tools/run_guest.py: Add --append-payload

Add command-line option for appending a separate file (or files) to
the payload.

proxyclient/tools/run_guest.py

@@ -12,6 +12,7 @@
 parser.add_argument('-S', '--shell', action="store_true")
 parser.add_argument('-e', '--hook-exceptions', action="store_true")
 parser.add_argument('-d', '--debug-xnu', action="store_true")
+parser.add_argument('-a', '--append-payload', type=pathlib.Path, action="append", default=[])
 parser.add_argument('payload', type=pathlib.Path)
 parser.add_argument('boot_args', default=[], nargs="*")
 args = parser.parse_args()
@@ -43,7 +44,71 @@
 symfile = None
 if args.symbols:
     symfile = args.symbols.open("rb")
-hv.load_macho(args.payload.open("rb"), symfile=symfile)
+
+
+class Concat:
+    '''
+    Present the file interface over a concatenation of couple
+    component files.
+    '''
+    def __init__(self, *components):
+        self._components = components
+        sizes = [f.seek(0, 2) for f in components]
+        for f in components:
+            f.seek(0, 0)
+        self._offsets = [sum(sizes[:n]) for n in range(len(components))]
+        self._ends = [sum(sizes[:n+1]) for n in range(len(components))]
+        self._size = sum(sizes)
+        self._pos = 0
+        self._remaining = list(components)
+
+    def read(self, size=-1):
+        ret = bytearray()
+        if not len(self._remaining):
+            return bytes() # EOF
+        current = self._remaining[0]
+        while size:
+            read = current.read(size if size>=0 else -1)
+            if len(read):
+                self._pos += len(read)
+                size -= len(read)
+                ret += read
+            else:
+                self._remaining.pop(0)
+                if not len(self._remaining):
+                    break
+                else:
+                    current = self._remaining[0]
+        return bytes(ret)
+
+    def tell(self):
+        return self._pos
+
+    def seek(self, pos, whence=0):
+        if whence == 1:
+            pos += self._pos
+        elif whence == 2:
+            pos += self._size
+        remaining = []
+        for i, f in enumerate(self._components):
+            if self._ends[i] <= pos:
+                continue
+            if self._offsets[i] < pos:
+                f.seek(pos - self._offsets[i])
+            else:
+                f.seek(0)
+            remaining.append(f)
+        self._pos = pos
+        self._remaining = remaining
+        return pos
+
+
+payload = args.payload.open("rb")
+
+if args.append_payload:
+    payload = Concat(payload, *[p.open("rb") for p in args.append_payload])
+
+hv.load_macho(payload, symfile=symfile)
 
 for i in args.script:
     try: