Message-ID: <e6jybeg4y6q6zqyhqma7q4icw7jllieq5rwwi5pguy242wioyp@hkelxx7tnzlg>
Date: Fri, 28 Feb 2025 12:48:38 -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
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	"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
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	<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 Fri, Feb 28, 2025 at 10:27:17AM +0100, Daniel Gomez wrote:
>On Mon, Feb 24, 2025 at 08:43:45AM +0100, Lucas De Marchi wrote:
>> 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?
>
>The code to trigger the error injection still needs to exist with both
>approaches. My understanding from debugfs and the comment from Luis earlier in
>the thread is that with eBPF you can mitigate how intrusive in-kernel code
>changes are to allow error injection. Luis added the example here [1] for
>debugfs.
>
>[1] https://lore.kernel.org/all/20210512064629.13899-9-mcgrof@kernel.org/
>
>To compare patch 8 in [1] with eBPF approach: the patch describes
>all the necessary changes required to allow error injection on the
>add_disk() path. With eBPF one would simply annotate the function(s) with
>ALLOW_ERROR_INJECTION(), e.g. device_add() and replace the return value
>in eBPF code with bpf_override_return() as implemented in moderr tool for
>module_enable_rdata_after_init() for example.

but that is all that we need with the fail_function in debugfs too:

$ git grep ALLOW_ERROR_INJECTION -- drivers/gpu/drm/xe
drivers/gpu/drm/xe/xe_device.c:ALLOW_ERROR_INJECTION(xe_device_create, ERRNO); /* See xe_pci_probe() */
drivers/gpu/drm/xe/xe_device.c:ALLOW_ERROR_INJECTION(wait_for_lmem_ready, ERRNO); /* See xe_pci_probe() */
drivers/gpu/drm/xe/xe_exec_queue.c:ALLOW_ERROR_INJECTION(xe_exec_queue_create_bind, ERRNO);
drivers/gpu/drm/xe/xe_ggtt.c:ALLOW_ERROR_INJECTION(xe_ggtt_init_early, ERRNO); /* See xe_pci_probe() */
drivers/gpu/drm/xe/xe_guc_ads.c:ALLOW_ERROR_INJECTION(xe_guc_ads_init, ERRNO); /* See xe_pci_probe() */
drivers/gpu/drm/xe/xe_guc_ct.c:ALLOW_ERROR_INJECTION(xe_guc_ct_init, ERRNO); /* See xe_pci_probe() */
drivers/gpu/drm/xe/xe_guc_log.c:ALLOW_ERROR_INJECTION(xe_guc_log_init, ERRNO); /* See xe_pci_probe() */
drivers/gpu/drm/xe/xe_guc_relay.c:ALLOW_ERROR_INJECTION(xe_guc_relay_init, ERRNO); /* See xe_pci_probe() */
drivers/gpu/drm/xe/xe_pci.c: * ALLOW_ERROR_INJECTION() is used to conditionally skip function execution
drivers/gpu/drm/xe/xe_pci.c: * ALLOW_ERROR_INJECTION() macro but this is acceptable because for those
drivers/gpu/drm/xe/xe_pm.c:ALLOW_ERROR_INJECTION(xe_pm_init_early, ERRNO); /* See xe_pci_probe() */
drivers/gpu/drm/xe/xe_pt.c:ALLOW_ERROR_INJECTION(xe_pt_create, ERRNO);
drivers/gpu/drm/xe/xe_pt.c:ALLOW_ERROR_INJECTION(xe_pt_update_ops_prepare, ERRNO);
drivers/gpu/drm/xe/xe_pt.c:ALLOW_ERROR_INJECTION(xe_pt_update_ops_run, ERRNO);
drivers/gpu/drm/xe/xe_sriov.c:ALLOW_ERROR_INJECTION(xe_sriov_init, ERRNO); /* See xe_pci_probe() */
drivers/gpu/drm/xe/xe_sync.c:ALLOW_ERROR_INJECTION(xe_sync_entry_parse, ERRNO);
drivers/gpu/drm/xe/xe_tile.c:ALLOW_ERROR_INJECTION(xe_tile_init_early, ERRNO); /* See xe_pci_probe() */
drivers/gpu/drm/xe/xe_uc_fw.c:ALLOW_ERROR_INJECTION(xe_uc_fw_init, ERRNO); /* See xe_pci_probe() */
drivers/gpu/drm/xe/xe_vm.c:ALLOW_ERROR_INJECTION(xe_vma_ops_alloc, ERRNO);
drivers/gpu/drm/xe/xe_vm.c:ALLOW_ERROR_INJECTION(xe_vm_create_scratch, ERRNO);
drivers/gpu/drm/xe/xe_vm.c:ALLOW_ERROR_INJECTION(vm_bind_ioctl_ops_create, ERRNO);
drivers/gpu/drm/xe/xe_vm.c:ALLOW_ERROR_INJECTION(vm_bind_ioctl_ops_execute, ERRNO);
drivers/gpu/drm/xe/xe_wa.c:ALLOW_ERROR_INJECTION(xe_wa_init, ERRNO); /* See xe_pci_probe() */
drivers/gpu/drm/xe/xe_wopcm.c:ALLOW_ERROR_INJECTION(xe_wopcm_init, ERRNO); /* See xe_pci_probe() */

That is different from the patch you are pointing to because that patch
is trying to add arbitrary/named error injection points throughout the
code. However via debugfs it's still possible to add error injection to
the beginning of a function by annotating that function with
ALLOW_ERROR_INJECTION. If a function is annotated with that, then if you
do e.g.

	echo xe_device_create > /sys/kernel/debug/fail_function/inject

it will cause that function to fail. There are some additional files to
control _when_ that function should fail, but I'm failing to see a clear
benefit. See this example in the docs:

	Documentation/fault-injection/fault-injection.rst:- Inject open_ctree error while btrfs mount::

Lucas De Marchi

>
>New error injection users such modules or block/disk would benefit of the eBPF
>approach with having a more simpler in-kernel code for the same purpose. Current
>users of debugfs/errorinj would have to remove all the upstream intrusive error
>injection related code if they want to adopt the eBPF approach.
>
>Daniel
>
>>
>> Lucas De Marchi
>>
>> >
>> > >
>> > >   Luis

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