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

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Message-ID: <b5066ff8-9bfc-635f-a4fd-2ed3e20271b6@redhat.com>
Date:   Thu, 23 Jan 2020 11:45:58 +0000
From:   Julien Thierry <jthierry@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 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