[PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y

[Sami Tolvanen]

On Tue, Jun 30, 2020 at 12:47 PM Marco Elver <elver at google.com> wrote:
>
> On Tue, 30 Jun 2020 at 19:39, Will Deacon <will at kernel.org> wrote:
> >
> > When building with LTO, there is an increased risk of the compiler
> > converting an address dependency headed by a READ_ONCE() invocation
> > into a control dependency and consequently allowing for harmful
> > reordering by the CPU.
> >
> > Ensure that such transformations are harmless by overriding the generic
> > READ_ONCE() definition with one that provides acquire semantics when
> > building with LTO.
> >
> > Signed-off-by: Will Deacon <will at kernel.org>
> > ---
> >  arch/arm64/include/asm/rwonce.h   | 63 +++++++++++++++++++++++++++++++
> >  arch/arm64/kernel/vdso/Makefile   |  2 +-
> >  arch/arm64/kernel/vdso32/Makefile |  2 +-
> >  3 files changed, 65 insertions(+), 2 deletions(-)
> >  create mode 100644 arch/arm64/include/asm/rwonce.h
>
> This seems reasonable, given we can't realistically tell the compiler
> about dependent loads. What (if any), is the performance impact? I
> guess this also heavily depends on the actual silicon.
>
> I do wonder, though, if there is some way to make the compiler do
> something better for us. Clearly, implementing real
> memory_order_consume hasn't worked out until today. But maybe the
> compiler could promote dependent loads to acquires if it recognizes it
> lost dependencies during optimizations. Just thinking out loud, it
> probably still has some weird corner case that will break. ;-)
>
> The other thing is that I'd be cautious blaming LTO, as I tried to
> summarize here:
> https://lore.kernel.org/kernel-hardening/20200630191931.GA884155@elver.google.com/
>
> The main thing is that, yes, this might be something to be worried
> about, but if we are worried about it, we need to be worried about it
> in *all* builds (LTO or not). My guess is that's not acceptable. Would
> it be better to just guard the promotion of READ_ONCE() to acquire
> behind a config option like CONFIG_ACQUIRE_READ_DEPENDENCIES, and then
> make LTO select that (or maybe leave it optional?). In future, for
> very aggressive non-LTO compilers even, one may then also select that
> if there is substantiated worry things do actually break.

I agree, a separate config option would be better here.

Also Will, the LTO patches use CONFIG_LTO_CLANG instead of CLANG_LTO.

Sami