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

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