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

[Peter Zijlstra <peterz@infradead.org>]

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@roeck-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.