[PATCH v2] efi_loader: Fix warnings for unaligned accesses

[Heinrich Schuchardt]

On 5/11/23 18:40, 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 and thus has 1-byte alignment but efi_guid_t is a type that
> requires greater alignment than that.
>
> However the EFI spec describes the EFI_GUID as
> "128-bit buffer containing a unique identifier value.
> Unless otherwise specified"
>
> So convert the efi_guid_t -> u8 b[16] here and skip the alignment
> requirements.  Since the struct is packed to begin with, it makes no
> difference on the final memory layout.
>
> Suggested-by: Heinrich Schuchardt <heinrich.schuchardt at canonical.com>
> Reported-by: Tom Rini <trini at konsulko.com>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> ---
> Changes since v1:
> - Adjust the commit message and add a comment on why this happens
>
>   include/efi_api.h | 28 +++++++++++++++++++++++++++-
>   1 file changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/include/efi_api.h b/include/efi_api.h
> index 2fd0221c1c77..55a4c989fc7c 100644
> --- a/include/efi_api.h
> +++ b/include/efi_api.h
> @@ -1170,7 +1170,33 @@ struct efi_key_descriptor {
>
>   struct efi_hii_keyboard_layout {
>   	u16 layout_length;
> -	efi_guid_t guid;
> +	/*
> +	 * The EFI spec defines this as efi_guid_t.
> +	 * clang and gcc both report alignment problems here.
> +	 * clang with -Wunaligned-access
> +	 * 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
> +	 *
> +	 * GCC with -Wpacked-not-aligned -Waddress-of-packed-member
> +	 * 'efi_guid_t' offset 2 in 'struct efi_hii_keyboard_layout'
> +	 * isn't aligned to 4
> +	 *
> +	 * Removing the alignment from efi_guid_t is not an option, since
> +	 * it is also used in non-packed structs and that would break
> +	 * calculations with offsetof
> +	 *
> +	 * This is the only place we get a report for. That happens because
> +	 * all other declarations of efi_guid_t within a packed struct happens
> +	 * to be 4-byte aligned.  i.e a u32, a u64 a 2 * u16 or any combination
> +	 * that ends up landing efi_guid_t on a 4byte boundary precedes.
> +	 *
> +	 * Replace this with a 1-byte aligned counterpart of b[16].  This is a
> +	 * packed struct so the memory  placement of efi_guid_t should not change
> +	 *
> +	 */
> +	u8 guid[16];
>   	u32 layout_descriptor_string_offset;
>   	u8 descriptor_count;
>   	/* struct efi_key_descriptor descriptors[]; follows here */
> --
> 2.39.2
>

Thank you for investigating this in depth.

Commit messages should preferably limited to 75 characters per line, see
scripts/checkpatch.pl. No need to resubmit.

Reviewed-by: Heinrich Schuchardt <xypron.glpk at gmx.de>