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

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Date:   Tue, 11 Dec 2018 19:13:36 -0800
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Stanislav Fomichev <sdf@...gle.com>, 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 12/12, 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 <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.
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.

> >>  	.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
> >>
>