[PATCH v2] efi_loader: Fix warnings for unaligned accesses

[Ilias Apalodimas]

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