[Cake] access to cmsg from go?

[Cake] access to cmsg from go?

[Dave Taht]

anyone have any good ideas here? https://github.com/golang/go/issues/46831

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Dave Täht CTO, TekLibre, LLC

Re: [Cake] access to cmsg from go?

[Matt Mathis]

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Is there running code in C?

Thanks,
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On Sat, Jun 19, 2021 at 2:59 PM Dave Taht <dave.taht@gmail.com> wrote:

> anyone have any good ideas here? https://github.com/golang/go/issues/46831
>
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>
> Dave Täht CTO, TekLibre, LLC
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Re: [Cake] [Bloat] access to cmsg from go?

[Hal Murray]

> Is there running code in C?

Yes.  The NTPsec code is full of #ifdef kludgery.

The C API started with SO_TIMESTAMP for microsecond precision then added 
SO_TIMESTAMPNS for nanosecond precision: timeval vs timespec

There is also SO_TIMESTAMPNS vs SCM_TIMESTAMPNS

The basic idea is to use recvmsg rather than recv.  In addition to the data 
buffer, you feed it another buffer where it can put meta data like timestamps. 
 Then you have to scan that buffer to find the part you want.  Both buffers 
are passed to the kernel via a single msghdr parameter (which also has a 
pointer for the addr you get via recvfrom).

Details are in man recvmsg and man cmsg
I thought they were reasonably clear.

Without the ifdefs:

You have to start by using setsockopt to turn on SO_TIMESTAMPNS.
  setsockopt(fd, SOL_SOCKET, SO_TIMESTAMPNS,
                               (const void *)&on, sizeof(on)));

The setup code for recvmsg:
        struct msghdr msghdr;
        struct iovec iovec;
        char control[100];   /* FIXME: Need space for time stamp plus overhead 
*/

        iovec.iov_base        = &rb->recv_buffer;
        iovec.iov_len         = sizeof(rb->recv_buffer);
        memset(&msghdr, '\0', sizeof(msghdr));
        msghdr.msg_name       = &rb->recv_srcadr;
        msghdr.msg_namelen    = fromlen;
        msghdr.msg_iov        = &iovec;
        msghdr.msg_iovlen     = 1;
        msghdr.msg_flags      = 0;
        msghdr.msg_control    = (void *)&control;
        msghdr.msg_controllen = sizeof(control);

The actual call:
        buflen                = recvmsg(fd, &msghdr, 0);

The extract code:
        struct timespec *       tsp;
        cmsghdr = CMSG_FIRSTHDR(msghdr);
        if (NULL == cmsghdr) {
             extra checking because this code is in a subroutine
             error
        }
        if (SCM_TIMESTAMPNS != cmsghdr->cmsg_type) {
             There is only one -- no need to loop
             error
        }
        tsp = (struct timespec *)CMSG_DATA(cmsghdr);


Actual code is in:
  read_network_packet in ntpd/ntp_io.c
  https://gitlab.com/NTPsec/ntpsec/-/blob/master/ntpd/ntp_io.c
and
  fetch_packetstamp() in ntpd/ntp_packetstamp.c
  https://gitlab.com/NTPsec/ntpsec/-/blob/master/ntpd/ntp_packetstamp.c


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Re: [Cake] [Cerowrt-devel] access to cmsg from go?

[David P. Reed]

(They closed the issue on the golang link.)


I'm not a golang user. One language too many for me. It sounds like a library issue.

My suggestion would be to use the openness of open source. Generate a patchset that extends the interface properly. Don't try to "improve" what you don't like - communities like stability and backward compatibility. Explain the added semantics in documentation.

Then, maintain your fork. I don't know how the golang community works with versioning of libraries, but Python, Rust, Haskell, and NodeJS all have ways to let projects use variants of libraries.

Then, submit that patchset upstream to the golang community. Advocate for upstreaming it, and develop a community that uses the patched library. Eventually, you may be able to stop maintaining your variant toolset. Or you will develop an alternat library user base that disagrees with upstream's decisions.

(Analogy, Android's Linux Kernel vs. Linus Torvalds's. Google Android rejects to some extent Linus's crew's unwillingness to accept what Android needs as improvements.)

This is "modern open source community" practice for getting things done. Pragmatic innovations in shared codebases sometimes have to wait for the original egos to die off.

Re: [Cake] [Cerowrt-devel] access to cmsg from go?

[Etienne Champetier]

Hello Dave,

Le sam. 19 juin 2021 à 17:59, Dave Taht <dave.taht@gmail.com> a écrit :
>
> anyone have any good ideas here? https://github.com/golang/go/issues/46831

(with the list in cc)

Some googling that might (or not) help:
https://github.com/google/gopacket/blob/master/pcapgo/capture.go
https://github.com/gracig/goping/blob/master/pingers/icmpv4/icmpv4_linux.go#L217

Best
Etienne

>
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