Re: [RFC v5 11/57] objtool: Abstract alternative special case handling

[Julien Thierry]

On 1/20/20 2:54 PM, Peter Zijlstra wrote:
> On Thu, Jan 09, 2020 at 04:02:14PM +0000, Julien Thierry wrote:
> > diff --git a/tools/objtool/arch/x86/arch_special.c b/tools/objtool/arch/x86/arch_special.c
> > new file mode 100644
> > index 000000000000..6dba31f419d0
> > --- /dev/null
> > +++ b/tools/objtool/arch/x86/arch_special.c
> > @@ -0,0 +1,34 @@
> > +// SPDX-License-Identifier: GPL-2.0-or-later
> > +#include "../../special.h"
> > +#include "../../builtin.h"
> > +
> > +void arch_handle_alternative(unsigned short feature, struct special_alt *alt)
> > +{
> > +	/*
> > +	 * If UACCESS validation is enabled; force that alternative;
> > +	 * otherwise force it the other way.
> > +	 *
> > +	 * What we want to avoid is having both the original and the
> > +	 * alternative code flow at the same time, in that case we can
> > +	 * find paths that see the STAC but take the NOP instead of
> > +	 * CLAC and the other way around.
> > +	 */
>
> That comment ^,
>
> > +	switch (feature) {
> > +	case X86_FEATURE_SMAP:
>
> goes here >

Good catch, I'll fix that.

> > +		if (uaccess)
> > +			alt->skip_orig = true;
> > +		else
> > +			alt->skip_alt = true;
> > +		break;
>
> > +	case X86_FEATURE_POPCNT:
> > +		/*
> > +		 * It has been requested that we don't validate the !POPCNT
> > +		 * feature path which is a "very very small percentage of
> > +		 * machines".
> > +		 */
> > +		alt->skip_orig = true;
> > +		break;
> > +	default:
> > +		break;
> > +	}
> > +}

Thanks,

--
Julien Thierry