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

[Yonghong Song]

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.

> Signed-off-by: Krister Johansen <kjlx@xxxxxxxxxxxxxxxxxx>
> ---
>   .../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.

> +
> +	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;
> +}
> +
> +char _license[] SEC("license") = "GPL";