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gdbstub: only send stop-reply packets when allowed to 
GDB's remote serial protocol allows stop-reply messages to be sent by
the stub either as a notification packet or as a reply to a GDB command
(provided that the cmd accepts such a response). QEMU currently does not
implement notification packets, so it should only send stop-replies
synchronously and when requested. Nevertheless, it still issues
unsolicited stop messages through gdb_vm_state_change().

Although this behavior doesn't seem to cause problems with GDB itself
(the messages are just ignored), it can impact other debuggers that
implement the GDB remote serial protocol, like hexagon-lldb. Let's
change the gdbstub to send stop messages only as a response to a
previous GDB command that accepts such a reply.

Signed-off-by: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Message-Id: <a49c0897fc22a6a7827c8dfc32aef2e1d933ec6b.1683214375.git.quic_mathbern@quicinc.com>
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@quic-mathbern @taylorsimpson
quic-mathbern authored and taylorsimpson committed May 18, 2023
1 parent 9e6d493 commit 7583700
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Showing 4 changed files with 60 additions and 19 deletions.
37 changes: 28 additions & 9 deletions gdbstub/gdbstub.c
View file
Expand Up @@ -777,6 +777,10 @@ typedef void (*GdbCmdHandler)(GArray *params, void *user_ctx);
/*
* cmd_startswith -> cmd is compared using startswith
*
* allow_stop_reply -> true iff the gdbstub can respond to this command with a
* "stop reply" packet. The list of commands that accept such response is
* defined at the GDB Remote Serial Protocol documentation. see:
* https://sourceware.org/gdb/onlinedocs/gdb/Stop-Reply-Packets.html#Stop-Reply-Packets.
*
* schema definitions:
* Each schema parameter entry consists of 2 chars,
Expand All @@ -802,6 +806,7 @@ typedef struct GdbCmdParseEntry {
const char *cmd;
bool cmd_startswith;
const char *schema;
bool allow_stop_reply;
} GdbCmdParseEntry;

static inline int startswith(const char *string, const char *pattern)
Expand Down Expand Up @@ -835,6 +840,7 @@ static int process_string_cmd(void *user_ctx, const char *data,
}
}

gdbserver_state.allow_stop_reply = cmd->allow_stop_reply;
cmd->handler(params, user_ctx);
return 0;
}
Expand Down Expand Up @@ -1283,11 +1289,14 @@ static void handle_v_attach(GArray *params, void *user_ctx)
gdbserver_state.g_cpu = cpu;
gdbserver_state.c_cpu = cpu;

g_string_printf(gdbserver_state.str_buf, "T%02xthread:", GDB_SIGNAL_TRAP);
gdb_append_thread_id(cpu, gdbserver_state.str_buf);
g_string_append_c(gdbserver_state.str_buf, ';');
if (gdbserver_state.allow_stop_reply) {
g_string_printf(gdbserver_state.str_buf, "T%02xthread:", GDB_SIGNAL_TRAP);
gdb_append_thread_id(cpu, gdbserver_state.str_buf);
g_string_append_c(gdbserver_state.str_buf, ';');
gdbserver_state.allow_stop_reply = false;
cleanup:
gdb_put_strbuf();
gdb_put_strbuf();
}
}

static void handle_v_kill(GArray *params, void *user_ctx)
Expand All @@ -1310,12 +1319,14 @@ static const GdbCmdParseEntry gdb_v_commands_table[] = {
.handler = handle_v_cont,
.cmd = "Cont",
.cmd_startswith = 1,
.allow_stop_reply = true,
.schema = "s0"
},
{
.handler = handle_v_attach,
.cmd = "Attach;",
.cmd_startswith = 1,
.allow_stop_reply = true,
.schema = "l0"
},
{
Expand Down Expand Up @@ -1698,10 +1709,13 @@ static void handle_gen_set(GArray *params, void *user_ctx)

static void handle_target_halt(GArray *params, void *user_ctx)
{
g_string_printf(gdbserver_state.str_buf, "T%02xthread:", GDB_SIGNAL_TRAP);
gdb_append_thread_id(gdbserver_state.c_cpu, gdbserver_state.str_buf);
g_string_append_c(gdbserver_state.str_buf, ';');
gdb_put_strbuf();
if (gdbserver_state.allow_stop_reply) {
g_string_printf(gdbserver_state.str_buf, "T%02xthread:", GDB_SIGNAL_TRAP);
gdb_append_thread_id(gdbserver_state.c_cpu, gdbserver_state.str_buf);
g_string_append_c(gdbserver_state.str_buf, ';');
gdb_put_strbuf();
gdbserver_state.allow_stop_reply = false;
}
/*
* Remove all the breakpoints when this query is issued,
* because gdb is doing an initial connect and the state
Expand All @@ -1725,7 +1739,8 @@ static int gdb_handle_packet(const char *line_buf)
static const GdbCmdParseEntry target_halted_cmd_desc = {
.handler = handle_target_halt,
.cmd = "?",
.cmd_startswith = 1
.cmd_startswith = 1,
.allow_stop_reply = true,
};
cmd_parser = &target_halted_cmd_desc;
}
Expand All @@ -1736,6 +1751,7 @@ static int gdb_handle_packet(const char *line_buf)
.handler = handle_continue,
.cmd = "c",
.cmd_startswith = 1,
.allow_stop_reply = true,
.schema = "L0"
};
cmd_parser = &continue_cmd_desc;
Expand All @@ -1747,6 +1763,7 @@ static int gdb_handle_packet(const char *line_buf)
.handler = handle_cont_with_sig,
.cmd = "C",
.cmd_startswith = 1,
.allow_stop_reply = true,
.schema = "l0"
};
cmd_parser = &cont_with_sig_cmd_desc;
Expand Down Expand Up @@ -1785,6 +1802,7 @@ static int gdb_handle_packet(const char *line_buf)
.handler = handle_step,
.cmd = "s",
.cmd_startswith = 1,
.allow_stop_reply = true,
.schema = "L0"
};
cmd_parser = &step_cmd_desc;
Expand Down Expand Up @@ -1976,6 +1994,7 @@ void gdb_read_byte(uint8_t ch)
{
uint8_t reply;

gdbserver_state.allow_stop_reply = false;
#ifndef CONFIG_USER_ONLY
if (gdbserver_state.last_packet->len) {
/* Waiting for a response to the last packet. If we see the start
Expand Down
5 changes: 5 additions & 0 deletions gdbstub/internals.h
View file
Expand Up @@ -65,6 +65,11 @@ typedef struct GDBState {
GByteArray *mem_buf;
int sstep_flags;
int supported_sstep_flags;
/*
* Whether we are allowed to send a stop reply packet at this moment.
* Must be set off after sending the stop reply itself.
*/
bool allow_stop_reply;
} GDBState;

/* lives in main gdbstub.c */
Expand Down
13 changes: 11 additions & 2 deletions gdbstub/softmmu.c
View file
Expand Up @@ -43,6 +43,7 @@ static void reset_gdbserver_state(void)
g_free(gdbserver_state.processes);
gdbserver_state.processes = NULL;
gdbserver_state.process_num = 0;
gdbserver_state.allow_stop_reply = false;
}

/*
Expand Down Expand Up @@ -139,6 +140,10 @@ static void gdb_vm_state_change(void *opaque, bool running, RunState state)
return;
}

if (!gdbserver_state.allow_stop_reply) {
return;
}

gdb_append_thread_id(cpu, tid);

switch (state) {
Expand Down Expand Up @@ -205,6 +210,7 @@ static void gdb_vm_state_change(void *opaque, bool running, RunState state)

send_packet:
gdb_put_packet(buf->str);
gdbserver_state.allow_stop_reply = false;

/* disable single step if it was enabled */
cpu_single_step(cpu, 0);
Expand Down Expand Up @@ -422,8 +428,11 @@ void gdb_exit(int code)

trace_gdbstub_op_exiting((uint8_t)code);

snprintf(buf, sizeof(buf), "W%02x", (uint8_t)code);
gdb_put_packet(buf);
if (gdbserver_state.allow_stop_reply) {
snprintf(buf, sizeof(buf), "W%02x", (uint8_t)code);
gdb_put_packet(buf);
gdbserver_state.allow_stop_reply = false;
}

qemu_chr_fe_deinit(&gdbserver_system_state.chr, true);
}
Expand Down
24 changes: 16 additions & 8 deletions gdbstub/user.c
View file
Expand Up @@ -108,8 +108,11 @@ void gdb_exit(int code)

trace_gdbstub_op_exiting((uint8_t)code);

snprintf(buf, sizeof(buf), "W%02x", (uint8_t)code);
gdb_put_packet(buf);
if (gdbserver_state.allow_stop_reply) {
snprintf(buf, sizeof(buf), "W%02x", (uint8_t)code);
gdb_put_packet(buf);
gdbserver_state.allow_stop_reply = false;
}
}

int gdb_handlesig(CPUState *cpu, int sig)
Expand All @@ -127,11 +130,14 @@ int gdb_handlesig(CPUState *cpu, int sig)

if (sig != 0) {
gdb_set_stop_cpu(cpu);
g_string_printf(gdbserver_state.str_buf,
"T%02xthread:", gdb_target_signal_to_gdb(sig));
gdb_append_thread_id(cpu, gdbserver_state.str_buf);
g_string_append_c(gdbserver_state.str_buf, ';');
gdb_put_strbuf();
if (gdbserver_state.allow_stop_reply) {
g_string_printf(gdbserver_state.str_buf,
"T%02xthread:", gdb_target_signal_to_gdb(sig));
gdb_append_thread_id(cpu, gdbserver_state.str_buf);
g_string_append_c(gdbserver_state.str_buf, ';');
gdb_put_strbuf();
gdbserver_state.allow_stop_reply = false;
}
}
/*
* gdb_put_packet() might have detected that the peer terminated the
Expand Down Expand Up @@ -174,12 +180,14 @@ void gdb_signalled(CPUArchState *env, int sig)
{
char buf[4];

if (!gdbserver_state.init || gdbserver_user_state.fd < 0) {
if (!gdbserver_state.init || gdbserver_user_state.fd < 0 ||
!gdbserver_state.allow_stop_reply) {
return;
}

snprintf(buf, sizeof(buf), "X%02x", gdb_target_signal_to_gdb(sig));
gdb_put_packet(buf);
gdbserver_state.allow_stop_reply = false;
}

static void gdb_accept_init(int fd)
Expand Down

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