linux-kernel - Re: [PATCH] kcov: mark in_softirq_really() as __always

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Message-ID: <20241218084049.npa3zhkagbqp2khc@jpoimboe>
Date: Wed, 18 Dec 2024 00:40:49 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Marco Elver <elver@...gle.com>
Cc: Arnd Bergmann <arnd@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Konovalov <andreyknvl@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnd Bergmann <arnd@...db.de>, Dmitry Vyukov <dvyukov@...gle.com>,
	Aleksandr Nogikh <nogikh@...gle.com>, kasan-dev@...glegroups.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kcov: mark in_softirq_really() as __always_inline

On Tue, Dec 17, 2024 at 09:30:24AM +0100, Marco Elver wrote:
> On Tue, 17 Dec 2024 at 08:18, Arnd Bergmann <arnd@...nel.org> wrote:
> >
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > If gcc decides not to inline in_softirq_really(), objtool warns about
> > a function call with UACCESS enabled:
> >
> > kernel/kcov.o: warning: objtool: __sanitizer_cov_trace_pc+0x1e: call to in_softirq_really() with UACCESS enabled
> > kernel/kcov.o: warning: objtool: check_kcov_mode+0x11: call to in_softirq_really() with UACCESS enabled
> >
> > Mark this as __always_inline to avoid the problem.
> >
> > Fixes: 7d4df2dad312 ("kcov: properly check for softirq context")
> > Signed-off-by: Arnd Bergmann <arnd@...db.de>
> 
> __always_inline is the usual approach for code that can be
> instrumented - but I thought we explicitly never instrument
> kernel/kcov.c with anything. So I'm rather puzzled why gcc would not
> inline this function. In any case "inline" guarantees nothing, so:

I'm guessing CONFIG_DEBUG_SECTION_MISMATCH was enabled, which enables
-fno-inline-functions-called-once which ends up being the cause of a lot
of these __always_inline patches.

I had a patch to get rid of that at some point, guess it got lost...

-- 
Josh