Re: KUBSan & alignment

[christos%astron.com@localhost (Christos Zoulas)]

In article <76d02b7c-6408-1836-b247-0b5951c8acc9%gmx.com@localhost>,
Kamil Rytarowski  <n54%gmx.com@localhost> wrote:
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>On 18.05.2019 17:21, Martin Husemann wrote:
>> On Fri, May 17, 2019 at 12:15:16PM -0500, David Young wrote:
>>> On Fri, May 17, 2019 at 05:19:40PM +0100, Patrick Welche wrote:
>>>> What should one do about
>>>>
>>>> UBSan: Undefined Behavior in
>../../../../external/bsd/acpica/dist/events/evregion.c:611:14, load of
>misaligned address 0xffffffff8302d4f3 for type 'const ACPI_NAME' which
>requires 4 byte alignment
>>>> UBSan: Undefined Behavior in
>../../../../external/bsd/acpica/dist/resources/rsaddr.c:331:22, member
>access within misaligned address 0xffffe967d71420e2 for type 'union
>AML_RESOURCE' which requires 4 byte alignment
>> 
>> Have not looked at those...
>> 
>>>> UBSan: Undefined Behavior in
>../../../../arch/x86/pci/pci_machdep.c:1134:15, member access within
>misaligned address 0xffffffff85a87c7c for type 'struct
>btinfo_framebuffer' which requires 8 byte alignment
>>>
>>> Supposing the authors really intended for the objects to be accessed in
>>> this way, label each declaration with __aligned(n) for some n?
>> 
>> Since this code is in pci *machdep* and the behaviour is well defined for
>> those CPUs, just ignore it (or fix UBSan).
>
>Strictly speaking it's still UB and a compiler is allowed to miscompile
>it. x86 is also sensitive to alignment in certain operations (movaps vs
>movups; stack alignment, ...).
>
>My 80386 manual says that misalignment operations are slower than
>alignment ones on x86.

I have already fixed it. And although misaligned access are slower than
aligned ones, the act of aligning them is even slower :-)

christos