Hi Steffen, Ard,
On 14.08.23 21:59, Oliver Steffen wrote:
Quoting Vladimir 'phcoder' Serbinenko (2023-08-13 09:46:45)
Full analysis: gpt_partentry can be marked as aligned8. But following are
problem:
* protocols_per_handle may return unaligned guids
* configuration_tables array may be unaligned. On efi32 every entry is 20
bytes, so guids can't be 8-byte aligned
* The worst offender is device path as it's packed, unpredictable and contains
uuids.
* Efiemu gets guid as argument and probably should be able to handle unaligned
case. So far it's x86 only and we have no mmx and co so it's not a problem
right now unless we enable ubsan.
All in all we do need packed guid type unless those are resolved. I attach
patches for testing. If they work, I'll rearrange them a bit
Looking at https://uefi.org/specs/UEFI/2.10/02_Overview.html#data-types,
the problem is the alignment, not the packing.
As far as EFI is concerned, GUIDs are always 128bit in size. This means
there shall be no padding between the struct members. To be on the safe
side one could add the "packed" attribute to the struct, but the
original grub_efi_guid struct was not marked like that and it worked.
Whenever we had over GUIDs to EFI, they have to be aligned to 64bit.
Ard thinks that a 64 bit alignment for EFI GUIDs is a mistake - see [1]
and [2]. Hence Linux uses `__aligned(__alignof__(u32))` for "efi_guid_t"
since 2019.
[1]:
https://github.com/torvalds/linux/blob/master/include/linux/efi.h#L60-L78
[2]:
https://github.com/torvalds/linux/commit/494c704f9af0a0cddf593b381ea44320888733e6
I know I mentioned [1] earlier, but it sounds like it might have slipped
through and it might be a good idea to have both GRUB and Linux in sync
in this regard.
Cheers,
Frank
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