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SubjectRe: x86-64: Maintain 16-byte stack alignment

* Herbert Xu <herbert@gondor.apana.org.au> wrote:

> > But if we can't do this with automatic verification, then I'm not sure
> > I want to do it at all. The asm is already more precarious than I'd
> > like, and having a code path that is misaligned is asking for obscure
> > bugs down the road.
>
> I understand the need for automated checks at this point in time.
> But longer term this is just part of the calling ABI. After all,
> we don't add checks everywhere to ensure people preserve rbx.

The intelligent and responsible way to introduce such post facto ABI changes is
via a smarter assembler: which would detect the obvious cases where assembly code
generates a misaligned stack, at build time.

Assembly code can obviously still mess up in a hard to detect fashion if it tries
- but that's OK, as most assembly code doesn't try to go outside regular stack
allocation patterns.

Such a static check is relatively straightforward to do in assembly tooling - and
perhaps objtool could do this too, as it already tracks the instructions that
change the stack offset.

( And yes, this is what the GCC guys should have done, instead of sloppily
introducing such silent breakages and making the whole application landscape
less robust ... )

Thanks,

Ingo

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