linux-kernel - Re: [PATCH 2/2] moderr: add module error injection tool

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Message-ID: <ccofyygi4rerybdmecqswldykihtabx6yco7ztylqnbmw4a5qw@ye7zoq7mcol2>
Date: Mon, 24 Feb 2025 08:43:45 -0600
From: Lucas De Marchi <lucas.demarchi@...el.com>
To: Daniel Gomez <da.gomez@...nel.org>
CC: Luis Chamberlain <mcgrof@...nel.org>, Alexei Starovoitov
	<alexei.starovoitov@...il.com>, Daniel Gomez <da.gomez@...sung.com>, "Petr
 Pavlu" <petr.pavlu@...e.com>, Sami Tolvanen <samitolvanen@...gle.com>,
	"Alexei Starovoitov" <ast@...nel.org>, Daniel Borkmann
	<daniel@...earbox.net>, "Andrii Nakryiko" <andrii@...nel.org>, Martin KaFai
 Lau <martin.lau@...ux.dev>, "Eduard Zingerman" <eddyz87@...il.com>, Song Liu
	<song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, John Fastabend
	<john.fastabend@...il.com>, "KP Singh" <kpsingh@...nel.org>, Stanislav
 Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa
	<jolsa@...nel.org>, Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers
	<ndesaulniers@...gle.com>, "Bill Wendling" <morbo@...gle.com>, Justin Stitt
	<justinstitt@...gle.com>, <linux-modules@...r.kernel.org>, LKML
	<linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>, clang-built-linux
	<llvm@...ts.linux.dev>, iovisor-dev <iovisor-dev@...ts.iovisor.org>,
	<gost.dev@...sung.com>
Subject: Re: [PATCH 2/2] moderr: add module error injection tool

On Sat, Feb 22, 2025 at 10:35:07PM +0100, Daniel Gomez wrote:
>On Fri, Feb 21, 2025 at 12:15:40PM +0100, Luis Chamberlain wrote:
>> On Wed, Feb 19, 2025 at 02:17:48PM -0600, Lucas De Marchi wrote:
>> > On Tue, Jan 28, 2025 at 12:57:05PM -0800, Luis Chamberlain wrote:
>> > > On Wed, Jan 22, 2025 at 09:02:19AM -0800, Alexei Starovoitov wrote:
>> > > > On Wed, Jan 22, 2025 at 5:12 AM Daniel Gomez <da.gomez@...sung.com> wrote:
>> > > > >
>> > > > > Add support for a module error injection tool. The tool
>> > > > > can inject errors in the annotated module kernel functions
>> > > > > such as complete_formation(), do_init_module() and
>> > > > > module_enable_rodata_after_init(). Module name and module function are
>> > > > > required parameters to have control over the error injection.
>> > > > >
>> > > > > Example: Inject error -22 to module_enable_rodata_ro_after_init for
>> > > > > brd module:
>> > > > >
>> > > > > sudo moderr --modname=brd --modfunc=module_enable_rodata_ro_after_init \
>> > > > > --error=-22 --trace
>> > > > > Monitoring module error injection... Hit Ctrl-C to end.
>> > > > > MODULE     ERROR FUNCTION
>> > > > > brd        -22   module_enable_rodata_after_init()
>> > > > >
>> > > > > Kernel messages:
>> > > > > [   89.463690] brd: module loaded
>> > > > > [   89.463855] brd: module_enable_rodata_ro_after_init() returned -22,
>> > > > > ro_after_init data might still be writable
>> > > > >
>> > > > > Signed-off-by: Daniel Gomez <da.gomez@...sung.com>
>> > > > > ---
>> > > > >  tools/bpf/Makefile            |  13 ++-
>> > > > >  tools/bpf/moderr/.gitignore   |   2 +
>> > > > >  tools/bpf/moderr/Makefile     |  95 +++++++++++++++++
>> > > > >  tools/bpf/moderr/moderr.bpf.c | 127 +++++++++++++++++++++++
>> > > > >  tools/bpf/moderr/moderr.c     | 236 ++++++++++++++++++++++++++++++++++++++++++
>> > > > >  tools/bpf/moderr/moderr.h     |  40 +++++++
>> > > > >  6 files changed, 510 insertions(+), 3 deletions(-)
>> > > >
>> > > > The tool looks useful, but we don't add tools to the kernel repo.
>> > > > It has to stay out of tree.
>> > >
>> > > For selftests we do add random tools.
>> > >
>> > > > The value of error injection is not clear to me.
>> > >
>> > > It is of great value, since it deals with corner cases which are
>> > > otherwise hard to reproduce in places which a real error can be
>> > > catostrophic.
>> > >
>> > > > Other places in the kernel use it to test paths in the kernel
>> > > > that are difficult to do otherwise.
>> > >
>> > > Right.
>> > >
>> > > > These 3 functions don't seem to be in this category.
>> > >
>> > > That's the key here we should focus on. The problem is when a maintainer
>> > > *does* agree that adding an error injection entry is useful for testing,
>> > > and we have a developer willing to do the work to help test / validate
>> > > it. In this case, this error case is rare but we do want to strive to
>> > > test this as we ramp up and extend our modules selftests.
>> > >
>> > > Then there is the aspect of how to mitigate how instrusive code changes
>> > > to allow error injection are. In 2021 we evaluated the prospect of error
>> > > injection in-kernel long ago for other areas like the block layer for
>> > > add_disk() failures [0] but the minimal interface to enable this from
>> > > userspace with debugfs was considered just too intrusive.
>> > >
>> > > This effort tried to evaluate what this could look like with eBPF to
>> > > mitigate the required in-kernel code, and I believe the light weight
>> > > nature of it by just requiring a sprinkle with ALLOW_ERROR_INJECTION()
>> > > suffices to my taste.
>> > >
>> > > So, perhaps the tools aspect can just go in:
>> > >
>> > > tools/testing/selftests/module/
>> >
>> > but why would it be module-specific?
>>
>> Gotta start somewhere.
>>
>> > Based on its current implementation
>> > and discussion about inject.py it seems to be generic enough to be
>> > useful to test any function annotated with ALLOW_ERROR_INJECTION().
>> >
>> > As xe driver maintainer, it may be interesting to use such a tool:
>> >
>> > 	$ git grep ALLOW_ERROR_INJECT -- drivers/gpu/drm/xe | wc -l  	23
>> >
>> > How does this approach compare to writing the function name on debugfs
>> > (the current approach in xe's testsuite)?
>> >
>> > 	fail_function @ https://docs.kernel.org/fault-injection/fault-injection.html#fault-injection-capabilities-infrastructure
>> > 	https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/blob/master/tests/intel/xe_fault_injection.c?ref_type=heads#L108
>> >
>> > If you decide to have the tool to live somewhere else, then kmod repo
>> > could be a candidate.
>>
>> Would we install this upon install target?
>>
>> Danny can decide on this :)
>>
>> > Although I think having it in kernel tree is
>> > simpler maintenance-wise.
>>
>> I think we have at least two users upstream who can make use of it. If
>> we end up going through tools/testing/selftests/module/ first, can't
>> you make use of it later?
>
>What are the features in debugfs required to be useful for xe that we can
>port to an eBPF version? I see from the link provided the use of probability,
>interval, times and space but these are configured to allways trigger the error.
>Is that right?

I don't think we use them... we just set them to "always trigger" and
then create the conditions for that to happen.  But my question still
remains:  what is the benefit of using the bpf approach over writing
these files?

Lucas De Marchi

>
>>
>>   Luis