segfault due to race in ebpf initialisation

[rade]

Apply

diff --git a/probe/endpoint/ebpf.go b/probe/endpoint/ebpf.go
index ff23b4e2..f9d470b5 100644
--- a/probe/endpoint/ebpf.go
+++ b/probe/endpoint/ebpf.go
@@ -7,6 +7,7 @@ import (
        "strconv"
        "sync"
        "syscall"
+       "time"
 
        log "github.com/Sirupsen/logrus"
        "github.com/weaveworks/common/fs"
@@ -96,6 +97,9 @@ func newEbpfTracker() (eventTracker, error) {
                tracer:          t,
        }
 
+       log.Infof("delaying setting of ebpfTracker")
+       time.Sleep(5 * time.Second)
+
        ebpfTracker = tracker
        return tracker, nil
 }

and then start a probe

$ sudo prog/scope --mode=probe --weave=false
INFO[0000] publishing to: 127.0.0.1:4040                
<probe> INFO: 2017/07/06 14:06:01.763599 command line args: --mode=probe --weave=false
<probe> INFO: 2017/07/06 14:06:01.763676 probe starting, version 0dbea45e, ID 3f9c8145f01926a1
<probe> ERRO: 2017/07/06 14:06:01.766805 Error fetching app details: Get http://127.0.0.1:4040/api: dial tcp 127.0.0.1:4040: getsockopt: connection refused
<probe> INFO: 2017/07/06 14:06:02.267125 delaying setting of ebpfTracker
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x176e70a]

goroutine 73 [running]:
github.com/weaveworks/scope/probe/endpoint.(*EbpfTracker).handleConnection(0x0, 0xc400000003, 0xc4219699f0, 0xc, 0xc421969a10, 0xd, 0x1bba04a, 0x5315, 0xc421969a30, 0xa)
	/go/src/github.com/weaveworks/scope/probe/endpoint/ebpf.go:208 +0x3a
github.com/weaveworks/scope/probe/endpoint.tcpEventCbV4(0x94fc54310ad1, 0x3, 0x531500000003, 0xc421969978, 0x5, 0xc421969990, 0x10, 0x10, 0xc4219699b0, 0x10, ...)
	/go/src/github.com/weaveworks/scope/probe/endpoint/ebpf.go:126 +0x217
github.com/weaveworks/scope/vendor/github.com/weaveworks/tcptracer-bpf/pkg/tracer.NewTracer.func1(0xc422e689c0, 0xc420468240, 0x287bf60, 0xc420468300, 0x287bf58)
	/go/src/github.com/weaveworks/scope/vendor/github.com/weaveworks/tcptracer-bpf/pkg/tracer/tracer.go:84 +0x1d6
created by github.com/weaveworks/scope/vendor/github.com/weaveworks/tcptracer-bpf/pkg/tracer.NewTracer
	/go/src/github.com/weaveworks/scope/vendor/github.com/weaveworks/tcptracer-bpf/pkg/tracer/tracer.go:89 +0x6d3

That's because the tracer invokes callback functions that dispatch via a global(!) var which we can only set after initialising the tracer. Chicken and egg. Easy to fix.

[2opremio]

CC @alban

[rade]

I'm on it, @fons.