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

Josh Poimboeuf
2015-06-10T14:32:21Z

On Wed, Jun 10, 2015 at 04:11:04PM +0200, Andi Kleen wrote:
> > In most cases there are ways to keep the optimizations. For example:
> >
> > - grow the function bounds to keep the jump internal
>
> So you mean moving it after the ret? That still means icache bloat.

No, in most cases it just means changing the ELF annotations. See patch
9 for an example.

> > - 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?

> > Also note that these rules only affect _callable_ functions, so the
> > entry code and other non-function asm code can still be a pile of
> > spaghetti (though I think Andy is working on improving that).
>
> Thank you for your kind words.

Don't like spaghetti? :-)

> > > In fact even gcc with the right options can generate code that violates
> > > this. Standard Linux constructions, such as exception handling,
> > > also violate this.
> > >
> > > If your tool needs that your tool is broken.
> >
> > This tool only validates asm code, so I don't see how whatever gcc does
> > is relevant.
>
> Whoever needs it would need it everywhere, right? If it's not needed
> for gcc then it shouldn't be needed for assembler code either.

Well, I don't see how that's really a logical conclusion. But we're
probably being too vague here... Do you have any examples where you
really need to jump outside of a callable function?

If we ignore C++, then 99% of the time, C functions are self-contained.
The only exception I can think of is for switch statements, which
sometimes have an external jump table.

--
Josh