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

[Will Deacon]

On Tue, Jun 30, 2020 at 03:57:54PM -0700, Sami Tolvanen wrote:
> On Tue, Jun 30, 2020 at 12:47 PM Marco Elver <elver@xxxxxxxxxx> wrote:
> >
> > On Tue, 30 Jun 2020 at 19:39, Will Deacon <will@xxxxxxxxxx> 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@xxxxxxxxxx>
> > > ---
> > >  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@xxxxxxxxxxxxxxxx/
> >
> > 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.

D'oh, sorry. I'll fix that (I had that #ifdef commented out for my testing).

Will