Add new code template

modules/encoders/x86/alpha_mixed.rb

@@ -31,6 +31,7 @@ class Metasploit3 < Msf::Encoder::Alphanum
   # being encoded.
   #
   def decoder_stub(state)
+    modified_registers = []
     reg = datastore['BufferRegister']
     off = (datastore['BufferOffset'] || 0).to_i
     buf = ''
@@ -52,7 +53,14 @@ class Metasploit3 < Msf::Encoder::Alphanum
       reg.upcase!
     end
 
-    buf + Rex::Encoder::Alpha2::AlphaMixed::gen_decoder(reg, off)
+    stub = buf + Rex::Encoder::Alpha2::AlphaMixed::gen_decoder(reg, off)
+
+    # Sanity check that saved_registers doesn't overlap with modified_registers
+    if (modified_registers & saved_registers).length > 0
+      raise BadGenerateError
+    end
+
+    stub
   end
 
   #
@@ -69,4 +77,14 @@ class Metasploit3 < Msf::Encoder::Alphanum
   def encode_end(state)
     state.encoded += Rex::Encoder::Alpha2::AlphaMixed::add_terminator()
   end
+
+  # Indicate that this module can preserve some registers
+  def can_preserve_registers?
+    true
+  end
+
+  # Convert the SaveRegisters to an array of x86 register constants
+  def saved_registers
+    Rex::Arch::X86.register_names_to_ids(datastore['SaveRegisters'])
+  end
 end