Re: [PATCH bpf-next v5 6/9] selftests/bpf: Add tests to verify global percpu data

[Leon Hwang <leon.hwang@linux.dev>]

On 8/6/26 23:20, sashiko-bot@kernel.org wrote:
[...]
> 
>> +	/* run on every CPU */
>> +	for (i = 0; i < num_online; i++) {
>> +		__u64 flags;
>> +
>> +		if (!online[i])
>> +			continue;
>> +
>> +		topts.cpu = i;
>> +		topts.retval = 0;
>> +		err = bpf_prog_test_run_opts(prog_fd, &topts);
>> +		ASSERT_OK(err, "bpf_prog_test_run_opts");
>> +		ASSERT_EQ(topts.retval, 0, "bpf_prog_test_run_opts retval");
>> +
>> +		flags = ((__u64) i << 32) | BPF_F_CPU;
>> +		err = bpf_map__lookup_elem(map, &key, sizeof(key), data, elem_sz, flags);
> 
> [Severity: Medium]
> When looking up a per-CPU map element for a specific CPU using the BPF_F_CPU
> flag, libbpf's validate_map_op() strictly requires that value_sz exactly matches
> map->def.value_size.
> 
> By rounding up sizeof(*data) to a multiple of 8 and passing it as elem_sz
> here, doesn't this make the test brittle?
> 
> It currently passes because the underlying .percpu section happens to be
> exactly 72 bytes (a multiple of 8). If the variables in the BPF program were
> changed such that the section size was 76 bytes, the roundup would yield 80,
> which wouldn't match map->def.value_size and would cause
> bpf_map__lookup_elem() to fail with -EINVAL.
> 
> Could we use the un-rounded size here instead?
> 
Good catch.

Will drop the roundup().

Thanks,
Leon