linux-kernel - Re: [PATCH -rcu/kcsan 04/23] kcsan: Add core support for a subset of weak memory modeling

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Message-ID: <YVxKplLAMJJUlg/w@hirez.programming.kicks-ass.net>
Date:   Tue, 5 Oct 2021 14:52:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
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>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        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 -rcu/kcsan 04/23] kcsan: Add core support for a subset of
 weak memory modeling

On Tue, Oct 05, 2021 at 12:58:46PM +0200, Marco Elver wrote:
> +#if !defined(CONFIG_ARCH_WANTS_NO_INSTR) || defined(CONFIG_STACK_VALIDATION)
> +/*
> + * Arch does not rely on noinstr, or objtool will remove memory barrier
> + * instrumentation, and no instrumentation of noinstr code is expected.
> + */
> +#define kcsan_noinstr

I think this still wants to be at the very least:

#define kcsan_noinstr noinline notrace

without noinline it is possible LTO (or similarly daft things) will end
up inlining the calls, and since we rely on objtool to NOP out CALLs
this must not happen.

And since you want to mark these functions as uaccess_safe, there must
not be any tracing on, hence notrace.

> +static inline bool within_noinstr(unsigned long ip) { return false; }
> +#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