Re: [PATCH 2/2] x86/speculation: Simplify indirect_branch_prediction_barrier()

[Josh Poimboeuf]

On Tue, Feb 06, 2018 at 11:31:18PM +0000, David Woodhouse wrote:
> 
> 
> On Tue, 2018-02-06 at 17:25 -0600, Josh Poimboeuf wrote:
> > On Tue, Feb 06, 2018 at 07:44:52PM +0000, David Woodhouse wrote:
> > > 
> > > On Fri, 2018-01-26 at 21:08 +0100, Borislav Petkov wrote:
> > > > 
> > > > Make it all a function which does the WRMSR instead of having a hairy
> > > > inline asm.
> > > ...
> > > 
> > > > 
> > > > +	alternative_input("",
> > > > +			Â"call __ibp_barrier",
> > > > +			ÂX86_FEATURE_IBPB,
> > > > +			ÂASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));
> > > > Â}
> > > Dammit. I know the best time to comment is *before* I add my own sign-
> > > off to it and before Linus has merged it but... I think this is broken.
> > > 
> > > If you're calling a C function then you have to mark *all* the call-
> > > clobbered registers as, well, clobbered.
> > > 
> > > If you really really really want to *call* something out of line, then
> > > it would need to be implemented in asm.
> >
> > Hm.ÂÂIn theory I agree this seems like a bug.ÂÂOn x86_64 I believe we
> > would need to mark the following registers as clobbered: r8-r11, ax, cx,
> > dx, si, di, plus "memory" and "cc".
> > 
> > But I'm scratching my head a bit, because we seem to have this bug all
> > over the kernel.ÂÂ(Grep for ASM_CALL_CONSTRAINT to see them.)
> > 
> > Many of those inline asm calls have been around a long time.ÂÂSo why
> > hasn't it ever bitten us?
> 
> How many are actually calling C functions, not asm or other special
> cases like firmware entry points?

I think many, and maybe even most, are calling normal C functions.

-- 
Josh