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SubjectRe: [PATCH 2/2] x86/speculation: Simplify indirect_branch_prediction_barrier()
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("",
> > +  "call __ibp_barrier",
> > +  X86_FEATURE_IBPB,
> > +  ASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));
> >  }
>
> 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. In theory I agree this seems like a bug. On 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. (Grep for ASM_CALL_CONSTRAINT to see them.)

Many of those inline asm calls have been around a long time. So why
hasn't it ever bitten us?

--
Josh

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