add containerd cli opts

Signed-off-by: Avi Deitcher <avi@deitcher.net>

docs/faq.md

@@ -37,6 +37,29 @@ If you're not seeing `containerd` logs in the console during boot, make sure tha
 
 `init` and other processes like `containerd` will use the last defined console in the kernel `cmdline`. When using `qemu`, to see the console you need to list `ttyS0` as the last console to properly see the output.
 
+## Enabling debug or trace log levels on containerd
+
+On startup, linuxkit looks for and parses a file `/etc/containerd/cli-opts`. If it exists, the content is used as arguments to containerd. Thus, to enable
+a higher log level, for example `debug`, create a file whose contents are `--log-level debug` and place it on the image:
+
+```yml
+files:
+  - path: /etc/containerd/cli-opts
+    contents: "--log-level debug"
+```
+
+Note that the package that parses the contents splits on _all_ whitespace. It does not, as of this writing, support shell-like parsing, so the following will work:
+
+```
+--log-level debug --arg abcd
+```
+
+while the following will not:
+
+```
+--log-level debug --arg 'abcd def'
+```
+
 ## Troubleshooting containers
 
 Linuxkit runs all services in a specific `containerd` namespace called `services.linuxkit`. To list all the defined containers:

pkg/init/cmd/service/system_init.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"strings"
 	"syscall"
 	"time"
 
@@ -17,6 +18,10 @@ import (
 	log "github.com/sirupsen/logrus"
 )
 
+const (
+	containerdOptsFile = "/etc/containerd/cli-opts"
+)
+
 func cleanupTask(ctx context.Context, ctr containerd.Container) error {
 	task, err := ctr.Task(ctx, nil)
 	if err != nil {
@@ -78,8 +83,14 @@ func systemInitCmd(ctx context.Context, args []string) {
 	// remove (unlikely) old containerd socket
 	_ = os.Remove(*sock)
 
+	// look for containerd options
+	ctrdArgs := []string{}
+	if b, err := ioutil.ReadFile(containerdOptsFile); err != nil {
+		ctrdArgs = strings.Fields(string(b))
+	}
+
 	// start up containerd
-	cmd := exec.Command(*binary)
+	cmd := exec.Command(*binary, ctrdArgs...)
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 	if err := cmd.Start(); err != nil {