Re: [PATCH 2/2] efi_loader: Fix warnings on unaligned accesses

[AKASHI Takahiro <takahiro.akashi@linaro.org>]

Hi Ilias,

On Thu, Apr 06, 2023 at 10:37:07PM +0300, Ilias Apalodimas wrote:
> Tom reports that when building with clang we see this warning:
> field guid within 'struct efi_hii_keyboard_layout' is less aligned than 'efi_guid_t' and is usually due to 'struct efi_hii_keyboard_layout' being packed, which can lead to unaligned accesses [-Wunaligned-access]
> 
> This happens because 'struct efi_hii_keyboard_layout' is defined as
> packed while efi_guid_t is 32bit aligned.

There are a couple of 'struct' definitions which are *packed*
and contain an 'efi_guid_t' member in efi_api.h.
If 'efi_hii_keyboard_layout' is the only place that causes a clang warning,
we need a more specific explanation to clarify the problem.

> However the EFI spec describes the EFI_GUID as
> "128-bit buffer containing a unique identifier value.
> Unless otherwise specified aligned on a 64-bit boundary"

That's right, but this text in this context may sound misleading.
(It doesn't explain why 'efi_guid_t' is 32-bit aligned.)

-Takahiro Akashi

> 
> So convert the efi_guid_t -> u8 b[16] here and skip the alignment
> requirements.
> 
> Reported-by: Tom Rini <trini@konsulko.com>
> Suggested-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> ---
>  include/efi_api.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/efi_api.h b/include/efi_api.h
> index 2fd0221c1c77..b84b577bd7b5 100644
> --- a/include/efi_api.h
> +++ b/include/efi_api.h
> @@ -1170,7 +1170,7 @@ struct efi_key_descriptor {
>  
>  struct efi_hii_keyboard_layout {
>  	u16 layout_length;
> -	efi_guid_t guid;
> +	u8 guid[16];
>  	u32 layout_descriptor_string_offset;
>  	u8 descriptor_count;
>  	/* struct efi_key_descriptor descriptors[]; follows here */
> -- 
> 2.39.2
>