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. Sami
