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>

gdbstub/gdbstub.c

@@ -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,
@@ -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)
@@ -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;
     }
@@ -1283,12 +1289,15 @@ static void handle_v_attach(GArray *params, void *user_ctx)
     gdbserver_state.g_cpu = cpu;
     gdbserver_state.c_cpu = cpu;
 
+    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();
     }
+}
 
 static void handle_v_kill(GArray *params, void *user_ctx)
 {
@@ -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"
     },
     {
@@ -1698,10 +1709,13 @@ static void handle_gen_set(GArray *params, void *user_ctx)
 
 static void handle_target_halt(GArray *params, void *user_ctx)
 {
+    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
@@ -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;
         }
@@ -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;
@@ -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;
@@ -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;
@@ -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

gdbstub/internals.h

@@ -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 */

gdbstub/softmmu.c

@@ -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;
 }
 
 /*
@@ -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) {
@@ -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);
@@ -422,8 +428,11 @@ void gdb_exit(int code)
 
     trace_gdbstub_op_exiting((uint8_t)code);
 
+    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);
 }

gdbstub/user.c

@@ -108,8 +108,11 @@ void gdb_exit(int code)
 
     trace_gdbstub_op_exiting((uint8_t)code);
 
+    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)
@@ -127,11 +130,14 @@ int gdb_handlesig(CPUState *cpu, int sig)
 
     if (sig != 0) {
         gdb_set_stop_cpu(cpu);
+        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
@@ -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)