Add alarm option for process() (#704)

* Add `alarm` option for process()

* Add signal to toplevel import

pwn/toplevel.py

@@ -6,6 +6,7 @@
 import os
 import re
 import socks
+import signal
 import string
 import struct
 import subprocess

pwnlib/tubes/process.py

@@ -6,6 +6,7 @@
 import pty
 import resource
 import select
+import signal
 import subprocess
 import tty
 
@@ -97,6 +98,8 @@ class process(tube):
             Where the process is running, used for logging purposes.
         display(list):
             List of arguments to display, instead of the main executable name.
+        alarm(int):
+            Set a SIGALRM alarm timeout on the process.
 
     Attributes:
         proc(subprocess)
@@ -183,6 +186,10 @@ class process(tube):
 
         >>> process(['sh','-c','ulimit -s'], aslr=0).recvline()
         'unlimited\n'
+
+        >>> io = process(['sh','-c','sleep 10; exit 7'], alarm=2)
+        >>> io.poll(block=True) == -signal.SIGALRM
+        True
     """
 
     PTY = PTY
@@ -206,7 +213,8 @@ def __init__(self, argv,
                  aslr = None,
                  setuid = None,
                  where = 'local',
-                 display = None):
+                 display = None,
+                 alarm = None):
         super(process, self).__init__(timeout, level = level)
 
         #: `subprocess.Popen` object
@@ -253,6 +261,9 @@ def __init__(self, argv,
         #: Directory the process was created in
         self.cwd          = cwd or os.path.curdir
 
+        #: Alarm timeout of the process
+        self.alarm        = alarm
+
         self.preexec_fn = preexec_fn
         self.display    = display or self.program
 
@@ -357,6 +368,9 @@ def __preexec_fn(self):
             except:
                 pass
 
+        if self.alarm is not None:
+            signal.alarm(self.alarm)
+
         self.preexec_fn()
 
     @property