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

[Ard Biesheuvel <ardb@kernel.org>]

On Wed, 10 Mar 2021 at 23:21, Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Wed, Mar 10, 2021 at 09:12:10AM +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
> >
> > Instead of bodging this further, let's simply switch to our own definition
> > of efi_guid_t that carries a uint32_t as well. Since efi_guid_t is used as
> > an opaque type everywhere in the EFI code, this is only a minor code change.
> >
> > Reported-by: Nathan Chancellor <nathan@kernel.org>
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>
> I ran this through my series of 32-bit and 64-bit x86 builds and I did
> not see any additional warnings added because of it.
>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> Tested-by: Nathan Chancellor <nathan@kernel.org>
>

Thanks all, but I am going to drop these, as I have decided to fix it
in a different way after all.


> > ---
> >
> > I am currently testing this change via my for-kernelci branch. Please give
> > this some soak time in the other CIs that we have access to.
> >
> >  include/linux/efi.h | 15 ++++++++++-----
> >  1 file changed, 10 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/efi.h b/include/linux/efi.h
> > index 8710f5710c1d..f39e9ec7485f 100644
> > --- a/include/linux/efi.h
> > +++ b/include/linux/efi.h
> > @@ -63,17 +63,22 @@ typedef void *efi_handle_t;
> >   * is 32 bits not 8 bits like our guid_t. In some cases (i.e., on 32-bit ARM),
> >   * this means that firmware services invoked by the kernel may assume that
> >   * efi_guid_t* arguments are 32-bit aligned, and use memory accessors that
> > - * do not tolerate misalignment. So let's set the minimum alignment to 32 bits.
> > + * do not tolerate misalignment.
> >   *
> >   * Note that the UEFI spec as well as some comments in the EDK2 code base
> >   * suggest that EFI_GUID should be 64-bit aligned, but this appears to be
> >   * a mistake, given that no code seems to exist that actually enforces that
> >   * or relies on it.
> >   */
> > -typedef guid_t efi_guid_t __aligned(__alignof__(u32));
> > +typedef struct {
> > +     u32     a;
> > +     u16     b;
> > +     u16     c;
> > +     u8      d[8];
> > +} efi_guid_t;
> >
> >  #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)
> > +     (efi_guid_t){ a, b, c, { d0,d1,d2,d3,d4,d5,d6,d7 }}
> >
> >  /*
> >   * Generic EFI table header
> > @@ -598,8 +603,8 @@ efi_guidcmp (efi_guid_t left, efi_guid_t right)
> >  static inline char *
> >  efi_guid_to_str(efi_guid_t *guid, char *out)
> >  {
> > -     sprintf(out, "%pUl", guid->b);
> > -        return out;
> > +     sprintf(out, "%pUl", guid);
> > +     return out;
> >  }
> >
> >  extern void efi_init (void);
> > --
> > 2.30.1
> >

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