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Date:   Tue, 5 Oct 2021 17:13:35 +0200
From:   Marco Elver <elver@google.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Paul E . McKenney" <paulmck@kernel.org>,
        Alexander Potapenko <glider@google.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Dmitry Vyukov <dvyukov@google.com>,
        Ingo Molnar <mingo@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Waiman Long <longman@redhat.com>,
        Will Deacon <will@kernel.org>, kasan-dev@googlegroups.com,
        linux-arch@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, x86@kernel.org
Subject: Re: [PATCH -rcu/kcsan 23/23] objtool, kcsan: Remove memory barrier
 instrumentation from noinstr
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On Tue, Oct 05, 2021 at 04:37PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 05, 2021 at 12:59:05PM +0200, Marco Elver wrote:
> > Teach objtool to turn instrumentation required for memory barrier
> > modeling into nops in noinstr text.
> > 
> > The __tsan_func_entry/exit calls are still emitted by compilers even
> > with the __no_sanitize_thread attribute. The memory barrier
> > instrumentation will be inserted explicitly (without compiler help), and
> > thus needs to also explicitly be removed.
> 
> How is arm64 and others using kernel/entry + noinstr going to fix this?
> 
> ISTR they fully rely on the compilers not emitting instrumentation,
> since they don't have objtool to fix up stray issues like this.

So this is where I'd like to hear if the approach of:

 | #if !defined(CONFIG_ARCH_WANTS_NO_INSTR) || defined(CONFIG_STACK_VALIDATION)
 | ...
 | #else
 | #define kcsan_noinstr noinstr
 | static __always_inline bool within_noinstr(unsigned long ip)
 | {
 | 	return (unsigned long)__noinstr_text_start <= ip &&
 | 	       ip < (unsigned long)__noinstr_text_end;
 | }
 | #endif

and then (using the !STACK_VALIDATION definitions)

 | kcsan_noinstr void instrumentation_may_appear_in_noinstr(void)
 | {
 | 	if (within_noinstr(_RET_IP_))
 | 		return;

works for the non-x86 arches that select ARCH_WANTS_NO_INSTR.

If it doesn't I can easily just remove kcsan_noinstr/within_noinstr, and
add a "depends on !ARCH_WANTS_NO_INSTR || STACK_VALIDATION" to the
KCSAN_WEAK_MEMORY option.

Looking at a previous discussion [1], however, I was under the
impression that this would work.

[1] https://lkml.kernel.org/r/CANpmjNMAZiW-Er=2QDgGP+_3hg1LOvPYcbfGSPMv=aR6MVTB-g@mail.gmail.com