On 12/12/2018 04:13 AM, Stanislav Fomichev wrote:
> On 12/12, Daniel Borkmann wrote:
>> On 12/11/2018 10:49 PM, Stanislav Fomichev wrote:
>>> On 12/11, Stanislav Fomichev wrote:
>>>> We're using bpf_htons in test_progs.c to initialize some static
>>>> global data and I think I hit some weird case on an older compiler
>>>> which doesn't have __builtin_bswap16 (and __builtin_constant_p
>>>> expands to false).
>>>>
>>>> In this case I see:
>>>> error: implicit declaration of function '__builtin_bswap16'
>>
>> Is that gcc < 4.8?
> Yes.
>
>>>> Let's explicitly use __constant_htons which should be exposed by the
>>>> linux/byteorder.h uapi header.
>>>
>>> Forgot to mention, that using simple htons produces the following:
>>> test_progs.c:54:17: error: braced-group within expression allowed only
>>> inside a function
>>> .eth.h_proto = htons(ETH_P_IP),
>>>
>>>> Signed-off-by: Stanislav Fomichev <s...@google.com>
>>>> ---
>>>> tools/testing/selftests/bpf/test_progs.c | 8 ++++----
>>>> 1 file changed, 4 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/tools/testing/selftests/bpf/test_progs.c
>>>> b/tools/testing/selftests/bpf/test_progs.c
>>>> index 26f1fdf3e2bf..61593d319c0e 100644
>>>> --- a/tools/testing/selftests/bpf/test_progs.c
>>>> +++ b/tools/testing/selftests/bpf/test_progs.c
>>>> @@ -51,10 +51,10 @@ static struct {
>>>> struct iphdr iph;
>>>> struct tcphdr tcp;
>>>> } __packed pkt_v4 = {
>>>> - .eth.h_proto = bpf_htons(ETH_P_IP),
>>>> + .eth.h_proto = __constant_htons(ETH_P_IP),
>>
>> If the __builtin_constant_p() evaluated to false on the constants (?),
>> wouldn't using the __bpf_constant_htons() directly work as well given
>> it's not using a builtin either? Should be fine either way though using
>> the same api/header might be slightly nicer.
> I got it wrong, __builtin_constant_p() evaluates correctly, I played
> with it a bit. But for some reason it still complains about that branch
> that it doesn't take :-/
>
> Using __bpf_constant_htons() is a good idea, I'll follow up with a v2.
Ok, sounds good, thanks!
