gdbstub: fixes cases where wrong threads were reported to GDB on SIGINT

This fix is implemented by having the vCont handler set the value of
`gdbserver_state.c_cpu` if any threads are to be resumed. The specific
CPU picked is arbitrarily from the ones to be resumed, but it should
be okay, as all GDB cares about is that it is a resumed thread.

Signed-off-by: Matheus Branco Borella <dark.ryu.550@gmail.com>
Message-Id: <20230804182633.47300-2-dark.ryu.550@gmail.com>
[AJB: style and whitespace fixes]
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1725
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230829161528.2707696-9-alex.bennee@linaro.org>

gdbstub/gdbstub.c

@@ -597,6 +597,15 @@ static int gdb_handle_vcont(const char *p)
      *  or incorrect parameters passed.
      */
     res = 0;
+
+    /*
+     * target_count and last_target keep track of how many CPUs we are going to
+     * step or resume, and a pointer to the state structure of one of them,
+     * respectivelly
+     */
+    int target_count = 0;
+    CPUState *last_target = NULL;
+
     while (*p) {
         if (*p++ != ';') {
             return -ENOTSUP;
@@ -637,6 +646,9 @@ static int gdb_handle_vcont(const char *p)
             while (cpu) {
                 if (newstates[cpu->cpu_index] == 1) {
                     newstates[cpu->cpu_index] = cur_action;
+
+                    target_count++;
+                    last_target = cpu;
                 }
 
                 cpu = gdb_next_attached_cpu(cpu);
@@ -654,6 +666,9 @@ static int gdb_handle_vcont(const char *p)
             while (cpu) {
                 if (newstates[cpu->cpu_index] == 1) {
                     newstates[cpu->cpu_index] = cur_action;
+
+                    target_count++;
+                    last_target = cpu;
                 }
 
                 cpu = gdb_next_cpu_in_process(cpu);
@@ -671,11 +686,25 @@ static int gdb_handle_vcont(const char *p)
             /* only use if no previous match occourred */
             if (newstates[cpu->cpu_index] == 1) {
                 newstates[cpu->cpu_index] = cur_action;
+
+                target_count++;
+                last_target = cpu;
             }
             break;
         }
     }
 
+    /*
+     * if we're about to resume a specific set of CPUs/threads, make it so that
+     * in case execution gets interrupted, we can send GDB a stop reply with a
+     * correct value. it doesn't really matter which CPU we tell GDB the signal
+     * happened in (VM pauses stop all of them anyway), so long as it is one of
+     * the ones we resumed/single stepped here.
+     */
+    if (target_count > 0) {
+        gdbserver_state.c_cpu = last_target;
+    }
+
     gdbserver_state.signal = signal;
     gdb_continue_partial(newstates);
     return res;

tests/tcg/multiarch/gdbstub/interrupt.py

@@ -0,0 +1,97 @@
+from __future__ import print_function
+#
+# Test some of the softmmu debug features with the multiarch memory
+# test. It is a port of the original vmlinux focused test case but
+# using the "memory" test instead.
+#
+# This is launched via tests/guest-debug/run-test.py
+#
+
+import gdb
+import sys
+
+failcount = 0
+
+
+def report(cond, msg):
+    "Report success/fail of test"
+    if cond:
+        print("PASS: %s" % (msg))
+    else:
+        print("FAIL: %s" % (msg))
+        global failcount
+        failcount += 1
+
+
+def check_interrupt(thread):
+    """
+    Check that, if thread is resumed, we go back to the same thread when the
+    program gets interrupted.
+    """
+
+    # Switch to the thread we're going to be running the test in.
+    print("thread ", thread.num)
+    gdb.execute("thr %d" % thread.num)
+
+    # Enter the loop() function on this thread.
+    #
+    # While there are cleaner ways to do this, we want to minimize the number of
+    # side effects on the gdbstub's internal state, since those may mask bugs.
+    # Ideally, there should be no difference between what we're doing here and
+    # the program reaching the loop() function on its own.
+    #
+    # For this to be safe, we only need the prologue of loop() to not have
+    # instructions that may have problems with what we're doing here. We don't
+    # have to worry about anything else, as this function never returns.
+    gdb.execute("set $pc = loop")
+
+    # Continue and then interrupt the task.
+    gdb.post_event(lambda: gdb.execute("interrupt"))
+    gdb.execute("c")
+
+    # Check whether the thread we're in after the interruption is the same we
+    # ran continue from.
+    return (thread.num == gdb.selected_thread().num)
+
+
+def run_test():
+    """
+    Test if interrupting the code always lands us on the same thread when
+    running with scheduler-lock enabled.
+    """
+
+    gdb.execute("set scheduler-locking on")
+    for thread in gdb.selected_inferior().threads():
+        report(check_interrupt(thread),
+               "thread %d resumes correctly on interrupt" % thread.num)
+
+
+#
+# This runs as the script it sourced (via -x, via run-test.py)
+#
+try:
+    inferior = gdb.selected_inferior()
+    arch = inferior.architecture()
+    print("ATTACHED: %s" % arch.name())
+except (gdb.error, AttributeError):
+    print("SKIPPING (not connected)", file=sys.stderr)
+    exit(0)
+
+if gdb.parse_and_eval('$pc') == 0:
+    print("SKIP: PC not set")
+    exit(0)
+if len(gdb.selected_inferior().threads()) == 1:
+    print("SKIP: set to run on a single thread")
+    exit(0)
+
+try:
+    # Run the actual tests
+    run_test()
+except (gdb.error):
+    print("GDB Exception: %s" % (sys.exc_info()[0]))
+    failcount += 1
+    pass
+
+# Finally kill the inferior and exit gdb with a count of failures
+gdb.execute("kill")
+exit(failcount)

tests/tcg/multiarch/system/Makefile.softmmu-target

@@ -27,7 +27,15 @@ run-gdbstub-memory: memory
 		"-monitor none -display none -chardev file$(COMMA)path=$<.out$(COMMA)id=output $(QEMU_OPTS)" \
 		--bin $< --test $(MULTIARCH_SRC)/gdbstub/memory.py, \
 	softmmu gdbstub support)
-
+run-gdbstub-interrupt: interrupt
+	$(call run-test, $@, $(GDB_SCRIPT) \
+		--gdb $(HAVE_GDB_BIN) \
+		--qemu $(QEMU) \
+		--output $<.gdb.out \
+		--qargs \
+		"-smp 2 -monitor none -display none -chardev file$(COMMA)path=$<.out$(COMMA)id=output $(QEMU_OPTS)" \
+		--bin $< --test $(MULTIARCH_SRC)/gdbstub/interrupt.py, \
+	softmmu gdbstub support)
 run-gdbstub-untimely-packet: hello
 	$(call run-test, $@, $(GDB_SCRIPT) \
 		--gdb $(HAVE_GDB_BIN) \
@@ -50,4 +58,4 @@ run-gdbstub-%:
 	$(call skip-test, "gdbstub test $*", "need working gdb")
 endif
 
-MULTIARCH_RUNS += run-gdbstub-memory run-gdbstub-untimely-packet
+MULTIARCH_RUNS += run-gdbstub-memory run-gdbstub-interrupt run-gdbstub-untimely-packet

tests/tcg/multiarch/system/interrupt.c

@@ -0,0 +1,28 @@
+/*
+ * External interruption test. This test is structured in such a way that it
+ * passes the cases that require it to exit, but we can make it enter an
+ * infinite loop from GDB.
+ *
+ * We don't have the benefit of libc, just builtin C primitives and
+ * whatever is in minilib.
+ */
+
+#include <minilib.h>
+
+void loop(void)
+{
+    do {
+        /*
+         * Loop forever. Just make sure the condition is always a constant
+         * expression, so that this loop is not UB, as per the C
+         * standard.
+         */
+    } while (1);
+}
+
+int main(void)
+{
+    return 0;
+}
+
+