Re: [PATCH 6/8] i915,uaccess: Fix redundant CLAC

From: Peter Zijlstra
Date: Thu Feb 28 2019 - 14:01:29 EST

On Thu, Feb 28, 2019 at 10:29:25AM -0800, Linus Torvalds wrote:
> On Thu, Feb 28, 2019 at 10:02 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > Weird, that jump is from C, not from a .fixup table. objtool _should_
> > see that and complain if there is a AC=1 path that reaches RET.
> No, unsafe_put_user() actually does the "asm goto" thing, so the jump
> is literally hidden as an exception entry. And apparently objtool
> doesn't follow exceptions (which *normally* doesn't matter for code
> liveness analysis since they normally jump back to right after the
> excepting instruction, but maybe it misses some exception handling
> code because of it?).
> You may have looked at unsafe_get_user(), which does indeed make the
> branch as C code, because gcc currently does not allow outputs from
> "asm goto" statements (which "get" obviously needs).

Indeed I did. But it looks like objtool actually does parse .fixup. What
appears to go wrong is the 'visited' marker for backward jumps.

If we've been there with AC=0 first, and then backjump with AC=1, things
go missing.

I've also now confused myself on how it branches from alternatives. It
looks like it now considers paths that take the STAC alternative, and
exit through the NOP alternative (which should be CLAC) and then hit
RET with AC=1.

I'll get this sorted, eventually..