linux-kernel - Re: [PATCH v2 23/23] objtool, kcsan: Remove memory barrier instrumentation from noinstr

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Date:   Fri, 19 Nov 2021 12:31:35 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Marco Elver <elver@...gle.com>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Waiman Long <longman@...hat.com>,
        Will Deacon <will@...nel.org>, kasan-dev@...glegroups.com,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org
Subject: Re: [PATCH v2 23/23] objtool, kcsan: Remove memory barrier
 instrumentation from noinstr

On Thu, Nov 18, 2021 at 09:10:27AM +0100, Marco Elver wrote:
> @@ -1071,12 +1071,7 @@ static void annotate_call_site(struct objtool_file *file,
>  		return;
>  	}
>  
> -	/*
> -	 * Many compilers cannot disable KCOV with a function attribute
> -	 * so they need a little help, NOP out any KCOV calls from noinstr
> -	 * text.
> -	 */
> -	if (insn->sec->noinstr && sym->kcov) {
> +	if (insn->sec->noinstr && sym->removable_instr) {
>  		if (reloc) {
>  			reloc->type = R_NONE;
>  			elf_write_reloc(file->elf, reloc);

I'd love to have a clearer name than 'removable_instr', though I'm
having trouble coming up with something.

'profiling_func'?

Profiling isn't really accurate but maybe it gets the point across.  I'm
definitely open to other suggestions.

Also, the above code isn't very self-evident so there still needs to be
a comment there, like:

	/*
	 * Many compilers cannot disable KCOV or sanitizer calls with a
	 * function attribute so they need a little help, NOP out any
	 * such calls from noinstr text.
	 */

> @@ -1991,6 +1986,32 @@ static int read_intra_function_calls(struct objtool_file *file)
>  	return 0;
>  }
>  
> +static bool is_removable_instr(const char *name)


> +{
> +	/*
> +	 * Many compilers cannot disable KCOV with a function attribute so they
> +	 * need a little help, NOP out any KCOV calls from noinstr text.
> +	 */
> +	if (!strncmp(name, "__sanitizer_cov_", 16))
> +		return true;

A comment is good here, but the NOP-ing bit seems out of place.

-- 
Josh