Re: [PATCH v5 00/10] x86/asm: Compile-time asm code validation

Andy Lutomirski
2015-06-10T19:39:02Z

On Wed, Jun 10, 2015 at 12:36 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> On Wed, Jun 10, 2015 at 11:40:06AM -0700, Andi Kleen wrote:
>> Josh Poimboeuf <jpoimboe@redhat.com> writes:
>>
>> > On Wed, Jun 10, 2015 at 05:04:12PM +0200, Andi Kleen wrote:
>> >> > > > - duplicate the destination code inside the function
>> >> > > > - convert the jump to a call
>> >> > >
>> >> > > That all won't work for a lot of cases.
>> >> >
>> >> > Hm, could you give an example?
>> >>
>> >> Just a standard *_user exception handler.
>> >
>> > I'm afraid I don't follow. Exception handlers don't work via jump
>> > instructions, but rather via CPU exceptions.
>> >
>> > Or are you talking about something else?
>>
>> Let's take an example:
>>
>> 102:
>> .section .fixup,"ax"
>> 103: addl %ecx,%edx /* ecx is zerorest also */
>> jmp copy_user_handle_tail
>> .previous
>>
>> _ASM_EXTABLE(100b,103b)
>> _ASM_EXTABLE(101b,103b)
>>
>> The exception handling code is part of the function, but it's out of line.
>
> The jump instruction is in the .fixup section, not in the callable
> function itself. So it doesn't violate the asmvalidate rules.

It still won't unwind correctly unless .pushsection somehow magically
propagates CFI state. (Does it?)

>
>> > Are you suggesting that we implement this gcc optimization in kernel asm
>> > code?
>>
>> It was how Linux traditionally implemented locking code for example.
>> Have the hot path handle the uncontended fast path, and the slow path
>> call.
>>
>> I don't know if there is much left of it (a lot of it was removed because
>> it was hard to describe in dwarf3, needs dwarf4). But it seems bad
>> to completely disallow it.
>>
>> But yes eventually gcc generated code should use it again, because it's
>> great for icache usage if you measure it correctly at run time
>> (not the broken "size" approach that is unfortunately far too common)
>
> This patch set has no relationship to gcc generated code whatsoever. So
> it doesn't disallow anything there.
>
> For kernel asm code, AFAIK, such a mechanism for hot/cold path
> separation in separate sections doesn't exist today. So it's not
> "disallowed" there either. It's just apparently not currently done.
>
> If somebody were to create such a mechanism, I think we could
> standardize it in such a way that it could be compatible with
> asmvalidate.

Hopefully true. The entry code is full of tail calls, though.

--Andy

>
> --
> Josh

--
Andy Lutomirski
AMA Capital Management, LLC