Add option for configuring the location of the container's host files…

…ystem (#2187)

This allows the system module to read metrics related to the host machine rather than the container. When using the -system.hostfs flag, directories like /proc and /sys are assumed to be mounted relative to the path given in the value. Here's an example usage.

`docker run --volume=/:/hostfs:ro --net=host --name=metricbeat -d metricbeat -system.hostfs=/hostfs`

And here is a more restrictive example which does not mount all of the host's filesystems inside the container. system/filesystem will not be able to report metrics for the host's filesystem.

`docker run --volume=/proc:/hostfs/proc:ro --volume=/sys/fs/cgroup:/hostfs/sys/fs/cgroup --net=host --name=metricbeat -d metricbeat -system.hostfs=/hostfs`

metricbeat/module/system/process/process.go

@@ -3,12 +3,14 @@
 package process
 
 import (
+	"fmt"
 	"runtime"
 	"strconv"
 
 	"github.com/elastic/beats/libbeat/common"
 	"github.com/elastic/beats/libbeat/logp"
 	"github.com/elastic/beats/metricbeat/mb"
+	"github.com/elastic/beats/metricbeat/module/system"
 
 	"github.com/elastic/gosigar/cgroup"
 	"github.com/pkg/errors"
@@ -54,7 +56,12 @@ func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
 	}
 
 	if runtime.GOOS == "linux" {
-		m.cgroup, err = cgroup.NewReader("", true)
+		systemModule, ok := base.Module().(*system.Module)
+		if !ok {
+			return nil, fmt.Errorf("unexpected module type")
+		}
+
+		m.cgroup, err = cgroup.NewReader(systemModule.HostFS, true)
 		if err != nil {
 			return nil, errors.Wrap(err, "error initializing cgroup reader")
 		}

metricbeat/module/system/system.go

@@ -1,6 +1,39 @@
 package system
 
-import "math"
+import (
+	"flag"
+	"math"
+	"sync"
+
+	"github.com/elastic/beats/metricbeat/mb"
+)
+
+var (
+	HostFS = flag.String("system.hostfs", "", "mountpoint of the host's filesystem for use in monitoring a host from within a container")
+)
+
+var once sync.Once
+
+func init() {
+	// Register the ModuleFactory function for the "system" module.
+	if err := mb.Registry.AddModule("system", NewModule); err != nil {
+		panic(err)
+	}
+}
+
+type Module struct {
+	mb.BaseModule
+	HostFS string // Mountpoint of the host's filesystem for use in monitoring inside a container.
+}
+
+func NewModule(base mb.BaseModule) (mb.Module, error) {
+	// This only needs to be configured once for all system modules.
+	once.Do(func() {
+		configureHostFS()
+	})
+
+	return &Module{BaseModule: base, HostFS: *HostFS}, nil
+}
 
 func Round(val float64, roundOn float64, places int) (newVal float64) {
 	var round float64

metricbeat/module/system/system_linux.go

@@ -0,0 +1,23 @@
+package system
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/elastic/gosigar"
+)
+
+func configureHostFS() {
+	dir := *HostFS
+	if dir == "" {
+		dir = "/"
+	}
+
+	// Set environment variables for gopsutil.
+	os.Setenv("HOST_PROC", filepath.Join(dir, "/proc"))
+	os.Setenv("HOST_SYS", filepath.Join(dir, "/sys"))
+	os.Setenv("HOST_ETC", filepath.Join(dir, "/etc"))
+
+	// Set proc location for gosigar.
+	gosigar.Procd = filepath.Join(dir, "/proc")
+}

metricbeat/module/system/system_other.go

@@ -0,0 +1,7 @@
+// +build !linux
+
+package system
+
+func configureHostFS() {
+	// Stub method for non-linux.
+}