libpayload: Add remote GDB support

This patch adds the ability to attach a GDB host through the UART to a
running payload. Libpayload implements a small stub that can parse and
respond to the GDB remote protocol and provide the required primitives
(reading/writing registers/memory, etc.) to allow GDB to control
execution.

The goal of this implementation is to be as small and uninvasive as
possible. It implements only the minimum amount of primitives required,
and relies on GDB's impressive workaround capabilities (such as
emulating breakpoints by temporarily replacing instructions) for the
more complicated features. This way, a relatively tiny amount of code on
the firmware side opens a vast range of capabilities to the user, not
just in debugging but also in remote-controlling the firmware to change
its behavior (e.g. through GDBs ability to modify variables and call
functions).

By default, a system with the REMOTEGDB Kconfig will only trap into GDB
when executing halt() (including the calls from die_if(), assert(), and
exception handlers). In addition, payloads can manually call gdb_enter()
if desired. It will print a final "Ready for GDB connection." on the
serial, detach the normal serial output driver and wait for the commands
that GDB starts sending on attach.

Based on original implementation by Gabe Black <gabeblack@chromium.org>.

BUG=chrome-os-partner:18390
TEST=Boot a GDB enabled image in recovery mode (or get it to hit a
halt()), close your terminal, execute '<toolchain>-gdb --symbols
/build/<board>/firmware/depthcharge_gdb/depthcharge.elf --directory
~/trunk/src/third_party/coreboot/payloads/libpayload --directory
~/trunk/src/platform/depthcharge --directory
~/trunk/src/platform/vboot_reference --ex "target remote
<cpu_uart_pty>"' and behold the magic.
(You can also SIGSTOP your terminal's parent shell and the terminal
itself, and SIGCONT them in reverse order after GDB exits. More
convenient wrapper tools to do all this automatically coming soon.)

Original-Change-Id: Ib440d1804126cdfdac4a8801f5015b4487e25269
Original-Signed-off-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/202563
Original-Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
(cherry picked from commit 9c4a642c7be2faf122fef39bdfaddd64aec68b77)
Signed-off-by: Marc Jones <marc.jones@se-eng.com>

Change-Id: I9238b4eb19d3ab2c98e4e1c5946cd7d252ca3c3b
Reviewed-on: http://review.coreboot.org/8119
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

1 files changed, 101 insertions, 0 deletions

diff --git a/payloads/libpayload/gdb/commands.c b/payloads/libpayload/gdb/commands.c
new file mode 100644
index 0000000000..5137dfd9f5
--- /dev/null
+++ b/payloads/libpayload/gdb/commands.c
@@ -0,0 +1,101 @@
+/*
+ * Copyright 2014 Google Inc.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of
+ * the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but without any warranty; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <arch/cache.h>
+#include <gdb.h>
+#include <libpayload.h>
+
+static void gdb_get_last_signal(struct gdb_message *command,
+				int offset, struct gdb_message *reply)
+{
+	gdb_message_add_string(reply, "S");
+	gdb_message_encode_bytes(reply, &gdb_state.signal, 1);
+}
+
+static void gdb_read_general_registers(struct gdb_message *command,
+				       int offset, struct gdb_message *reply)
+{
+	gdb_arch_encode_regs(reply);
+}
+
+static void gdb_write_general_registers(struct gdb_message *command,
+					int offset, struct gdb_message *reply)
+{
+	gdb_arch_decode_regs(offset, command);
+	gdb_message_add_string(reply, "OK");
+}
+
+static void gdb_read_memory(struct gdb_message *command,
+			    int offset, struct gdb_message *reply)
+{
+	int tok = gdb_message_tokenize(command, &offset);
+	uintptr_t addr = gdb_message_decode_int(command, tok, offset - 1 - tok);
+	size_t length = gdb_message_decode_int(command, offset,
+					       command->used - offset);
+
+	gdb_message_encode_bytes(reply, (void *)addr, length);
+}
+
+static void gdb_write_memory(struct gdb_message *command,
+			     int offset, struct gdb_message *reply)
+{
+	int tok = gdb_message_tokenize(command, &offset);
+	uintptr_t addr = gdb_message_decode_int(command, tok, offset - 1 - tok);
+	tok = gdb_message_tokenize(command, &offset);
+	size_t length = gdb_message_decode_int(command, tok, offset - 1 - tok);
+
+	die_if(length * 2 != command->used - offset, "Invalid length field in "
+	       "GDB command: %.*s", command->used, command->buf);
+
+	gdb_message_decode_bytes(command, offset, (void *)addr, length);
+	cache_sync_instructions();
+	gdb_message_add_string(reply, "OK");
+}
+
+static void gdb_continue(struct gdb_message *command,
+			 int offset, struct gdb_message *reply)
+{
+	/* Disable single step if it's still on. */
+	gdb_arch_set_single_step(0);
+
+	/* No need to support the extension that passes in new EIP/PC. */
+	if (command->used > offset)
+		gdb_message_add_string(reply, "E00");
+	else
+		gdb_state.resumed = 1;
+}
+
+static void gdb_single_step(struct gdb_message *command,
+			    int offset, struct gdb_message *reply)
+{
+	if (command->used > offset || gdb_arch_set_single_step(1))
+		gdb_message_add_string(reply, "E00");
+	else
+		gdb_state.resumed = 1;
+}
+
+struct gdb_command gdb_commands[] = {
+	{ "?", &gdb_get_last_signal },
+	{ "g", &gdb_read_general_registers },
+	{ "G", &gdb_write_general_registers },
+	{ "m", &gdb_read_memory },
+	{ "M", &gdb_write_memory },
+	{ "c", &gdb_continue },
+	{ "s", &gdb_single_step }
+};
+const int gdb_command_count = ARRAY_SIZE(gdb_commands);