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

[Leon Hwang]

On 18/7/26 06:49, Emil Tsalapatis wrote:
> 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@xxxxxxxxx>
>> ---
>>  .../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.


We are unable to implement it with a syscall example, because libbpf
does not support creating a rdonly internal map for global percpu data.
We have no '.ropercpu' section support in libbpf.

Will add comment to explain the reason of hardcoded insns.

Thanks,
Leon

> [...]