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Date:   Thu, 14 May 2020 13:01:04 +0100
From:   Will Deacon <will@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Marco Elver <elver@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen
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On Thu, May 14, 2020 at 01:21:42PM +0200, Peter Zijlstra wrote:
> On Wed, May 13, 2020 at 03:58:30PM +0200, Marco Elver wrote:
> > On Wed, 13 May 2020 at 15:24, Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Wed, May 13, 2020 at 03:15:55PM +0200, Marco Elver wrote:
> > > > So far so good, except: both __no_sanitize_or_inline and
> > > > __no_kcsan_or_inline *do* avoid KCSAN instrumenting plain accesses, it
> > > > just doesn't avoid explicit kcsan_check calls, like those in
> > > > READ/WRITE_ONCE if KCSAN is enabled for the compilation unit. That's
> > > > just because macros won't be redefined just for __no_sanitize
> > > > functions. Similarly, READ_ONCE_NOCHECK does work as expected, and its
> > > > access is unchecked.
> > > >
> > > > This will have the expected result:
> > > > __no_sanitize_or_inline void foo(void) { x++; } // no data races reported
> > > >
> > > > This will not work as expected:
> > > > __no_sanitize_or_inline void foo(void) { READ_ONCE(x); }  // data
> > > > races are reported
> > > >
> > > > All this could be fixed if GCC devs would finally take my patch to
> > > > make -fsanitize=thread distinguish volatile [1], but then we have to
> > > > wait ~years for the new compilers to reach us. So please don't hold
> > > > your breath for this one any time soon.
> > > > [1] https://gcc.gnu.org/pipermail/gcc-patches/2020-April/544452.html
> > >
> > > Right, but that does not address the much larger issue of the attribute
> > > vs inline tranwreck :/
> > 
> > Could you check if Clang is equally broken for you? I think GCC and
> > Clang have differing behaviour on this. No idea what it takes to fix
> > GCC though.
> 
> So I have some good and some maybe not so good news.
> 
> Given the patch below (on top of tglx's entry-v5-the-rest tag); I did
> find that I could actually build alternative.o for gcc-{8,9,10} and
> indeed clang-10. Any earlier gcc (I tried, 5,6,7) does not build:
> 
> ../arch/x86/include/asm/ptrace.h:126:28: error: inlining failed in call to always_inline ‘user_mode’: function attribute mismatch
> 
> I dumped the poke_int3_handler output using objdump, find the attached
> files.
> 
> It looks like clang-10 doesn't want to turn UBSAN off :/ The GCC files
> look OK, no funny calls in those.
> 
> (the config has KASAN/UBSAN on, it looks like KCSAN and KASAN are
> mutually exclusive)
> 
> ---
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 77c83833d91e..06d8db612efc 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -990,7 +990,7 @@ static __always_inline int patch_cmp(const void *key, const void *elt)
>  	return 0;
>  }
>  
> -int noinstr poke_int3_handler(struct pt_regs *regs)
> +int noinstr __no_kcsan __no_sanitize_address __no_sanitize_undefined poke_int3_handler(struct pt_regs *regs)
>  {
>  	struct bp_patching_desc *desc;
>  	struct text_poke_loc *tp;
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index 2cb42d8bdedc..5e83aada6553 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -15,6 +15,13 @@
>  /* all clang versions usable with the kernel support KASAN ABI version 5 */
>  #define KASAN_ABI_VERSION 5
>  
> +#if __has_feature(undefined_sanitizer)

Hmm, this might want to be __has_feature(undefined_behavior_sanitizer)
(and damn is that hard for a Brit to type out!)

Will