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

Josh Poimboeuf
2015-06-10T19:51:49Z

On Wed, Jun 10, 2015 at 12:38:32PM -0700, Andy Lutomirski wrote:
> 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?)

I don't think it does. We'll probably need some intelligence in the
CFI generation tooling to deal properly with the extable stuff.

> >> > 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.

Well, I wasn't talking specifically about tail calls here. But either
way, as long as they're not in a callable function (which is the case
for most of the entry code), asmvalidate doesn't care.

--
Josh