linux-kernel - Re: [PATCH v5 1/5] bug/kunit: Core support for suppressing warning backtraces

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Message-ID: <20250529090219.GA24938@noisy.programming.kicks-ass.net>
Date: Thu, 29 May 2025 11:02:19 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Kees Cook <kees@...nel.org>
Cc: Alessandro Carminati <acarmina@...hat.com>,
	linux-kselftest@...r.kernel.org,
	Dan Carpenter <dan.carpenter@...aro.org>,
	Daniel Diaz <daniel.diaz@...aro.org>,
	David Gow <davidgow@...gle.com>,
	Arthur Grillo <arthurgrillo@...eup.net>,
	Brendan Higgins <brendan.higgins@...ux.dev>,
	Naresh Kamboju <naresh.kamboju@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Maxime Ripard <mripard@...nel.org>,
	Ville Syrjala <ville.syrjala@...ux.intel.com>,
	Daniel Vetter <daniel@...ll.ch>, Guenter Roeck <linux@...ck-us.net>,
	Alessandro Carminati <alessandro.carminati@...il.com>,
	Jani Nikula <jani.nikula@...el.com>,
	Jeff Johnson <jeff.johnson@....qualcomm.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Shuah Khan <skhan@...uxfoundation.org>,
	Linux Kernel Functional Testing <lkft@...aro.org>,
	dri-devel@...ts.freedesktop.org, kunit-dev@...glegroups.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/5] bug/kunit: Core support for suppressing warning
 backtraces

On Wed, May 28, 2025 at 03:47:42PM -0700, Kees Cook wrote:
> On Mon, May 26, 2025 at 01:27:51PM +0000, Alessandro Carminati wrote:
> > Some unit tests intentionally trigger warning backtraces by passing bad
> > parameters to kernel API functions. Such unit tests typically check the
> > return value from such calls, not the existence of the warning backtrace.
> > 
> > Such intentionally generated warning backtraces are neither desirable
> > nor useful for a number of reasons:
> > - They can result in overlooked real problems.
> > - A warning that suddenly starts to show up in unit tests needs to be
> >   investigated and has to be marked to be ignored, for example by
> >   adjusting filter scripts. Such filters are ad hoc because there is
> >   no real standard format for warnings. On top of that, such filter
> >   scripts would require constant maintenance.
> > 
> > Solve the problem by providing a means to identify and suppress specific
> > warning backtraces while executing test code. Support suppressing multiple
> > backtraces while at the same time limiting changes to generic code to the
> > absolute minimum.
> > 
> > Implementation details:
> > Check suppression directly in the `WARN()` Macros.
> > This avoids the need for function symbol resolution or ELF section
> > modification.
> > Suppression is implemented directly in the `WARN*()` macros.
> > 
> > A helper function, `__kunit_is_suppressed_warning()`, is used to determine
> > whether suppression applies. It is marked as `noinstr`, since some `WARN*()`
> > sites reside in non-instrumentable sections. As it uses `strcmp`, a
> > `noinstr` version of `strcmp` was introduced.
> > The implementation is deliberately simple and avoids architecture-specific
> > optimizations to preserve portability. Since this mechanism compares
> > function names and is intended for test usage only, performance is not a
> > primary concern.
> > 
> > Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> 
> I like this -- it's very simple, it doesn't need to be fast-path, so
> a linear list walker with strcmp is fine. Nice!

But it is on the fast path! This is still bloating every UD2 site
instead of doing it on the other end.