gdb.py: add a 'memdump' command.

This makes it easier for me to examine the contents of binary memory
buffers, while debugging through code that does crypto or packet
marshalling.

contrib/gdb.py

@@ -35,3 +35,62 @@ rcpp.add_printer(PuTTYBignumPrettyPrinter.name, "^Bignum$",
                  PuTTYBignumPrettyPrinter)
 
 gdb.printing.register_pretty_printer(None, rcpp)
+
+class MemDumpCommand(gdb.Command):
+    """Print a hex+ASCII dump of object EXP.
+
+EXP must be an expression whose value resides in memory. The
+contents of the memory it occupies are printed in a standard hex
+dump format, with each line showing an offset relative to the
+address of EXP, then the hex byte values of the memory at that
+offset, and then a translation into ASCII of the same bytes (with
+values outside the printable ASCII range translated as '.').
+
+To dump a number of bytes from a particular address, it's useful
+to use the gdb expression extensions {TYPE} and @LENGTH. For
+example, if 'ptr' and 'len' are variables giving an address and a
+length in bytes, then the command
+
+    memdump {char} ptr @ len
+
+will dump the range of memory described by those two variables."""
+
+    def __init__(self):
+        super(MemDumpCommand, self).__init__(
+            "memdump", gdb.COMMAND_DATA, gdb.COMPLETE_EXPRESSION)
+
+    def invoke(self, cmdline, from_tty):
+        expr = gdb.parse_and_eval(cmdline)
+        try:
+            start, size = int(expr.address), expr.type.sizeof
+        except gdb.error as e:
+            sys.stderr.write(str(e))
+            return
+        except (TypeError, AttributeError):
+            sys.stderr.write("expression must identify an object in memory")
+            return
+
+        width = 16
+        line_ptr_type = gdb.lookup_type(
+            "unsigned char").const().array(width).pointer()
+
+        dumpaddr = 0
+        while size > 0:
+            line = gdb.Value(start).cast(line_ptr_type).dereference()
+            thislinelen = min(size, width)
+            start += thislinelen
+            size -= thislinelen
+
+            dumpline = [None, " "] + ["   "] * width + ["  "] + [""] * width
+
+            dumpline[0] = "{:08x}".format(dumpaddr)
+            dumpaddr += thislinelen
+
+            for i in range(thislinelen):
+                ch = int(line[i]) & 0xFF
+                dumpline[2+i] = " {:02x}".format(ch)
+                dumpline[3+width+i] = chr(ch) if 0x20 <= ch < 0x7F else "."
+
+            sys.stdout.write("".join(dumpline) + "\n")
+
+MemDumpCommand()