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

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Date:   Mon, 8 Jun 2020 19:32:37 +0200
From:   Martin Liška <mliska@...e.cz>
To:     Marco Elver <elver@...gle.com>, 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>, ndesaulniers@...gle.com
Subject: Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen

On 6/3/20 9:23 PM, Marco Elver wrote:
> 
> 
> 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

Hello.

Thank you for the example. It seems to me that Clang does not inline a no_sanitize_* function
into one which is instrumented. Is it a documented behavior ([1] doesn't mention that)?
If so, we can do the same in GCC.

Thanks,
Martin

[1] https://clang.llvm.org/docs/AttributeReference.html#no-sanitize

> 
> 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
>