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

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Date:   Mon, 20 Jan 2020 15:54:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Julien Thierry <jthierry@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        jpoimboe@...hat.com, raphael.gault@....com,
        catalin.marinas@....com, will@...nel.org
Subject: Re: [RFC v5 11/57] objtool: Abstract alternative special case
 handling

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 >

> +		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;
> +	}
> +}