Support HOST_PROC in processes and linux_sysctl_fs inputs (#2924)

plugins/inputs/system/PROCESSES_README.md

@@ -14,6 +14,11 @@ it requires access to execute `ps`.
   # no configuration
 ```
 
+Another possible configuration is to define an alternative path for resolving the /proc location.
+Using the environment variable `HOST_PROC` the plugin will retrieve process information from the specified location.
+
+`docker run -v /proc:/rootfs/proc:ro -e HOST_PROC=/rootfs/proc`
+
 ### Measurements & Fields:
 
 - processes

plugins/inputs/system/linux_sysctl_fs.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/plugins/inputs"
+	"path"
 )
 
 // https://www.kernel.org/doc/Documentation/sysctl/fs.txt
@@ -80,9 +81,10 @@ func (sfs *SysctlFS) Gather(acc telegraf.Accumulator) error {
 }
 
 func init() {
+
 	inputs.Add("linux_sysctl_fs", func() telegraf.Input {
 		return &SysctlFS{
-			path: "/proc/sys/fs",
+			path: path.Join(GetHostProc(), "/sys/fs"),
 		}
 	})
 }

plugins/inputs/system/processes.go

@@ -20,6 +20,7 @@ import (
 
 type Processes struct {
 	execPS       func() ([]byte, error)
+	getHostProc  func() string
 	readProcFile func(filename string) ([]byte, error)
 
 	forcePS   bool
@@ -62,6 +63,14 @@ func (p *Processes) Gather(acc telegraf.Accumulator) error {
 	return nil
 }
 
+func GetHostProc() string {
+	procPath := "/proc"
+	if os.Getenv("HOST_PROC") != "" {
+		procPath = os.Getenv("HOST_PROC")
+	}
+	return procPath
+}
+
 // Gets empty fields of metrics based on the OS
 func getEmptyFields() map[string]interface{} {
 	fields := map[string]interface{}{
@@ -132,14 +141,14 @@ func (p *Processes) gatherFromPS(fields map[string]interface{}) error {
 
 // get process states from /proc/(pid)/stat files
 func (p *Processes) gatherFromProc(fields map[string]interface{}) error {
-	filenames, err := filepath.Glob("/proc/[0-9]*/stat")
+	filenames, err := filepath.Glob(GetHostProc() + "/[0-9]*/stat")
+
 	if err != nil {
 		return err
 	}
 
 	for _, filename := range filenames {
 		_, err := os.Stat(filename)
-
 		data, err := p.readProcFile(filename)
 		if err != nil {
 			return err