Re: [PATCHv3 bpf 2/2] selftests/bpf: Add test for early update in prog_array_map_poke_run

[Yonghong Song <yonghong.song@linux.dev>]

On 12/5/23 3:43 AM, Jiri Olsa wrote:
> On Mon, Dec 04, 2023 at 09:16:52PM -0800, Yonghong Song wrote:
>> On 12/3/23 3:48 PM, Jiri Olsa wrote:
>>> Adding test that tries to trigger the BUG_ON during early map update
>>> in prog_array_map_poke_run function.
>>>
>>> The idea is to share prog array map between thread that constantly
>>> updates it and another one loading a program that uses that prog
>>> array.
>>>
>>> Eventually we will hit a place where the program is ok to be updated
>>> (poke->tailcall_target_stable check) but the address is still not
>>> registered in kallsyms, so the bpf_arch_text_poke returns -EINVAL
>>> and cause imbalance for the next tail call update check, which will
>>> fail with -EBUSY in bpf_arch_text_poke as described in previous fix.
>>>
>>> Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
>>> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
>>> ---
>>>    .../selftests/bpf/prog_tests/tailcall_poke.c  | 74 +++++++++++++++++++
>>>    .../selftests/bpf/progs/tailcall_poke.c       | 32 ++++++++
>>>    2 files changed, 106 insertions(+)
>>>    create mode 100644 tools/testing/selftests/bpf/prog_tests/tailcall_poke.c
>>>    create mode 100644 tools/testing/selftests/bpf/progs/tailcall_poke.c
>>>
>>> diff --git a/tools/testing/selftests/bpf/prog_tests/tailcall_poke.c b/tools/testing/selftests/bpf/prog_tests/tailcall_poke.c
>>> new file mode 100644
>>> index 000000000000..f7e2c09fd772
>>> --- /dev/null
>>> +++ b/tools/testing/selftests/bpf/prog_tests/tailcall_poke.c
>>> @@ -0,0 +1,74 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +#include <unistd.h>
>>> +#include <test_progs.h>
>>> +#include "tailcall_poke.skel.h"
>>> +
>>> +#define JMP_TABLE "/sys/fs/bpf/jmp_table"
>>> +
>>> +static int thread_exit;
>>> +
>>> +static void *update(void *arg)
>>> +{
>>> +	__u32 zero = 0, prog1_fd, prog2_fd, map_fd;
>>> +	struct tailcall_poke *call = arg;
>>> +
>>> +	map_fd = bpf_map__fd(call->maps.jmp_table);
>>> +	prog1_fd = bpf_program__fd(call->progs.call1);
>>> +	prog2_fd = bpf_program__fd(call->progs.call2);
>>> +
>>> +	while (!thread_exit) {
>>> +		bpf_map_update_elem(map_fd, &zero, &prog1_fd, BPF_ANY);
>>> +		bpf_map_update_elem(map_fd, &zero, &prog2_fd, BPF_ANY);
>>> +	}
>>> +
>>> +	return NULL;
>>> +}
>>> +
>>> +void test_tailcall_poke(void)
>>> +{
>>> +	struct tailcall_poke *call, *test;
>>> +	int err, cnt = 10;
>>> +	pthread_t thread;
>>> +
>>> +	unlink(JMP_TABLE);
>>> +
>>> +	call = tailcall_poke__open_and_load();
>>> +	if (!ASSERT_OK_PTR(call, "tailcall_poke__open"))
>>> +		return;
>>> +
>>> +	err = bpf_map__pin(call->maps.jmp_table, JMP_TABLE);
>>> +	if (!ASSERT_OK(err, "bpf_map__pin"))
>>> +		goto out;
>> Just curious. What is the reason having bpf_map__pin() here
>> and below? I tried and it looks like removing bpf_map__pin()
>> and below bpf_map__set_pin_path() will make reproducing
>> the failure hard/impossible.
> yes, it's there to share the jmp_table map between the two
> skeleton instances, so the update thread changes the same
> jmp_table map that's used in the skeleton we load in the
> while loop below

This does make sense.

>
> I'll add some comments to the test

Thanks for explanation. Some comments are definitely helpful!

>
> jirka
>
>>> +
>>> +	err = pthread_create(&thread, NULL, update, call);
>>> +	if (!ASSERT_OK(err, "new toggler"))
>>> +		goto out;
>>> +
>>> +	while (cnt--) {
>>> +		test = tailcall_poke__open();
>>> +		if (!ASSERT_OK_PTR(test, "tailcall_poke__open"))
>>> +			break;
>>> +
>>> +		err = bpf_map__set_pin_path(test->maps.jmp_table, JMP_TABLE);
>>> +		if (!ASSERT_OK(err, "bpf_map__pin")) {
>>> +			tailcall_poke__destroy(test);
>>> +			break;
>>> +		}
>>> +
>>> +		bpf_program__set_autoload(test->progs.test, true);
>>> +		bpf_program__set_autoload(test->progs.call1, false);
>>> +		bpf_program__set_autoload(test->progs.call2, false);
>>> +
>>> +		err = tailcall_poke__load(test);
>>> +		tailcall_poke__destroy(test);
>>> +		if (!ASSERT_OK(err, "tailcall_poke__load"))
>>> +			break;
>>> +	}
>>> +
>>> +	thread_exit = 1;
>>> +	ASSERT_OK(pthread_join(thread, NULL), "pthread_join");
>>> +
>>> +out:
>>> +	bpf_map__unpin(call->maps.jmp_table, JMP_TABLE);
>>> +	tailcall_poke__destroy(call);
>>> +}
> SNIP