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

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

On Thu, Apr 20, 2023 at 09:35:42AM +0300, Ilias Apalodimas wrote:
> Akashi-san
> 
> On Fri, 7 Apr 2023 at 10:33, Ilias Apalodimas
> <ilias.apalodimas@linaro.org> wrote:
> >
> > On Fri, 7 Apr 2023 at 04:46, AKASHI Takahiro <takahiro.akashi@linaro.org> wrote:
> > >
> > > 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.
> >
> > I assumed that all other definitions are aligned regardless of packed,
> > i.e they are defined right after a u32, u64, 2xu16 etc, but I'll have
> > a closer look
> 
> So I did look closer and my assumption was indeed correct.
> IOW the warning is the only place in the struct definition where
> efi_guid_t happens to be be aligned.

My concern is that we use char[] in one place and efi_guid_t elsewhere.
It sounds arbitrary without any clear explanation.

-Takahiro Akashi


> Tom would you like me to send a v2 on this?
> I think what happens here is that struct efi_hii_keyboard_layout is
> defined as packed, and efi_guid_t is aligned(4).
> So clang is trying to tell us: I will generate safe code for unaligned
> accesses, but one of the struct members requires specific alignment.
> 
> Regards
> /Ilias
> >
> > >
> > > > 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.)
> >
> > commit 1dd705cf9903 ("efi: use 32-bit alignment for efi_guid_t")
> > explains why, but it's a bit orthogonal to this commit.  In any case
> > I'll include it in v2
> >
> > Thanks
> > /Ilias
> > >
> > > -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
> > > >