Re: [PATCH 00/14] Add support for suppressing warning backtraces

[Geert Uytterhoeven]

Hi Günter,

On Tue, Mar 12, 2024 at 6:03 PM Guenter Roeck <linux@xxxxxxxxxxxx> 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.
>
> One option to address problem would be to add messages such as "expected
> warning backtraces start / end here" to the kernel log.  However, that
> would again require filter scripts, it might result in missing real
> problematic warning backtraces triggered while the test is running, and
> the irrelevant backtrace(s) would still clog the kernel log.
>
> 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. Architecture specific changes are kept at minimum by
> retaining function names only if both CONFIG_DEBUG_BUGVERBOSE and
> CONFIG_KUNIT are enabled.
>
> The first patch of the series introduces the necessary infrastructure.
> The second patch introduces support for counting suppressed backtraces.
> This capability is used in patch three to implement unit tests.
> Patch four documents the new API.
> The next two patches add support for suppressing backtraces in drm_rect
> and dev_addr_lists unit tests. These patches are intended to serve as
> examples for the use of the functionality introduced with this series.
> The remaining patches implement the necessary changes for all
> architectures with GENERIC_BUG support.

Thanks for your series!

I gave it a try on m68k, just running backtrace-suppression-test,
and that seems to work fine.

> Design note:
>   Function pointers are only added to the __bug_table section if both
>   CONFIG_KUNIT and CONFIG_DEBUG_BUGVERBOSE are enabled to avoid image
>   size increases if CONFIG_KUNIT=n. There would be some benefits to
>   adding those pointers all the time (reduced complexity, ability to
>   display function names in BUG/WARNING messages). That change, if
>   desired, can be made later.

Unfortunately this also increases kernel size in the CONFIG_KUNIT=m
case (ca. 80 KiB for atari_defconfig), making it less attractive to have
kunit and all tests enabled as modules in my standard kernel.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68korg

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds