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

[Linus Torvalds]

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).

[ One of these days I really should look at the gcc sources to try to
figure out why gcc doesn't like them. I wish we could have a rule like
"it's an output only for the fallthrough case, not for the goto
cases". Because I wonder if the gcc peoples aversion to "asm goto" and
outputs comes from "we can't set outputs in multiple places". But my
gcc-foo is not strong enough that I've felt confident enough to really
go take a deep dive into something that feels pretty subtle, so I've
_thought_ about doing it for a long time, but have never actually
built up the confidence to do so ]

                    Linus