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 11:15:18 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Krister Johansen <kjlx@...pleofstupid.com>,
        Ilya Leoshkevich <iii@...ux.ibm.com>
Cc:     bpf <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>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf v3 2/2] selftests/bpf: add a test for subprogram extables

On Thu, Jun 8, 2023 at 5:11 PM Krister Johansen <kjlx@...pleofstupid.com> 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.
>
> 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;
> +       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;
> +
> +       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");
> +
> +       test_subprogs_extable__detach(skel);
> +
> +cleanup:
> +       test_subprogs_extable__destroy(skel);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/test_subprogs_extable.c b/tools/testing/selftests/bpf/progs/test_subprogs_extable.c
> new file mode 100644
> index 000000000000..c3ff66bf4cbe
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/test_subprogs_extable.c
> @@ -0,0 +1,46 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include "vmlinux.h"
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +
> +struct {
> +       __uint(type, BPF_MAP_TYPE_ARRAY);
> +       __uint(max_entries, 8);
> +       __type(key, __u32);
> +       __type(value, __u64);
> +} test_array SEC(".maps");
> +
> +static __u64 test_cb(struct bpf_map *map, __u32 *key, __u64 *val, void *data)
> +{
> +       return 1;
> +}
> +
> +SEC("fexit/bpf_testmod_return_ptr")
> +int BPF_PROG(handle_fexit_ret_subprogs, int arg, struct file *ret)
> +{
> +       *(volatile long *)ret;
> +       *(volatile int *)&ret->f_mode;
> +       bpf_for_each_map_elem(&test_array, test_cb, NULL, 0);
> +       return 0;
> +}
> +
> +SEC("fexit/bpf_testmod_return_ptr")
> +int BPF_PROG(handle_fexit_ret_subprogs2, int arg, struct file *ret)
> +{
> +       *(volatile long *)ret;
> +       *(volatile int *)&ret->f_mode;
> +       bpf_for_each_map_elem(&test_array, test_cb, NULL, 0);
> +       return 0;
> +}
> +
> +SEC("fexit/bpf_testmod_return_ptr")
> +int BPF_PROG(handle_fexit_ret_subprogs3, int arg, struct file *ret)
> +{
> +       *(volatile long *)ret;
> +       *(volatile int *)&ret->f_mode;
> +       bpf_for_each_map_elem(&test_array, test_cb, NULL, 0);
> +       return 0;
> +}

What is the point of attaching 3 the same progs to the same hook?
One would be enough to test it, no?

In other news...
Looks like this test is triggering a bug on s390.

Ilya,
please take a look:
https://github.com/kernel-patches/bpf/actions/runs/5216942096/jobs/9416404780

bpf_prog_78c0d4c618ed2df7_handle_fexit_ret_subprogs3
is crashing the kernel.
A bug in extable logic on s390?