[U-Boot] [PATCH v0 21/20] efi_loader: hack for archs that cannot do unaligned accesses

[Rob Clark]

On Tue, Aug 8, 2017 at 7:32 AM, Leif Lindholm <leif.lindholm at linaro.org> wrote:
> On Tue, Aug 08, 2017 at 09:11:14AM +0100, Ard Biesheuvel wrote:
>> On 8 August 2017 at 07:52, Alexander Graf <agraf at suse.de> wrote:
>> >
>> >
>> >> Am 07.08.2017 um 23:18 schrieb Rob Clark <robdclark at gmail.com>:
>> >>
>> >> This is problematic around file nodes in the device path.  Adding the
>> >> padding bytes to the end of each device-path struct would "solve"
>> >> that, and if pre-aarch64 we are aiming at "good enough to work", I
>> >> kinda think that this the approach we should go for.  Other than
>> >> file-path nodes, the rest of the issues in u-boot should be solved by
>> >> addition of missing __packed on 'struct efi_device_path' (which I've
>> >> added locally and will be in the next revision of the patchset).
>> >
>> > Let's ask Leif and Ard if that is actually correct. If I remember
>> > correctly, edk2 some times leverages automatic padding from the
>> > compiler on structs.
>>
>> I guess that that might work, if U-boot is the only agent
>> instantiating device path structures. But what do you mean by
>> 'correct' in this context?
>
> Well, if that is the case, are we risking the ability to in the future
> support loading drivers or protocols at runtime. (This would for
> example exclude Shim compatibility or running the UEFI Shell.)
>

My proposal (and this is only for <=armv6 and armv7 until someone gets
around to enabling mmu and disabling alignment faults) is to add
padding bytes at the end of the various device-path structs to at
least keep the structs (and things like utf16 string in file-path
struct) aligned, and rely on efi payload and u-boot to be compiled
with -mno-unaligned-access if it needs to access fields within the
device-path structs.

This is *essentially* what u-boot does implicitly at the moment (by
missing __packed attribute on certain structs).  I want to fix that on
aarch64, but without the padding bytes it causes a some unaligned
accesses in u-boot on armv7 devices.

I think the goal for armv7 is more to have enough uefi for grub and
OpenBSD's bootloader.  If fancy things like loading drivers and
protocols at runtime doesn't work, well these didn't work before so I
won't loose much sleep.  But I would like that to work properly on
aarch64.

BR,
-R