Defend against infinite loop when following symlinks

ideally it should never reach it, but there was already multiple issues with infinite loop
at following symlinks. this fixes hanging unit tests

Docker-DCO-1.1-Signed-off-by: Lajos Papp <lajos.papp@sequenceiq.com> (github: lalyos)

symlink/fs.go

@@ -3,10 +3,13 @@ package symlink
 import (
 	"fmt"
 	"os"
+	"path"
 	"path/filepath"
 	"strings"
 )
 
+const maxLoopCounter = 100
+
 // FollowSymlink will follow an existing link and scope it to the root
 // path provided.
 func FollowSymlinkInScope(link, root string) (string, error) {
@@ -30,7 +33,14 @@ func FollowSymlinkInScope(link, root string) (string, error) {
 		prev = filepath.Join(prev, p)
 		prev = filepath.Clean(prev)
 
+		loopCounter := 0
 		for {
+			loopCounter++
+
+			if loopCounter >= maxLoopCounter {
+				return "", fmt.Errorf("loopCounter reached MAX: %v", loopCounter)
+			}
+
 			if !strings.HasPrefix(prev, root) {
 				// Don't resolve symlinks outside of root. For example,
 				// we don't have to check /home in the below.