[PATCH v2] efi: use 32-bit alignment for efi_guid_t literals

[PATCH v2] efi: use 32-bit alignment for efi_guid_t literals

[Ard Biesheuvel]

Commit 494c704f9af0 ("efi: Use 32-bit alignment for efi_guid_t") updated
the type definition of efi_guid_t to ensure that it always appears
sufficiently aligned (the UEFI spec is ambiguous about this, but given
the fact that its EFI_GUID type is defined in terms of a struct carrying
a uint32_t, the natural alignment is definitely >= 32 bits).

However, we missed the EFI_GUID() macro which is used to instantiate
efi_guid_t literals: that macro is still based on the guid_t type,
which does not have a minimum alignment at all. This results in warnings
such as

  In file included from drivers/firmware/efi/mokvar-table.c:35:
  include/linux/efi.h:1093:34: warning: passing 1-byte aligned argument to
      4-byte aligned parameter 2 of 'get_var' may result in an unaligned pointer
      access [-Walign-mismatch]
          status = get_var(L"SecureBoot", &EFI_GLOBAL_VARIABLE_GUID, NULL, &size,
                                          ^
  include/linux/efi.h:1101:24: warning: passing 1-byte aligned argument to
      4-byte aligned parameter 2 of 'get_var' may result in an unaligned pointer
      access [-Walign-mismatch]
          get_var(L"SetupMode", &EFI_GLOBAL_VARIABLE_GUID, NULL, &size, &setupmode);

The distinction only matters on CPUs that do not support misaligned loads
fully, but 32-bit ARM's load-multiple instructions fall into that category,
and these are likely to be emitted by the compiler that built the firmware
for loading word-aligned 128-bit GUIDs from memory

So re-implement the initializer in terms of our own efi_guid_t type, so that
the alignment becomes a property of the literal's type.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 include/linux/efi.h | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/linux/efi.h b/include/linux/efi.h
index 8710f5710c1d..6b5d36babfcc 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -72,8 +72,10 @@ typedef void *efi_handle_t;
  */
 typedef guid_t efi_guid_t __aligned(__alignof__(u32));
 
-#define EFI_GUID(a,b,c,d0,d1,d2,d3,d4,d5,d6,d7) \
-	GUID_INIT(a, b, c, d0, d1, d2, d3, d4, d5, d6, d7)
+#define EFI_GUID(a, b, c, d...) (efi_guid_t){ {					\
+	(a) & 0xff, ((a) >> 8) & 0xff, ((a) >> 16) & 0xff, ((a) >> 24) & 0xff,	\
+	(b) & 0xff, ((b) >> 8) & 0xff,						\
+	(c) & 0xff, ((c) >> 8) & 0xff, d } }
 
 /*
  * Generic EFI table header
-- 
2.30.2

Re: [PATCH v2] efi: use 32-bit alignment for efi_guid_t literals

[Nick Desaulniers]

On Thu, Mar 18, 2021 at 11:20 AM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> Commit 494c704f9af0 ("efi: Use 32-bit alignment for efi_guid_t") updated
> the type definition of efi_guid_t to ensure that it always appears
> sufficiently aligned (the UEFI spec is ambiguous about this, but given
> the fact that its EFI_GUID type is defined in terms of a struct carrying
> a uint32_t, the natural alignment is definitely >= 32 bits).
>
> However, we missed the EFI_GUID() macro which is used to instantiate
> efi_guid_t literals: that macro is still based on the guid_t type,
> which does not have a minimum alignment at all. This results in warnings
> such as
>
>   In file included from drivers/firmware/efi/mokvar-table.c:35:
>   include/linux/efi.h:1093:34: warning: passing 1-byte aligned argument to
>       4-byte aligned parameter 2 of 'get_var' may result in an unaligned pointer
>       access [-Walign-mismatch]
>           status = get_var(L"SecureBoot", &EFI_GLOBAL_VARIABLE_GUID, NULL, &size,
>                                           ^
>   include/linux/efi.h:1101:24: warning: passing 1-byte aligned argument to
>       4-byte aligned parameter 2 of 'get_var' may result in an unaligned pointer
>       access [-Walign-mismatch]
>           get_var(L"SetupMode", &EFI_GLOBAL_VARIABLE_GUID, NULL, &size, &setupmode);
>
> The distinction only matters on CPUs that do not support misaligned loads
> fully, but 32-bit ARM's load-multiple instructions fall into that category,
> and these are likely to be emitted by the compiler that built the firmware
> for loading word-aligned 128-bit GUIDs from memory
>
> So re-implement the initializer in terms of our own efi_guid_t type, so that
> the alignment becomes a property of the literal's type.
>
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Nice use of a variadic macro, and thanks for the patch.

494c704f9af0 landed in v5.1-rc1, would a Fixes: tag be appropriate?

Also, a tag like
Link: https://github.com/ClangBuiltLinux/linux/issues/1327
would help us track when/where this lands, in case the issue ever
comes up again, in branches of the stable tree for example.

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

It might be further possible to share more code between EFI_GUID and
GUID_INIT if the cast for the literals are factored out, but this
looks fine to me as is.  Too much macro nesting gets hard to follow
anyways.

> ---
>  include/linux/efi.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index 8710f5710c1d..6b5d36babfcc 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -72,8 +72,10 @@ typedef void *efi_handle_t;
>   */
>  typedef guid_t efi_guid_t __aligned(__alignof__(u32));
>
> -#define EFI_GUID(a,b,c,d0,d1,d2,d3,d4,d5,d6,d7) \
> -       GUID_INIT(a, b, c, d0, d1, d2, d3, d4, d5, d6, d7)
> +#define EFI_GUID(a, b, c, d...) (efi_guid_t){ {                                        \
> +       (a) & 0xff, ((a) >> 8) & 0xff, ((a) >> 16) & 0xff, ((a) >> 24) & 0xff,  \
> +       (b) & 0xff, ((b) >> 8) & 0xff,                                          \
> +       (c) & 0xff, ((c) >> 8) & 0xff, d } }
>
>  /*
>   * Generic EFI table header
> --
> 2.30.2
>


-- 
Thanks,
~Nick Desaulniers

Re: [PATCH v2] efi: use 32-bit alignment for efi_guid_t literals

[Nathan Chancellor]

On Thu, Mar 18, 2021 at 07:20:46PM +0100, Ard Biesheuvel wrote:
> Commit 494c704f9af0 ("efi: Use 32-bit alignment for efi_guid_t") updated
> the type definition of efi_guid_t to ensure that it always appears
> sufficiently aligned (the UEFI spec is ambiguous about this, but given
> the fact that its EFI_GUID type is defined in terms of a struct carrying
> a uint32_t, the natural alignment is definitely >= 32 bits).
> 
> However, we missed the EFI_GUID() macro which is used to instantiate
> efi_guid_t literals: that macro is still based on the guid_t type,
> which does not have a minimum alignment at all. This results in warnings
> such as
> 
>   In file included from drivers/firmware/efi/mokvar-table.c:35:
>   include/linux/efi.h:1093:34: warning: passing 1-byte aligned argument to
>       4-byte aligned parameter 2 of 'get_var' may result in an unaligned pointer
>       access [-Walign-mismatch]
>           status = get_var(L"SecureBoot", &EFI_GLOBAL_VARIABLE_GUID, NULL, &size,
>                                           ^
>   include/linux/efi.h:1101:24: warning: passing 1-byte aligned argument to
>       4-byte aligned parameter 2 of 'get_var' may result in an unaligned pointer
>       access [-Walign-mismatch]
>           get_var(L"SetupMode", &EFI_GLOBAL_VARIABLE_GUID, NULL, &size, &setupmode);
> 
> The distinction only matters on CPUs that do not support misaligned loads
> fully, but 32-bit ARM's load-multiple instructions fall into that category,
> and these are likely to be emitted by the compiler that built the firmware
> for loading word-aligned 128-bit GUIDs from memory
> 
> So re-implement the initializer in terms of our own efi_guid_t type, so that
> the alignment becomes a property of the literal's type.
> 
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  include/linux/efi.h | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/efi.h b/include/linux/efi.h
> index 8710f5710c1d..6b5d36babfcc 100644
> --- a/include/linux/efi.h
> +++ b/include/linux/efi.h
> @@ -72,8 +72,10 @@ typedef void *efi_handle_t;
>   */
>  typedef guid_t efi_guid_t __aligned(__alignof__(u32));
>  
> -#define EFI_GUID(a,b,c,d0,d1,d2,d3,d4,d5,d6,d7) \
> -	GUID_INIT(a, b, c, d0, d1, d2, d3, d4, d5, d6, d7)
> +#define EFI_GUID(a, b, c, d...) (efi_guid_t){ {					\
> +	(a) & 0xff, ((a) >> 8) & 0xff, ((a) >> 16) & 0xff, ((a) >> 24) & 0xff,	\
> +	(b) & 0xff, ((b) >> 8) & 0xff,						\
> +	(c) & 0xff, ((c) >> 8) & 0xff, d } }
>  
>  /*
>   * Generic EFI table header
> -- 
> 2.30.2
>