linux-kernel - Re: [PATCH v5 00/18] Rework READ

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Date:   Wed, 3 Jun 2020 21:23:53 +0200
From:   Marco Elver <elver@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen



On Wed, 03 Jun 2020, Borislav Petkov wrote:

> On Thu, May 14, 2020 at 12:05:38PM +0100, Will Deacon wrote:
> > Talking off-list, Clang >= 7 is pretty reasonable wrt inlining decisions
> > and the behaviour for __always_inline is:
> > 
> >   * An __always_inline function inlined into a __no_sanitize function is
> >     not instrumented
> >   * An __always_inline function inlined into an instrumented function is
> >     instrumented
> >   * You can't mark a function as both __always_inline __no_sanitize, because
> >     __no_sanitize functions are never inlined
> > 
> > GCC, on the other hand, may still inline __no_sanitize functions and then
> > subsequently instrument them.
> 
> Yeah, about that: I've been looking for a way to trigger this so that
> I can show preprocessed source to gcc people. So do you guys have a
> .config or somesuch I can try?

For example take this:

	int x;

	static inline __attribute__((no_sanitize_thread)) void do_not_sanitize(void) {
	  x++;
	}

	void sanitize_this(void) {
	  do_not_sanitize();
	}

Then

	gcc-10 -O3 -fsanitize=thread -o example.o -c example.c
	objdump -D example.o

will show that do_not_sanitize() was inlined into sanitize_this() and is
instrumented. (With Clang this doesn't happen.)

Hope this is enough.

Thanks,
-- Marco