netdev - Re: [PATCH bpf-next] selftests/bpf: don't use bpf helpers in non-bpf environment

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Date:   Wed, 12 Dec 2018 02:56:39 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Stanislav Fomichev <sdf@...ichev.me>,
        Stanislav Fomichev <sdf@...gle.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, ast@...nel.org
Subject: Re: [PATCH bpf-next] selftests/bpf: don't use bpf helpers in non-bpf
 environment

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?

>> 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 <sdf@...gle.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.

>>  	.iph.ihl = 5,
>>  	.iph.protocol = 6,
>> -	.iph.tot_len = bpf_htons(MAGIC_BYTES),
>> +	.iph.tot_len = __constant_htons(MAGIC_BYTES),
>>  	.tcp.urg_ptr = 123,
>>  };
>>  
>> @@ -64,9 +64,9 @@ static struct {
>>  	struct ipv6hdr iph;
>>  	struct tcphdr tcp;
>>  } __packed pkt_v6 = {
>> -	.eth.h_proto = bpf_htons(ETH_P_IPV6),
>> +	.eth.h_proto = __constant_htons(ETH_P_IPV6),
>>  	.iph.nexthdr = 6,
>> -	.iph.payload_len = bpf_htons(MAGIC_BYTES),
>> +	.iph.payload_len = __constant_htons(MAGIC_BYTES),
>>  	.tcp.urg_ptr = 123,
>>  };
>>  
>> -- 
>> 2.20.0.rc2.403.gdbc3b29805-goog
>>