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

[David Woodhouse]

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?

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