allow 'force to MPTCP' mode: sysctl

[pabeni]

we need a system/per netns switch to allow forcing existing, unmodified, application to create by default MPTCP sockets instead of TCP one.

A current, naive, approach is via syscall hijacking:

https://github.com/pabeni/mptcp-tools/tree/master/use_mptcp

possible alternatives could be via systemtap, eBPF (do we have a suitable eBPF hook?) or per netns sysctl.

[mjmartineau]

We don't have a suitable eBPF hook yet. We've discussed adding BPF_CGROUP_SOCKET to allow per-cgroup override of proto in the socket() call.

[matttbe]

Another way to do that, here with systemtap, from @dcaratti :

#! /usr/bin/env stap

%{
#include <linux/in.h>
#include <linux/ip.h>
%}

/* according to [1], RSI contains 'type' and RDX
 * contains 'protocol'.
 * [1] https://github.com/torvalds/linux/blob/master/arch/x86/entry/entry_64.S#L79
 */

function mptcpify () %{
    if (CONTEXT->kregs->si == SOCK_STREAM &&
        (CONTEXT->kregs->dx == IPPROTO_TCP ||
         CONTEXT->kregs->dx == 0)) {
                CONTEXT->kregs->dx = IPPROTO_MPTCP;
                STAP_RETVALUE = 1;
    } else {
         STAP_RETVALUE = 0;
    }
%}

probe kernel.function("__sys_socket") {
        if (mptcpify() == 1) {
                printf("command %16s mptcpified\n", execname());
        }
}

[ericcurtin]

This is what I get when I try this on my linux system @matttbe

curtine@G5-5587:~/git$ uname -a
Linux G5-5587 5.8.5-050805-generic #202008270831 SMP Thu Aug 27 08:33:39 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
curtine@G5-5587:~/git$ sudo stap -vg mpctp.stap
Pass 1: parsed user script and 476 library scripts using 115492virt/90800res/7356shr/83308data kb, in 160usr/20sys/183real ms.
semantic error: while resolving probe point: identifier 'kernel' at mpctp.stap:24:7
        source: probe kernel.function("__sys_socket") {
                      ^

semantic error: missing x86_64 kernel/module debuginfo [man warning::debuginfo] under '/lib/modules/5.8.5-050805-generic/build'

Pass 2: analyzed script: 0 probes, 0 functions, 1 embed, 0 globals using 117340virt/93232res/7980shr/85156data kb, in 50usr/60sys/115real ms.
Pass 2: analysis failed.  [man error::pass2]
Tip: /usr/share/doc/systemtap/README.Debian should help you get started.

Any ideas?

[dcaratti]

On Mon, 2020-08-31 at 11:17 -0700, Eric Curtin wrote: This is what I get when I try this on my linux system @matttbe ***@***.***:~/git$ uname -a Linux G5-5587 5.8.5-050805-generic #202008270831 SMP Thu Aug 27 08:33:39 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux ***@***.***:~/git$ sudo stap -vg mpctp.stap Pass 1: parsed user script and 476 library scripts using 115492virt/90800res/7356shr/83308data kb, in 160usr/20sys/183real ms. semantic error: while resolving probe point: identifier 'kernel' at mpctp.stap:24:7 source: probe kernel.function("__sys_socket") { ^

hello,

semantic error: missing x86_64 kernel/module debuginfo [man warning::debuginfo] under '/lib/modules/5.8.5-050805-generic/build'

mptcp.stap builds a kernel probe and attachs to __sys_socket() to change IPPROTO_TCP into IPPROTO_MPTCP. In your case, I see that stap fails finding the debug information for the kernel executable. On Fedora, it's fixed installing kernel-debuginfo- stuff. On Ubuntu, I think you need to do this: $ sudo apt-get install linux-image-$(uname -r)-dbgsym (more instructions at https://wiki.ubuntu.com/Kernel/Systemtap) thanks! -- davide