Prefer Python3 for the remotely-executed Python script (#1746)

* Prefer Python3 for the remotely-executed Python script

This should be done as part of our general transition to Python3,
but should also be done because some systems with e.g. Python2.6
do not support OrderedDict, but DO have a Python3 version that works.

* Make the script actually work in py2 and py3

* Fix & update CHANGELOG.md

This actually is a behavior change, and can be a breaking change for some,
so must be stated prominently in the changelog.

Co-authored-by: Arusekk <arek_koz@o2.pl>

CHANGELOG.md

@@ -61,9 +61,11 @@ The table below shows which release corresponds to each branch, and what date th
 ## 4.6.0 (`dev`)
 
 - [#1739][1739] Add/fix shellcraft.linux.kill() / shellcraft.linux.killparent()
+- [#1746][1746] Prefer Python3 over Python2 for spawning remote processes over SSH
 - [#1776][1776] mips: do not use $t0 temporary variable in dupio
 
 [1739]: https://github.com/Gallopsled/pwntools/pull/1739
+[1746]: https://github.com/Gallopsled/pwntools/pull/1746
 [1776]: https://github.com/Gallopsled/pwntools/pull/1776
 
 ## 4.5.0 (`beta`)

pwnlib/tubes/ssh.py

@@ -864,7 +864,7 @@ def process(self, argv=None, executable=None, tty=True, cwd=None, env=None, time
             >>> print(s.process('false', preexec_fn=uses_globals).recvall().strip().decode()) # doctest: +ELLIPSIS
             Traceback (most recent call last):
             ...
-            NameError: ... name 'bar' is not defined
+            NameError: ...name 'bar' is not defined
 
             >>> s.process('echo hello', shell=True).recvall()
             b'hello\n'
@@ -932,7 +932,7 @@ def process(self, argv=None, executable=None, tty=True, cwd=None, env=None, time
         # Validate, since failures on the remote side will suck.
         if not isinstance(executable, (six.text_type, six.binary_type, bytearray)):
             self.error("executable / argv[0] must be a string: %r" % executable)
-        executable = packing._decode(executable)
+        executable = bytearray(packing._encode(executable))
 
         # Allow passing in sys.stdin/stdout/stderr objects
         handles = {sys.stdin: 0, sys.stdout:1, sys.stderr:2}
@@ -954,7 +954,7 @@ def func(): pass
 
         func_src  = inspect.getsource(func).strip()
         setuid = True if setuid is None else bool(setuid)
-        
+
         script = r"""
 #!/usr/bin/env python
 import os, sys, ctypes, resource, platform, stat
@@ -963,25 +963,27 @@ def func(): pass
     integer_types = int, long
 except NameError:
     integer_types = int,
-exe   = %(executable)r
+exe   = bytes(%(executable)r)
 argv  = [bytes(a) for a in %(argv)r]
 env   = %(env)r
 
 os.chdir(%(cwd)r)
 
+environ = getattr(os, 'environb', os.environ)
+
 if env is not None:
     env = OrderedDict((bytes(k), bytes(v)) for k,v in env)
     os.environ.clear()
-    getattr(os, 'environb', os.environ).update(env)
+    environ.update(env)
 else:
     env = os.environ
 
 def is_exe(path):
     return os.path.isfile(path) and os.access(path, os.X_OK)
 
-PATH = os.environ.get('PATH','').split(os.pathsep)
+PATH = environ.get(b'PATH',b'').split(os.pathsep.encode())
 
-if os.path.sep not in exe and not is_exe(exe):
+if os.path.sep.encode() not in exe and not is_exe(exe):
     for path in PATH:
         test_path = os.path.join(path, exe)
         if is_exe(test_path):
@@ -990,7 +992,7 @@ def is_exe(path):
 
 if not is_exe(exe):
     sys.stderr.write('3\n')
-    sys.stderr.write("{} is not executable or does not exist in $PATH: {}".format(exe,PATH))
+    sys.stderr.write("{!r} is not executable or does not exist in $PATH: {!r}".format(exe,PATH))
     sys.exit(-1)
 
 if not %(setuid)r:
@@ -1027,7 +1029,7 @@ def is_exe(path):
     sys.stdout.write(str(os.getgid()) + "\n")
     sys.stdout.write(str(suid) + "\n")
     sys.stdout.write(str(sgid) + "\n")
-    sys.stdout.write(os.path.realpath(exe) + '\x00')
+    getattr(sys.stdout, 'buffer', sys.stdout).write(os.path.realpath(exe) + b'\x00')
     sys.stdout.flush()
 
 for fd, newfd in {0: %(stdin)r, 1: %(stdout)r, 2:%(stderr)r}.items():
@@ -1098,7 +1100,7 @@ def is_exe(path):
 
         with self.progress(msg) as h:
 
-            script = 'echo PWNTOOLS; for py in python2.7 python2 python; do test -x "$(which $py 2>&1)" && echo $py && exec $py -c %s check; done; echo 2' % sh_string(script)
+            script = 'echo PWNTOOLS; for py in python3 python2.7 python2 python; do test -x "$(which $py 2>&1)" && echo $py && exec $py -c %s check; done; echo 2' % sh_string(script)
             with context.quiet:
                 python = ssh_process(self, script, tty=True, raw=True, level=self.level, timeout=timeout)
 
@@ -1108,8 +1110,10 @@ def is_exe(path):
                 result = safeeval.const(python.recvline())  # Status flag from the Python script
             except (EOFError, ValueError):
                 h.failure("Process creation failed")
-                self.warn_once('Could not find a Python interpreter on %s\n' % self.host \
-                               + "Use ssh.run() instead of ssh.process()")
+                self.warn_once('Could not find a Python interpreter on %s\n' % self.host
+                               + "Use ssh.run() instead of ssh.process()\n"
+                                 "The original error message:\n"
+                               + python.recvall().decode())
                 return None
 
             # If an error occurred, try to grab as much output