gdbremote: Add support for error packet handling

Add logic to detect error messages issued by the gdbserver and to
convert them into exceptions.

Signed-off-by: Daniel Thompson <[email protected]>

libdrgn/gdbremote.c

@@ -332,6 +332,30 @@ static struct drgn_error *gdb_send_and_receive(int fd, struct gdb_packet *pkt)
 	if (VERBOSE_PROTOCOL > 1)
 		fprintf(stderr, "-> +\n");
 
+	// We need to make sure we check for errors after sending the
+	// acknowledgement!
+	if (pkt->buffer[1] == 'E') {
+		// Error packets are naturally printable to we just have
+		// to deframe things slightly to make it look good in the
+		// error message.
+		//
+		// Format is either a two digit number (e.g. $E01#c5) or
+		// a textual message (e.g. $E.messaeg#c5). We can print
+		// either of these simply by terminating at the '#' and
+		// stripping of the . or leading '0' if it exists.
+
+		char *msg = (char *) pkt->buffer +
+			(pkt->buffer[2] == '.' || pkt->buffer[2] == '0' ? 3 :
+									  2);
+
+		// strrchr() will always find the '#' because the
+		// packet has already been subjected to framing checks
+		*strrchr(msg, '#') = '\0';
+
+		return drgn_error_format(DRGN_ERROR_OTHER,
+					 "gdbserver reported error: %s", msg);
+	}
+
 	return NULL;
 }
 

tests/test_gdbremote.py

@@ -22,8 +22,10 @@
     b"$m7fffffee40,16#63": b'$60eef* 7f0*"4077e1f77f0*"d8ef*!7f00#c6',
     b"$m7fffffee60,16#65": b'$70ef*!7f0*"1878e1f77f0*"f00cfdf77f00#47',
     b"$m7fffffef70,16#67": b'$0*,70065*"0*.#5c',
-}
 
+    # Null pointer read
+    b"$m0,64#33" : b'$E01#a6',
+}
 
 class GdbMockProcess(multiprocessing.Process):
     def __init__(self):
@@ -124,6 +126,15 @@ def test_gdbremote_read(self):
             val, b"`\xee\xff\xff\x7f\x00\x00\x00@w\xe1\xf7\x7f\x00\x00\x00"
         )
 
+    def test_gdbremote_read_with_error(self):
+        self.prog.set_gdbremote(self.conn_str)
+        self.assertRaisesRegex(
+            Exception,
+            "gdbserver reported error: 1",
+            self.prog.read,
+            0,
+            64)
+
     def test_gdbremote_getregs(self):
         self.prog.set_gdbremote(self.conn_str)
         if self.prog.platform.arch != Architecture.AARCH64: