Tell payloads and encoders how much space they have to work with

[hdm]

This change allows payload and encoder modules to become much more dynamic by providing an available_space accessor that is set by the EncodedPayload wrapper. As it stands today, this does nothing, but it opens the door for evasive encoders and payloads that can enable features and randomization based on available room. The actual payloads that use this are being implemented in the complex payload branch.

[bcook-r7]

Do we want available_space to be initialized to anything in the encoder/payload base class? Spot checking the generate method, I see it is called once with available_space=nil, then a second time with it set when generating with mfsvenom.

./msfvenom -p windows/meterpreter/reverse_https 2>&1 |grep bytes
 bytes available
1073741824 bytes available

Maybe something like this?

diff --git a/lib/msf/core/encoder.rb b/lib/msf/core/encoder.rb
index eea5de9..d45c273 100644
--- a/lib/msf/core/encoder.rb
+++ b/lib/msf/core/encoder.rb
@@ -17,6 +17,7 @@ class EncoderState
   def initialize(key = nil)
     @orig_buf = nil
     @buf = nil
+    @available_space = 1.gigabyte
     reset(key)
   end

diff --git a/lib/msf/core/payload.rb b/lib/msf/core/payload.rb
index 06146e4..01bdf80 100644
--- a/lib/msf/core/payload.rb
+++ b/lib/msf/core/payload.rb
@@ -86,6 +86,8 @@ class Payload < Msf::Module
       @staged = false
     end

+    @available_space = 1.gigabyte
+
     # Update the module info hash with the connection type
     # that is derived from the handler for this payload.  This is
     # used for compatibility filtering purposes.

[hdm]

@bcall-r7 This is somewhat intentional (it preserves the old behavior when the caller isn't aware of the option). The reason you see first nil then 1Gb is because of the call to generate() on module load [ something we can fix with #4894] before the second call to generate() from msfvenom. The msfvenom -s parameter controls the available space passed to this field.

[bcook-r7]

Alright. Tested the -s too, landing.

./msfvenom -p windows/meterpreter/reverse_https -s 4 2>&1 |grep bytes
4 bytes available