Re: [PATCH bpf-next v3 7/7] selftests/bpf: BPF task work scheduling tests

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2025-09-05 at 17:45 +0100, Mykyta Yatsenko wrote:
> From: Mykyta Yatsenko <yatsenko@meta.com>
> 
> Introducing selftests that check BPF task work scheduling mechanism.
> Validate that verifier does not accepts incorrect calls to
> bpf_task_work_schedule kfunc.
> 
> Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
> ---

The test cases in this patch check functional correctness, but there
is no attempt to do some stress testing of the state machine.
E.g. how hard/feasible would it be to construct a test that attempts
to exercise both branches of the (state == BPF_TW_SCHEDULED) in the
bpf_task_work_cancel_and_free()?

>  .../selftests/bpf/prog_tests/test_task_work.c | 149 ++++++++++++++++++
>  tools/testing/selftests/bpf/progs/task_work.c | 108 +++++++++++++
>  .../selftests/bpf/progs/task_work_fail.c      |  98 ++++++++++++
>  3 files changed, 355 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/test_task_work.c
>  create mode 100644 tools/testing/selftests/bpf/progs/task_work.c
>  create mode 100644 tools/testing/selftests/bpf/progs/task_work_fail.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/test_task_work.c b/tools/testing/selftests/bpf/prog_tests/test_task_work.c
> new file mode 100644
> index 000000000000..9c3c7a46a827
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/test_task_work.c
> @@ -0,0 +1,149 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
> +#include <test_progs.h>
> +#include <string.h>
> +#include <stdio.h>
> +#include "task_work.skel.h"
> +#include "task_work_fail.skel.h"
> +#include <linux/bpf.h>
> +#include <linux/perf_event.h>
> +#include <sys/syscall.h>
> +#include <time.h>
> +
> +static int perf_event_open(__u32 type, __u64 config, int pid)
> +{
> +	struct perf_event_attr attr = {
> +		.type = type,
> +		.config = config,
> +		.size = sizeof(struct perf_event_attr),
> +		.sample_period = 100000,
> +	};
> +
> +	return syscall(__NR_perf_event_open, &attr, pid, -1, -1, 0);
> +}
> +
> +struct elem {
> +	char data[128];
> +	struct bpf_task_work tw;
> +};
> +
> +static int verify_map(struct bpf_map *map, const char *expected_data)
> +{
> +	int err;
> +	struct elem value;
> +	int processed_values = 0;
> +	int k, sz;
> +
> +	sz = bpf_map__max_entries(map);
> +	for (k = 0; k < sz; ++k) {
> +		err = bpf_map__lookup_elem(map, &k, sizeof(int), &value, sizeof(struct elem), 0);
> +		if (err)
> +			continue;
> +		if (!ASSERT_EQ(strcmp(expected_data, value.data), 0, "map data")) {
> +			fprintf(stderr, "expected '%s', found '%s' in %s map", expected_data,
> +				value.data, bpf_map__name(map));
> +			return 2;
> +		}
> +		processed_values++;
> +	}
> +
> +	return processed_values == 0;

Nit: check for exact number of expected values here?

> +}
> +
> +static void task_work_run(const char *prog_name, const char *map_name)
> +{
> +	struct task_work *skel;
> +	struct bpf_program *prog;
> +	struct bpf_map *map;
> +	struct bpf_link *link;
> +	int err, pe_fd = 0, pid, status, pipefd[2];
> +	char user_string[] = "hello world";
> +
> +	if (!ASSERT_NEQ(pipe(pipefd), -1, "pipe"))
> +		return;
> +
> +	pid = fork();

Nit: check for negative return value?

> +	if (pid == 0) {
> +		__u64 num = 1;
> +		int i;
> +		char buf;
> +
> +		close(pipefd[1]);
> +		read(pipefd[0], &buf, sizeof(buf));
> +		close(pipefd[0]);
> +
> +		for (i = 0; i < 10000; ++i)
> +			num *= time(0) % 7;
> +		(void)num;
> +		exit(0);
> +	}
> +	skel = task_work__open();
> +	if (!ASSERT_OK_PTR(skel, "task_work__open"))
> +		return;
> +
> +	bpf_object__for_each_program(prog, skel->obj) {
> +		bpf_program__set_autoload(prog, false);
> +	}
> +
> +	prog = bpf_object__find_program_by_name(skel->obj, prog_name);
> +	if (!ASSERT_OK_PTR(prog, "prog_name"))
> +		goto cleanup;
> +	bpf_program__set_autoload(prog, true);
> +	bpf_program__set_type(prog, BPF_PROG_TYPE_PERF_EVENT);

Nit: this is not really necessary, the programs are already defined as
     SEC("perf_event").

> +	skel->bss->user_ptr = (char *)user_string;
> +
> +	err = task_work__load(skel);
> +	if (!ASSERT_OK(err, "skel_load"))
> +		goto cleanup;

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