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

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

> On Tue, Jan 10, 2017 at 09:05:28AM -0800, Linus Torvalds wrote:
> >
> > I'm pretty sure we have random asm code that may not maintain a
> > 16-byte stack alignment when it calls other code (including, in some
> > cases, calling C code).
> >
> > So I'm not at all convinced that this is a good idea. We shouldn't
> > expect 16-byte alignment to be something trustworthy.
>
> So what if we audited all the x86 assembly code to fix this? Would
> it then be acceptable to do a 16-byte aligned stack?

Audits for small but deadly details that isn't checked automatically by tooling
would inevitably bitrot again - and in this particular case there's a 50% chance
that a new, buggy change would test out to be 'fine' on a kernel developer's own
box - and break on different configs, different hw or with unrelated (and
innocent) kernel changes, sometime later - spreading the pain unnecessarily.

So my feeling is that we really need improved tooling for this (and yes, the GCC
toolchain should have handled this correctly).

But fortunately we have related tooling in the kernel: could objtool handle this?
My secret hope was always that objtool would grow into a kind of life insurance
against toolchain bogosities (which is a must for things like livepatching or a
DWARF unwinder - but I digress).

Thanks,

Ingo

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