From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y Date: Wed, 1 Jul 2020 11:25:31 +0100 Message-ID: <20200701102531.GE14959@willie-the-truck> References: <20200630173734.14057-1-will@kernel.org> <20200630173734.14057-19-will@kernel.org> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1593599137; bh=jhLhsiid/JHPTIQixozqrmVK7Z7yivTR5uDy/0fEI4U=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=2MIdgHcm8hZxwfkiuFehiZvcf3/Tr5GQA+93bWFROSIzCHdkqIW0C336sqCSrcoOe 9IGFq8THz2jtH5mtcNylSPiv8N8xlfrvpic2hj5lk+Rk4Eb7rCXBL3olxKyBxMCDvy 0GTZR7CuylPcMkoeFsVzizN2x+JiEUGVw6wBlss8= Content-Disposition: inline In-Reply-To: Sender: linux-alpha-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Sami Tolvanen Cc: Marco Elver , LKML , Nick Desaulniers , Kees Cook , "Paul E. McKenney" , Josh Triplett , Matt Turner , Ivan Kokshaysky , Richard Henderson , Peter Zijlstra , Alan Stern , "Michael S. Tsirkin" , Jason Wang , Arnd Bergmann , Boqun Feng , Catalin Marinas , Mark Rutland , linux-arm-kernel , linux-alpha@vger.kernel.org, virtualization@li On Tue, Jun 30, 2020 at 03:57:54PM -0700, Sami Tolvanen wrote: > On Tue, Jun 30, 2020 at 12:47 PM Marco Elver wrote: > > > > On Tue, 30 Jun 2020 at 19:39, Will Deacon 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 > > > --- > > > 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. D'oh, sorry. I'll fix that (I had that #ifdef commented out for my testing). Will