Subject: Re: [PATCH mptcp-next v13 4/4] selftests/bpf: Add mptcpify test

From: Martin KaFai Lau <[email-masked]>
Date: Tue, 15 Aug 2023 11:57:52 -0700

On 8/15/23 3:08 AM, Geliang Tang wrote:
> On Mon, Aug 14, 2023 at 11:23:49PM -0700, Martin KaFai Lau wrote:
>> On 8/11/23 7:54 PM, Geliang Tang wrote:
>>> +static int verify_mptcpify(int server_fd)
>>> +{
>>> +	socklen_t optlen;
>>> +	char cmd[256];
>>> +	int protocol;
>>> +	int err = 0;
>>> +
>>> +	optlen = sizeof(protocol);
>>> +	if (!ASSERT_OK(getsockopt(server_fd, SOL_SOCKET, SO_PROTOCOL, &protocol, &optlen),
>>> +		       "getsockopt(SOL_PROTOCOL)"))
>>> +		return -1;
>>> +
>>> +	if (!ASSERT_EQ(protocol, IPPROTO_MPTCP, "protocol isn't MPTCP"))
>>> +		err++;
>>> +
>>> +	/* Output of nstat:
>>> +	 *
>>> +	 * #kernel
>>> +	 * MPTcpExtMPCapableSYNACKRX       1                  0.0
>>> +	 */
>>> +	snprintf(cmd, sizeof(cmd),
>>> +		 "ip netns exec %s nstat -asz %s | awk '%s' | grep -q '%s'",
>>> +		 NS_TEST, "MPTcpExtMPCapableSYNACKRX",
>>> +		 "NR==1 {next} {print $2}", "1");
>>
>> Is the mp-capable something that the regular mptcp user want to learn from a
>> fd also? Does it have a simpler way like to learn this, eg. getsockopt(fd,
>> SOL_MPTCP, MPTCP_xxx), instead of parsing text output?
> 
> Thanks Martin. Yes, you're right. A better one is using getsockopt
> (MPTCP_INFO) to get the mptcpi_flags, then test the FALLBACK bit to make
> sure this MPTCP connection didn't fallback. This is, in other word, this
> MPTCP connection has been established correctly. Something like this:
> 
> +       optlen = sizeof(info);
> +       if (!ASSERT_OK(getsockopt(fd, SOL_MPTCP, MPTCP_INFO, &info, &optlen),
> +                      "getsockopt(MPTCP_INFO)"))
> +               return -1;
> +
> +       if (!ASSERT_FALSE(info.mptcpi_flags & MPTCP_INFO_FLAG_FALLBACK,
> +                         "MPTCP fallback"))
> +               err++;
> 
> It's necessary to add this further check after the MPTCP protocol check
> using getsockopt(SOL_PROTOCOL). Since in some cases, the MPTCP protocol
> check is not enough. Say, if we change TCP protocol into MPTCP using
> "cgroup/sock_create", the hook of BPF_CGROUP_RUN_PROG_INET_SOCK in
> inet_create(), this place is too late to change the protocol. Although
> sk->sk_protocol is set to MPTCP correctly, and the MPTCP protocol check
> using getsockopt(SOL_PROTOCOL) will pass. This MPTCP connection will
> fallback to TCP connection. So this further check is needed.

If I read it correctly, it sounds like the "ip netns... nstat.... awk...grep..." 
can be replaced with the getsockopt(MPTCP_INFO)?  If that is the case, could you 
respin one more time to do getsockopt(MPTCP_INFO) instead? I would like the test 
to avoid parsing text output and have less dependency on external binary/library 
if possible (On top of 'ip', the above uses nstat, awk, grep...). This will make 
the bpf CI and other developers' environment tricky to maintain. eg. fwiw, 
although unrelated to the commands used above, my dev environment suddenly like 
to give this text output when I run "e"grep: "egrep: warning: egrep is 
obsolescent; using grep -E"

>>
>>> +	if (!ASSERT_OK(system(cmd), "No MPTcpExtMPCapableSYNACKRX found!"))
>>
>>