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

* Herbert Xu [off-list ref] wrote:

quoted

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