[continue 912] strip rootfs prefix for run in docker (#1058)

* strip rootfs prefix for run in docker
* Use `/` as default value of path.rootfs, and parse mounts from `/proc/1/mounts`.
* No need to mount `/proc` and `/sys` because we share host's PID
namespace, which allows processes within the container to see all of the
processes on the system.

Closes: #66

Signed-off-by: Ivan Mikheykin <ivan.mikheykin@flant.com>
Signed-off-by: Yecheng Fu <cofyc.jackson@gmail.com>

4 files changed, 48 insertions, 6 deletions

diff --git a/README.md b/README.md
index b3940c5..320b54f 100644
--- a/README.md
+++ b/README.md
@@ -144,12 +144,17 @@ The `node_exporter` is designed to monitor the host system. It's not recommended
 to deploy it as a Docker container because it requires access to the host system.
 Be aware that any non-root mount points you want to monitor will need to be 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 access host filesystem.
 
 ```bash
 docker run -d \
   --net="host" \
   --pid="host" \
-  quay.io/prometheus/node-exporter
+  -v "/:/host:ro,rslave" \
+  quay.io/prometheus/node-exporter \
+  --path.rootfs /host
 ```
 
 On some systems, the `timex` collector requires an additional Docker flag,
diff --git a/collector/filesystem_linux.go b/collector/filesystem_linux.go
index 5e2e37b..74701d4 100644
--- a/collector/filesystem_linux.go
+++ b/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
                 }
 
@@ -133,7 +133,7 @@ func stuckMountWatcher(mountPoint string, success chan struct{}) {
 }
 
 func mountPointDetails() ([]filesystemLabels, error) {
-        file, err := os.Open(procFilePath("mounts"))
+        file, err := os.Open(procFilePath("1/mounts"))
         if err != nil {
                 return nil, err
         }
diff --git a/collector/fixtures/proc/1/mounts b/collector/fixtures/proc/1/mounts
new file mode 100644
index 0000000..7452d49
--- /dev/null
+++ b/collector/fixtures/proc/1/mounts
@@ -0,0 +1,32 @@
+rootfs / rootfs rw 0 0
+sysfs /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0
+proc /proc proc rw,nosuid,nodev,noexec,relatime 0 0
+udev /dev devtmpfs rw,relatime,size=10240k,nr_inodes=1008585,mode=755 0 0
+devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
+tmpfs /run tmpfs rw,nosuid,relatime,size=1617716k,mode=755 0 0
+/dev/dm-2 / ext4 rw,relatime,errors=remount-ro,data=ordered 0 0
+securityfs /sys/kernel/security securityfs rw,nosuid,nodev,noexec,relatime 0 0
+tmpfs /dev/shm tmpfs rw,nosuid,nodev 0 0
+tmpfs /run/lock tmpfs rw,nosuid,nodev,noexec,relatime,size=5120k 0 0
+tmpfs /sys/fs/cgroup tmpfs ro,nosuid,nodev,noexec,mode=755 0 0
+cgroup /sys/fs/cgroup/systemd cgroup rw,nosuid,nodev,noexec,relatime,xattr,release_agent=/lib/systemd/systemd-cgroups-agent,name=systemd 0 0
+pstore /sys/fs/pstore pstore rw,nosuid,nodev,noexec,relatime 0 0
+cgroup /sys/fs/cgroup/cpuset cgroup rw,nosuid,nodev,noexec,relatime,cpuset 0 0
+cgroup /sys/fs/cgroup/cpu,cpuacct cgroup rw,nosuid,nodev,noexec,relatime,cpu,cpuacct 0 0
+cgroup /sys/fs/cgroup/devices cgroup rw,nosuid,nodev,noexec,relatime,devices 0 0
+cgroup /sys/fs/cgroup/freezer cgroup rw,nosuid,nodev,noexec,relatime,freezer 0 0
+cgroup /sys/fs/cgroup/net_cls,net_prio cgroup rw,nosuid,nodev,noexec,relatime,net_cls,net_prio 0 0
+cgroup /sys/fs/cgroup/blkio cgroup rw,nosuid,nodev,noexec,relatime,blkio 0 0
+cgroup /sys/fs/cgroup/perf_event cgroup rw,nosuid,nodev,noexec,relatime,perf_event 0 0
+systemd-1 /proc/sys/fs/binfmt_misc autofs rw,relatime,fd=22,pgrp=1,timeout=300,minproto=5,maxproto=5,direct 0 0
+mqueue /dev/mqueue mqueue rw,relatime 0 0
+debugfs /sys/kernel/debug debugfs rw,relatime 0 0
+hugetlbfs /dev/hugepages hugetlbfs rw,relatime 0 0
+fusectl /sys/fs/fuse/connections fusectl rw,relatime 0 0
+/dev/sda3 /boot ext2 rw,relatime 0 0
+rpc_pipefs /run/rpc_pipefs rpc_pipefs rw,relatime 0 0
+binfmt_misc /proc/sys/fs/binfmt_misc binfmt_misc rw,relatime 0 0
+tmpfs /run/user/1000 tmpfs rw,nosuid,nodev,relatime,size=808860k,mode=700,uid=1000,gid=1000 0 0
+gvfsd-fuse /run/user/1000/gvfs fuse.gvfsd-fuse rw,nosuid,nodev,relatime,user_id=1000,group_id=1000 0 0
+/dev/sda /var/lib/kubelet/plugins/kubernetes.io/vsphere-volume/mounts/[vsanDatastore]\040bafb9e5a-8856-7e6c-699c-801844e77a4a/kubernetes-dynamic-pvc-3eba5bba-48a3-11e8-89ab-005056b92113.vmdk ext4 rw,relatime,data=ordered 0 0
+/dev/sda /var/lib/kubelet/plugins/kubernetes.io/vsphere-volume/mounts/[vsanDatastore]\011bafb9e5a-8856-7e6c-699c-801844e77a4a/kubernetes-dynamic-pvc-3eba5bba-48a3-11e8-89ab-005056b92113.vmdk ext4 rw,relatime,data=ordered 0 0
diff --git a/collector/paths.go b/collector/paths.go
index f8e6e6e..a3a61ad 100644
--- a/collector/paths.go
+++ b/collector/paths.go
@@ -22,8 +22,9 @@ import (
 
 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 +34,7 @@ func procFilePath(name string) string {
 func sysFilePath(name string) string {
         return path.Join(*sysPath, name)
 }
+
+func rootfsFilePath(name string) string {
+        return path.Join(*rootfsPath, name)
+}