Re: [PATCH bpf-next v10 7/9] selftests/bpf: Test direct reading/writing read-only percpu_array map

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Wed Jul 15, 2026 at 11:32 AM EDT, Leon Hwang wrote:
> Verify these two cases:
>
> 1. Direct reading the data of read-only percpu data's percpu_array map
>    is allowed.
> 2. Direct writing the data of read-only percpu data's percpu_array map
>    is disallowed.
>
> Assisted-by: Codex:gpt-5.5-xhigh
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> ---
>  .../bpf/prog_tests/global_data_init.c         | 83 +++++++++++++++++++
>  1 file changed, 83 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/global_data_init.c b/tools/testing/selftests/bpf/prog_tests/global_data_init.c
> index f7a56e3fdbce..d4405646847a 100644
> --- a/tools/testing/selftests/bpf/prog_tests/global_data_init.c
> +++ b/tools/testing/selftests/bpf/prog_tests/global_data_init.c
> @@ -199,6 +199,85 @@ static void test_global_percpu_data_lskel(void)
>  	test_global_percpu_data_lskel__destroy(lskel);
>  }
>  
> +static int create_rdonly_percpu_array(void)
> +{
> +	LIBBPF_OPTS(bpf_map_create_opts, map_opts,
> +		    .map_flags = BPF_F_RDONLY_PROG,
> +	);
> +	int key = 0, map_fd, err;
> +	__u64 value = 0;
> +
> +	map_fd = bpf_map_create(BPF_MAP_TYPE_PERCPU_ARRAY, "percpu_ro_map", sizeof(int),
> +				sizeof(__u64), 1, &map_opts);
> +	if (!ASSERT_GE(map_fd, 0, "bpf_map_create"))
> +		return -1;
> +
> +	err = bpf_map_update_elem(map_fd, &key, &value, BPF_F_ALL_CPUS);
> +	if (!ASSERT_OK(err, "bpf_map_update_elem"))
> +		goto out;
> +
> +	err = bpf_map_freeze(map_fd);
> +	if (!ASSERT_OK(err, "bpf_map_freeze"))
> +		goto out;
> +
> +	return map_fd;
> +
> +out:
> +	close(map_fd);
> +	return -1;
> +}
> +
> +static void test_global_percpu_data_rdonly_direct_read(void)
> +{
> +	struct bpf_insn insns[] = {
> +		BPF_LD_MAP_VALUE(BPF_REG_1, 0, 0),
> +		BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, 0),
> +		BPF_EXIT_INSN(),
> +	};

Why hardcode this as inline assembly? Can we do a syscall example in the
progs/? That way there will also be a concrete example of how we're
using the feature in practice.

> +	int map_fd, prog_fd;
> +
> +	map_fd = create_rdonly_percpu_array();
> +	if (map_fd < 0)
> +		return;
> +
> +	insns[0].imm = map_fd;
> +	prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, "percpu_ro_prog", "GPL", insns,
> +				ARRAY_SIZE(insns), NULL);
> +	if (ASSERT_GE(prog_fd, 0, "bpf_prog_load"))
> +		close(prog_fd);
> +	close(map_fd);
> +}
> +
> +static void test_global_percpu_data_rdonly_direct_write(void)
> +{
> +	LIBBPF_OPTS(bpf_prog_load_opts, prog_opts);
> +	struct bpf_insn insns[] = {
> +		BPF_LD_MAP_VALUE(BPF_REG_1, 0, 0),
> +		BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, 0),
> +		BPF_ST_MEM(BPF_DW, BPF_REG_1, 0, 0),
> +		BPF_EXIT_INSN(),
> +	};
> +	char log_buf[256] = {};
> +	int map_fd, prog_fd;
> +
> +	prog_opts.log_buf = log_buf;
> +	prog_opts.log_size = sizeof(log_buf);
> +	prog_opts.log_level = 1;
> +
> +	map_fd = create_rdonly_percpu_array();
> +	if (map_fd < 0)
> +		return;
> +
> +	insns[0].imm = map_fd;
> +	prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, "percpu_ro_prog", "GPL", insns,
> +				ARRAY_SIZE(insns), &prog_opts);
> +	if (!ASSERT_LT(prog_fd, 0, "bpf_prog_load"))
> +		close(prog_fd);
> +	else
> +		ASSERT_HAS_SUBSTR(log_buf, "write into map forbidden", "verifier log");
> +	close(map_fd);
> +}
> +
>  void test_global_percpu_data(void)
>  {
>  	if (!feat_supported(NULL, FEAT_PERCPU_DATA)) {
> @@ -210,4 +289,8 @@ void test_global_percpu_data(void)
>  		test_global_percpu_data_init();
>  	if (test__start_subtest("lskel"))
>  		test_global_percpu_data_lskel();
> +	if (test__start_subtest("rdonly_direct_read"))
> +		test_global_percpu_data_rdonly_direct_read();
> +	if (test__start_subtest("rdonly_direct_write"))
> +		test_global_percpu_data_rdonly_direct_write();
>  }