Since https://github.com/weaveworks/tcptracer-bpf/pull/39, tcptracer-bpf
can generate "fd_install" events when a process installs a new file
descriptor in its fd table. Those events must be requested explicitely
on a per-pid basis with tracer.AddFdInstallWatcher(pid).
This is useful to know about "accept" events that would otherwise be
missed because kretprobes are not triggered for functions that were
called before the installation of the kretprobe.
This patch find all the processes that are currently blocked on an
accept() syscall during the EbpfTracker initialization.
feedInitialConnections() will use tracer.AddFdInstallWatcher() to
subscribe to fd_install events. When a fd_install event is received,
synthesise an accept event with the connection tuple and the network
namespace (from /proc).
The header checking code was unsafe because:
1. It was accessing the byteslice at [2] without ensuring a length >= 3
2. It was assuming that the indentation of the 'sl' header is always 2 (which seems to be the case in recent kernels 8f18e4d03e/net/ipv4/tcp_ipv4.c (L2304) and 8f18e4d03e/net/ipv6/tcp_ipv6.c (L1831) ) but it's more robust to simply trim the byteslice.
