strip rootfs prefix for run in docker

- relates to prometheus#66

Signed-off-by: Ivan Mikheykin <ivan.mikheykin@flant.com>

README.md

@@ -144,12 +144,23 @@ The node\_exporter is designed to monitor the host system. It's not recommended
 to deploy it as Docker container because it requires access to the host system.
 Be aware that any non-root mount points you want to monitor will need bind-mounted
 into the container.
+If you start container for host monitoring, specify `path.rootfs` argument.
+This argument must match path in bind-mount of host root. The node\_exporter will use
+`path.rootfs` as prefix to filter entries in ${path.procfs}/mounts and to
+cleanup it from `mountpoint` label. You also need bind-mount host proc and host sys
+with `path.procfs` and `path.sysfs` arguments.
+Also you need to use `bind-propagation=rslave` option for bind-mount host rootfs to
+propagate host mounts changes to container (option available since 17.05.0).
 
 ```bash
 docker run -d \
   --net="host" \
   --pid="host" \
-  quay.io/prometheus/node-exporter
+  -v "/proc:/host/proc:ro" -v "/sys:/host/sys:ro" \
+  --mount "type=bind,source=/,target=/rootfs,readonly,bind-propagation=rslave" \
+  quay.io/prometheus/node-exporter \
+  --path.procfs /host/proc --path.sysfs /host/sys \
+  --path.rootfs /rootfs
 ```
 
 On some systems, the `timex` collector requires an additional Docker flag,

collector/filesystem_linux.go

@@ -70,7 +70,7 @@ func (c *filesystemCollector) GetStats() ([]filesystemStats, error) {
 		go stuckMountWatcher(labels.mountPoint, success)
 
 		buf := new(syscall.Statfs_t)
-		err = syscall.Statfs(labels.mountPoint, buf)
+		err = syscall.Statfs(rootfsFilePath(labels.mountPoint), buf)
 
 		stuckMountsMtx.Lock()
 		close(success)
@@ -86,7 +86,7 @@ func (c *filesystemCollector) GetStats() ([]filesystemStats, error) {
 				labels:      labels,
 				deviceError: 1,
 			})
-			log.Debugf("Error on statfs() system call for %q: %s", labels.mountPoint, err)
+			log.Debugf("Error on statfs() system call for %q: %s", rootfsFilePath(labels.mountPoint), err)
 			continue
 		}
 
@@ -143,6 +143,10 @@ func mountPointDetails() ([]filesystemLabels, error) {
 	scanner := bufio.NewScanner(file)
 	for scanner.Scan() {
 		parts := strings.Fields(scanner.Text())
+		// skip non rootfs paths if rootfsPath defined
+		if !rootfsPathDetect(parts[1]) {
+			continue
+		}
 
 		// Ensure we handle the translation of \040 and \011
 		// as per fstab(5).
@@ -151,7 +155,7 @@ func mountPointDetails() ([]filesystemLabels, error) {
 
 		filesystems = append(filesystems, filesystemLabels{
 			device:     parts[0],
-			mountPoint: parts[1],
+			mountPoint: rootfsStripPrefix(parts[1]),
 			fsType:     parts[2],
 			options:    parts[3],
 		})

collector/paths.go

@@ -15,15 +15,17 @@ package collector
 
 import (
 	"path"
+	"strings"
 
 	"github.com/prometheus/procfs"
 	"gopkg.in/alecthomas/kingpin.v2"
 )
 
 var (
 	// The path of the proc filesystem.
-	procPath = kingpin.Flag("path.procfs", "procfs mountpoint.").Default(procfs.DefaultMountPoint).String()
-	sysPath  = kingpin.Flag("path.sysfs", "sysfs mountpoint.").Default("/sys").String()
+	procPath   = kingpin.Flag("path.procfs", "procfs mountpoint.").Default(procfs.DefaultMountPoint).String()
+	sysPath    = kingpin.Flag("path.sysfs", "sysfs mountpoint.").Default("/sys").String()
+	rootfsPath = kingpin.Flag("path.rootfs", "rootfs mountpoint.").Default("").String()
 )
 
 func procFilePath(name string) string {
@@ -33,3 +35,30 @@ func procFilePath(name string) string {
 func sysFilePath(name string) string {
 	return path.Join(*sysPath, name)
 }
+
+func rootfsFilePath(name string) string {
+	if name == "/" {
+		return *rootfsPath
+	}
+	return path.Join(*rootfsPath, name)
+}
+
+func rootfsStripPrefix(path string) string {
+	if path == *rootfsPath {
+		return "/"
+	}
+	if strings.HasPrefix(path, *rootfsPath) {
+		return strings.TrimPrefix(path, *rootfsPath)
+	}
+	return path
+}
+
+func rootfsPathDetect(path string) bool {
+	if *rootfsPath == "" {
+		return true
+	}
+	if strings.HasPrefix(path, *rootfsPath) {
+		return true
+	}
+	return false
+}