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

On Wed, Jan 11, 2017 at 11:05 PM, Herbert Xu
[off-list ref] wrote:

On Tue, Jan 10, 2017 at 09:05:28AM -0800, Linus Torvalds wrote:

quoted

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?

On the face of it it doesn't seem to be a huge amount of code
assuming they mostly live under arch/x86.

The problem is that we have nasties like TRACE_IRQS_OFF.  Performance
doesn't really matter for these macros, so we could probably rig up a
helper for forcibly align the stack there.  Maybe
FRAME_BEGIN_FORCE_ALIGN?  I also think I'd rather not to modify
pt_regs.  We should just fix the small number of code paths that
create a pt_regs and then call into C code to align the stack.

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.

--Andy