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>

4 files changed, 480 insertions, 0 deletions

diff --git a/payloads/libpayload/gdb/Makefile.inc b/payloads/libpayload/gdb/Makefile.inc
new file mode 100644
index 0000000000..cacd0d02f3
--- /dev/null
+++ b/payloads/libpayload/gdb/Makefile.inc
@@ -0,0 +1,20 @@
+##
+## 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
+
+libgdb-y += commands.c
+libgdb-y += stub.c
+libgdb-y += transport.c
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);
diff --git a/payloads/libpayload/gdb/stub.c b/payloads/libpayload/gdb/stub.c
new file mode 100644
index 0000000000..73afa70c47
--- /dev/null
+++ b/payloads/libpayload/gdb/stub.c
@@ -0,0 +1,124 @@
+/*
+ * 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 <gdb.h>
+#include <libpayload.h>
+
+struct gdb_state gdb_state;
+
+static u8 reply_buf[2048];
+static u8 command_buf[2048];
+
+static struct gdb_message command = {
+	.buf = command_buf,
+	.used = 0,
+	.size = sizeof(command_buf),
+};
+static struct gdb_message reply = {
+	.buf = reply_buf,
+	.used = 0,
+	.size = sizeof(reply_buf),
+};
+
+void gdb_command_loop(u8 signal)
+{
+	if (gdb_state.resumed) {
+		/* We were just running. Send a stop reply. */
+		reply.used = 0;
+		gdb_message_add_string(&reply, "S");
+		gdb_message_encode_bytes(&reply, &signal, 1);
+		gdb_send_reply(&reply);
+
+	}
+	gdb_state.signal = signal;
+	gdb_state.resumed = 0;
+	gdb_state.connected = 1;
+
+	while (1) {
+		int i;
+
+		gdb_get_command(&command);
+
+		reply.used = 0;
+		for (i = 0; i < gdb_command_count; i++) {
+			int clen = strlen(gdb_commands[i].str);
+			if (!strncmp(gdb_commands[i].str, (char *)command.buf,
+				     MIN(clen, command.used))) {
+				gdb_commands[i].handler(&command, clen, &reply);
+				break;
+			}
+		}
+
+		/* If we're resuming, we won't send a reply until we stop. */
+		if (gdb_state.resumed)
+			return;
+
+		gdb_send_reply(&reply);
+	}
+}
+
+static void gdb_output_write(const void *buffer, size_t count)
+{
+	if (!gdb_state.resumed) {
+		/* Must be a die_if() in GDB (or a bug), so bail out and die. */
+		gdb_exit(-1);
+		video_console_init();
+		puts("GDB died, redirecting its last words to the screen:\n");
+		console_write(buffer, count);
+	} else {
+		reply.used = 0;
+		reply.buf[reply.used++] = 'O';
+		gdb_message_encode_bytes(&reply, buffer, count);
+		gdb_send_reply(&reply);
+	}
+}
+
+static struct console_output_driver gdb_output_driver = {
+	.write = &gdb_output_write
+};
+
+static void gdb_init(void)
+{
+	printf("Ready for GDB connection.\n");
+	gdb_transport_init();
+	gdb_arch_init();
+	console_add_output_driver(&gdb_output_driver);
+}
+
+void gdb_enter(void)
+{
+	if (!gdb_state.connected)
+		gdb_init();
+	gdb_arch_enter();
+}
+
+void gdb_exit(s8 exit_status)
+{
+	if (!gdb_state.connected)
+		return;
+
+	reply.used = 0;
+	gdb_message_add_string(&reply, "W");
+	gdb_message_encode_bytes(&reply, &exit_status, 1);
+	gdb_send_reply(&reply);
+
+	console_remove_output_driver(&gdb_output_write);
+	gdb_transport_teardown();
+	gdb_state.connected = 0;
+	printf("Detached from GDB connection.\n");
+}
diff --git a/payloads/libpayload/gdb/transport.c b/payloads/libpayload/gdb/transport.c
new file mode 100644
index 0000000000..596ceb5c6d
--- /dev/null
+++ b/payloads/libpayload/gdb/transport.c
@@ -0,0 +1,235 @@
+/*
+ * 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 <gdb.h>
+#include <libpayload.h>
+
+static const int timeout_us = 100 * 1000;
+static const char output_overrun[] = "GDB output buffer overrun (try "
+				     "increasing reply.size)!\n";
+static const char input_underrun[] = "GDB input message truncated (bug or "
+				     "communication problem)?\n";
+
+/* Serial-specific glue code... add more transport layers here when desired. */
+
+static void gdb_raw_putchar(u8 c)
+{
+	serial_putchar(c);
+}
+
+static int gdb_raw_getchar(void)
+{
+	u64 start = timer_us(0);
+
+	while (!serial_havechar())
+		if (timer_us(start) > timeout_us)
+			return -1;
+
+	return serial_getchar();
+}
+
+void gdb_transport_init(void)
+{
+	console_remove_output_driver(serial_putchar);
+}
+
+void gdb_transport_teardown(void)
+{
+	serial_console_init();
+}
+
+/* Hex digit character <-> number conversion (illegal chars undefined!). */
+
+static s8 from_hex(unsigned char c)
+{
+	static const s8 values[] = {
+		-1, 10, 11, 12, 13, 14, 15, -1,
+		-1, -1, -1, -1, -1, -1, -1, -1,
+		 0,  1,  2,  3,  4,  5,  6,  7,
+		 8,  9, -1, -1, -1, -1, -1, -1,
+	};
+
+	return values[c & 0x1f];
+}
+
+static char to_hex(u8 v)
+{
+	static const char digits[] = "0123456789abcdef";
+
+	return digits[v & 0xf];
+}
+
+/* Message encode/decode functions */
+
+void gdb_message_encode_bytes(struct gdb_message *message, const void *data,
+			      int length)
+{
+	const u8 *bytes = data;
+	die_if(message->used + length * 2 > message->size, output_overrun);
+	while (length--) {
+		message->buf[message->used++] = to_hex(*bytes >> 4);
+		message->buf[message->used++] = to_hex(*bytes & 0xf);
+		bytes++;
+	}
+}
+
+void gdb_message_decode_bytes(const struct gdb_message *message, int offset,
+			      void *data, int length)
+{
+	u8 *bytes = data;
+	die_if(offset + 2 * length > message->used, "Decode overrun in GDB "
+		"message: %.*s", message->used, message->buf);
+	while (length--) {
+		*bytes = from_hex(message->buf[offset++]) << 4;
+		*bytes += from_hex(message->buf[offset++]);
+		bytes++;
+	}
+}
+
+void gdb_message_encode_zero_bytes(struct gdb_message *message, int length)
+{
+	die_if(message->used + length * 2 > message->size, output_overrun);
+	memset(message->buf + message->used, '0', length * 2);
+	message->used += length * 2;
+}
+
+void gdb_message_add_string(struct gdb_message *message, const char *string)
+{
+	message->used += strlcpy((char *)message->buf + message->used,
+			     string, message->size - message->used);
+
+	/* Check >= instead of > to account for strlcpy's trailing '\0'. */
+	die_if(message->used >= message->size, output_overrun);
+}
+
+void gdb_message_encode_int(struct gdb_message *message, uintptr_t val)
+{
+	int length = sizeof(uintptr_t) * 2 - __builtin_clz(val) / 4;
+	die_if(message->used + length > message->size, output_overrun);
+	while (length--)
+		message->buf[message->used++] =
+			to_hex((val >> length * 4) & 0xf);
+}
+
+uintptr_t gdb_message_decode_int(const struct gdb_message *message, int offset,
+				 int length)
+{
+	uintptr_t val = 0;
+
+	die_if(length > sizeof(uintptr_t) * 2, "GDB decoding invalid number: "
+	       "%.*s", message->used, message->buf);
+
+	while (length--) {
+		val <<= 4;
+		val |= from_hex(message->buf[offset++]);
+	}
+
+	return val;
+}
+
+/* Like strtok/strsep: writes back offset argument, returns original offset. */
+int gdb_message_tokenize(const struct gdb_message *message, int *offset)
+{
+	int token = *offset;
+	while (!strchr(",;:", message->buf[(*offset)++]))
+		die_if(*offset >= message->used, "Undelimited token in GDB "
+				"message at offset %d: %.*s",
+				token, message->used, message->buf);
+	return token;
+}
+
+/* High-level send/receive functions. */
+
+void gdb_get_command(struct gdb_message *command)
+{
+	enum command_state {
+		STATE_WAITING,
+		STATE_COMMAND,
+		STATE_CHECKSUM0,
+		STATE_CHECKSUM1,
+	};
+
+	u8 checksum = 0;
+	u8 running_checksum = 0;
+	enum command_state state = STATE_WAITING;
+
+	while (1) {
+		int c = gdb_raw_getchar();
+		if (c < 0) {
+			/*
+			 * Timeout waiting for a byte. Reset the
+			 * state machine.
+			 */
+			state = STATE_WAITING;
+			continue;
+		}
+
+		switch (state) {
+		case STATE_WAITING:
+			if (c == '$') {
+				running_checksum = 0;
+				command->used = 0;
+				state = STATE_COMMAND;
+			}
+			break;
+		case STATE_COMMAND:
+			if (c == '#') {
+				state = STATE_CHECKSUM0;
+				break;
+			}
+			die_if(command->used >= command->size, "GDB input buf"
+			       "fer overrun (try increasing command.size)!\n");
+			command->buf[command->used++] = c;
+			running_checksum += c;
+			break;
+		case STATE_CHECKSUM0:
+			checksum = from_hex(c) << 4;
+			state = STATE_CHECKSUM1;
+			break;
+		case STATE_CHECKSUM1:
+			checksum += from_hex(c);
+			if (running_checksum == checksum) {
+				gdb_raw_putchar('+');
+				return;
+			} else {
+				state = STATE_WAITING;
+				gdb_raw_putchar('-');
+			}
+			break;
+		}
+	}
+}
+
+void gdb_send_reply(const struct gdb_message *reply)
+{
+	int i;
+	int retries = 1 * 1000 * 1000 / timeout_us;
+	u8 checksum = 0;
+
+	for (i = 0; i < reply->used; i++)
+		checksum += reply->buf[i];
+
+	do {
+		gdb_raw_putchar('$');
+		for (i = 0; i < reply->used; i++)
+			gdb_raw_putchar(reply->buf[i]);
+		gdb_raw_putchar('#');
+		gdb_raw_putchar(to_hex(checksum >> 4));
+		gdb_raw_putchar(to_hex(checksum & 0xf));
+	} while (gdb_raw_getchar() != '+' && retries--);
+}