util/pidowner: new package to map from process ID to its user ID

util/pidowner/pidowner_linux.go

@@ -0,0 +1,33 @@
+package pidowner
+
+import (
+	"fmt"
+	"os"
+	"strings"
+
+	"tailscale.com/util/lineread"
+)
+
+func ownerOfPID(pid int) (userID string, err error) {
+	file := fmt.Sprintf("/proc/%d/status", pid)
+	err = lineread.File(file, func(line []byte) error {
+		if len(line) < 4 || string(line[:4]) != "Uid:" {
+			return nil
+		}
+		f := strings.Fields(string(line))
+		if len(f) >= 2 {
+			userID = f[1] // real userid
+		}
+		return nil
+	})
+	if os.IsNotExist(err) {
+		return "", ErrProcessNotFound
+	}
+	if err != nil {
+		return
+	}
+	if userID == "" {
+		return "", fmt.Errorf("missing Uid line in %s", file)
+	}
+	return userID, nil
+}