULL,
> +				&key, desc_size, &desc_version);
> +	if (status != EFI_BUFFER_TOO_SMALL)
> +		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.
>  	 */
> -	*map_size += sizeof(*m);
> +	*map_size += *desc_size;
>  	status = efi_call_early(allocate_pool, EFI_LOADER_DATA,
>  				*map_size, (void **)&m);
>  	if (status != EFI_SUCCESS)
>  		goto fail;
>  
> -	*desc_size = 0;
> -	key = 0;
>  	status = efi_call_early(get_memory_map, map_size, m,
>  				&key, desc_size, &desc_version);
>  	if (status == EFI_BUFFER_TOO_SMALL) {
>  		efi_call_early(free_pool, m);
> -		goto again;
> +		return EFI_LOAD_ERROR;
>  	}
>  
>  	if (status != EFI_SUCCESS)
> -- 
> 1.8.3.2
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-efi" in
> the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>