linux-kernel - Re: [PATCH bpf v3 2/2] selftests/bpf: add a test for subprogram extables

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Date:   Fri, 9 Jun 2023 14:34:04 -0700
From:   Krister Johansen <kjlx@...pleofstupid.com>
To:     Yonghong Song <yhs@...a.com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf v3 2/2] selftests/bpf: add a test for subprogram
 extables

On Thu, Jun 08, 2023 at 08:52:36PM -0700, Yonghong Song wrote:
> On 6/8/23 5:11 PM, Krister Johansen wrote:
> > In certain situations a program with subprograms may have a NULL
> > extable entry.  This should not happen, and when it does, it turns a
> > single trap into multiple.  Add a test case for further debugging and to
> > prevent regressions.  N.b: without any other patches this can panic or
> > oops a kernel.
> 
> It would be great if you can add the panic call stack in the commit message.
> 
> Please also mention that three identical bpf programs in the test
> significantly increased the 'oops' chance. Just one program may
> not be able to trigger the issue.

Apologies, I mistakenly put the oops in the cover letter when you did tell
me to put it in the commit message for this patch.  That is fixed now.

> > Signed-off-by: Krister Johansen <kjlx@...pleofstupid.com>
> > ---
> >   .../bpf/prog_tests/subprogs_extable.c         | 31 +++++++++++++
> >   .../bpf/progs/test_subprogs_extable.c         | 46 +++++++++++++++++++
> >   2 files changed, 77 insertions(+)
> >   create mode 100644 tools/testing/selftests/bpf/prog_tests/subprogs_extable.c
> >   create mode 100644 tools/testing/selftests/bpf/progs/test_subprogs_extable.c
> > 
> > diff --git a/tools/testing/selftests/bpf/prog_tests/subprogs_extable.c b/tools/testing/selftests/bpf/prog_tests/subprogs_extable.c
> > new file mode 100644
> > index 000000000000..2201988274a4
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/prog_tests/subprogs_extable.c
> > @@ -0,0 +1,31 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +#include <test_progs.h>
> > +#include "test_subprogs_extable.skel.h"
> > +
> > +void test_subprogs_extable(void)
> > +{
> > +	const int READ_SZ = 456;
> 
> There is no need to use uppercase for READ_SZ.
> Just do
> 	const int read_sz = 456;
> is sufficient.
> 
> > +	struct test_subprogs_extable *skel;
> > +	int err;
> > +
> > +	skel = test_subprogs_extable__open();
> > +	if (!ASSERT_OK_PTR(skel, "skel_open"))
> > +		return;
> > +
> > +	err = test_subprogs_extable__load(skel);
> > +	if (!ASSERT_OK(err, "skel_load"))
> > +		goto cleanup;
> 
> You can combine the above open and load with a single one
>    test_subprogs_extable__open_and_load().
> 
> > +
> > +	err = test_subprogs_extable__attach(skel);
> > +	if (!ASSERT_OK(err, "skel_attach"))
> > +		goto cleanup;
> > +
> > +	/* trigger tracepoint */
> > +	ASSERT_OK(trigger_module_test_read(READ_SZ), "trigger_read");
> 
> I think we should at least ensure that the program is triggered. For
> example, add a global variable 'triggered' in the program and
> triggered will be set to 1 in the program if the program is running.
> Here check
> 	skel->bss->triggered
> must be 1.
 
Thanks for the additional feedback on cleaning this up.  I've
incorporated the requested changes and will send out a v4 shortly.

-K