Make ExecTainted easier to extend

To add a method that executes a command, you can now define a class
extending ExecMethod.

java/ql/src/semmle/code/java/JDK.qll

@@ -3,6 +3,7 @@
  */
 
 import Member
+import semmle.code.java.security.ExternalProcess
 
 // --- Standard types ---
 /** The class `java.lang.Object`. */
@@ -179,7 +180,7 @@ class TypeFile extends Class {
 /**
  * Any of the methods named `command` on class `java.lang.ProcessBuilder`.
  */
-class MethodProcessBuilderCommand extends Method {
+class MethodProcessBuilderCommand extends ExecMethod {
   MethodProcessBuilderCommand() {
     hasName("command") and
     getDeclaringType() instanceof TypeProcessBuilder
@@ -189,7 +190,7 @@ class MethodProcessBuilderCommand extends Method {
 /**
  * Any method named `exec` on class `java.lang.Runtime`.
  */
-class MethodRuntimeExec extends Method {
+class MethodRuntimeExec extends ExecMethod {
   MethodRuntimeExec() {
     hasName("exec") and
     getDeclaringType() instanceof TypeRuntime

java/ql/src/semmle/code/java/frameworks/apache/Exec.qll

@@ -1,18 +1,19 @@
 /* Definitions related to the Apache Commons Exec library. */
 import semmle.code.java.Type
+import semmle.code.java.security.ExternalProcess
 
 library class TypeCommandLine extends Class {
   TypeCommandLine() { hasQualifiedName("org.apache.commons.exec", "CommandLine") }
 }
 
-library class MethodCommandLineParse extends Method {
+library class MethodCommandLineParse extends ExecMethod {
   MethodCommandLineParse() {
     getDeclaringType() instanceof TypeCommandLine and
     hasName("parse")
   }
 }
 
-library class MethodCommandLineAddArguments extends Method {
+library class MethodCommandLineAddArguments extends ExecMethod {
   MethodCommandLineAddArguments() {
     getDeclaringType() instanceof TypeCommandLine and
     hasName("addArguments")

java/ql/src/semmle/code/java/security/ExternalProcess.qll

@@ -3,6 +3,11 @@ import semmle.code.java.Member
 import semmle.code.java.JDK
 import semmle.code.java.frameworks.apache.Exec
 
+/**
+ * A method that executes a command.
+ */
+abstract class ExecMethod extends Method { }
+
 /**
  * An expression used as an argument to a call that executes an external command. For calls to
  * varargs method calls, this only includes the first argument, which will be the command
@@ -10,15 +15,9 @@ import semmle.code.java.frameworks.apache.Exec
  */
 class ArgumentToExec extends Expr {
   ArgumentToExec() {
-    exists(MethodAccess execCall, Method method |
+    exists(MethodAccess execCall, ExecMethod method |
       execCall.getArgument(0) = this and
-      method = execCall.getMethod() and
-      (
-        method instanceof MethodRuntimeExec or
-        method instanceof MethodProcessBuilderCommand or
-        method instanceof MethodCommandLineParse or
-        method instanceof MethodCommandLineAddArguments
-      )
+      method = execCall.getMethod()
     )
     or
     exists(ConstructorCall expr, Constructor cons |