gdbstub: Implement follow-fork-mode child

Currently it's not possible to use gdbstub for debugging linux-user
code that runs in a forked child, which is normally done using the `set
follow-fork-mode child` GDB command. Purely on the protocol level, the
missing piece is the fork-events feature.

However, a deeper problem is supporting $Hg switching between different
processes - right now it can do only threads. Implementing this for the
general case would be quite complicated, but, fortunately, for the
follow-fork-mode case there are a few factors that greatly simplify
things: fork() happens in the exclusive section, there are only two
processes involved, and before one of them is resumed, the second one
is detached.

This makes it possible to implement a simplified scheme: the parent and
the child share the gdbserver socket, it's used only by one of them at
any given time, which is coordinated through a separate socketpair. The
processes can read from the gdbserver socket only one byte at a time,
which is not great for performance, but, fortunately, the
follow-fork-mode handling involves only a few messages.

Advertise the fork-events support, and remember whether GDB has it
as well. Implement the state machine that is initialized on fork(),
decides the current owner of the gdbserver socket, and is terminated
when one of the two processes is detached. The logic for the parent and
the child is the same, only the initial state is different.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20240219141628.246823-12-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240305121005.3528075-13-alex.bennee@linaro.org>

gdbstub/user.c

@@ -25,6 +25,61 @@
 #define GDB_NR_SYSCALLS 1024
 typedef unsigned long GDBSyscallsMask[BITS_TO_LONGS(GDB_NR_SYSCALLS)];
 
+/*
+ * Forked child talks to its parent in order to let GDB enforce the
+ * follow-fork-mode. This happens inside a start_exclusive() section, so that
+ * the other threads, which may be forking too, do not interfere. The
+ * implementation relies on GDB not sending $vCont until it has detached
+ * either from the parent (follow-fork-mode child) or from the child
+ * (follow-fork-mode parent).
+ *
+ * The parent and the child share the GDB socket; at any given time only one
+ * of them is allowed to use it, as is reflected in the respective fork_state.
+ * This is negotiated via the fork_sockets pair as a reaction to $Hg.
+ *
+ * Below is a short summary of the possible state transitions:
+ *
+ *     ENABLED                     : Terminal state.
+ *     DISABLED                    : Terminal state.
+ *     ACTIVE                      : Parent initial state.
+ *     INACTIVE                    : Child initial state.
+ *     ACTIVE       -> DEACTIVATING: On $Hg.
+ *     ACTIVE       -> ENABLING    : On $D.
+ *     ACTIVE       -> DISABLING   : On $D.
+ *     ACTIVE       -> DISABLED    : On communication error.
+ *     DEACTIVATING -> INACTIVE    : On gdb_read_byte() return.
+ *     DEACTIVATING -> DISABLED    : On communication error.
+ *     INACTIVE     -> ACTIVE      : On $Hg in the peer.
+ *     INACTIVE     -> ENABLE      : On $D in the peer.
+ *     INACTIVE     -> DISABLE     : On $D in the peer.
+ *     INACTIVE     -> DISABLED    : On communication error.
+ *     ENABLING     -> ENABLED     : On gdb_read_byte() return.
+ *     ENABLING     -> DISABLED    : On communication error.
+ *     DISABLING    -> DISABLED    : On gdb_read_byte() return.
+ */
+enum GDBForkState {
+    /* Fully owning the GDB socket. */
+    GDB_FORK_ENABLED,
+    /* Working with the GDB socket; the peer is inactive. */
+    GDB_FORK_ACTIVE,
+    /* Handing off the GDB socket to the peer. */
+    GDB_FORK_DEACTIVATING,
+    /* The peer is working with the GDB socket. */
+    GDB_FORK_INACTIVE,
+    /* Asking the peer to close its GDB socket fd. */
+    GDB_FORK_ENABLING,
+    /* Asking the peer to take over, closing our GDB socket fd. */
+    GDB_FORK_DISABLING,
+    /* The peer has taken over, our GDB socket fd is closed. */
+    GDB_FORK_DISABLED,
+};
+
+enum GDBForkMessage {
+    GDB_FORK_ACTIVATE = 'a',
+    GDB_FORK_ENABLE = 'e',
+    GDB_FORK_DISABLE = 'd',
+};
+
 /* User-mode specific state */
 typedef struct {
     int fd;
@@ -36,6 +91,10 @@ typedef struct {
      */
     bool catch_all_syscalls;
     GDBSyscallsMask catch_syscalls_mask;
+    bool fork_events;
+    enum GDBForkState fork_state;
+    int fork_sockets[2];
+    pid_t fork_peer_pid, fork_peer_tid;
 } GDBUserState;
 
 static GDBUserState gdbserver_user_state;
@@ -358,6 +417,18 @@ int gdbserver_start(const char *port_or_path)
 
 void gdbserver_fork_start(void)
 {
+    if (!gdbserver_state.init || gdbserver_user_state.fd < 0) {
+        return;
+    }
+    if (!gdbserver_user_state.fork_events ||
+            qemu_socketpair(AF_UNIX, SOCK_STREAM, 0,
+                            gdbserver_user_state.fork_sockets) < 0) {
+        gdbserver_user_state.fork_state = GDB_FORK_DISABLED;
+        return;
+    }
+    gdbserver_user_state.fork_state = GDB_FORK_INACTIVE;
+    gdbserver_user_state.fork_peer_pid = getpid();
+    gdbserver_user_state.fork_peer_tid = qemu_get_thread_id();
 }
 
 static void disable_gdbstub(CPUState *thread_cpu)
@@ -376,23 +447,160 @@ static void disable_gdbstub(CPUState *thread_cpu)
 
 void gdbserver_fork_end(CPUState *cpu, pid_t pid)
 {
-    if (pid != 0 || !gdbserver_state.init || gdbserver_user_state.fd < 0) {
+    char b;
+    int fd;
+
+    if (!gdbserver_state.init || gdbserver_user_state.fd < 0) {
         return;
     }
-    disable_gdbstub(cpu);
+
+    if (pid == -1) {
+        if (gdbserver_user_state.fork_state != GDB_FORK_DISABLED) {
+            g_assert(gdbserver_user_state.fork_state == GDB_FORK_INACTIVE);
+            close(gdbserver_user_state.fork_sockets[0]);
+            close(gdbserver_user_state.fork_sockets[1]);
+        }
+        return;
+    }
+
+    if (gdbserver_user_state.fork_state == GDB_FORK_DISABLED) {
+        if (pid == 0) {
+            disable_gdbstub(cpu);
+        }
+        return;
+    }
+
+    if (pid == 0) {
+        close(gdbserver_user_state.fork_sockets[0]);
+        fd = gdbserver_user_state.fork_sockets[1];
+        g_assert(gdbserver_state.process_num == 1);
+        g_assert(gdbserver_state.processes[0].pid ==
+                     gdbserver_user_state.fork_peer_pid);
+        g_assert(gdbserver_state.processes[0].attached);
+        gdbserver_state.processes[0].pid = getpid();
+    } else {
+        close(gdbserver_user_state.fork_sockets[1]);
+        fd = gdbserver_user_state.fork_sockets[0];
+        gdbserver_user_state.fork_state = GDB_FORK_ACTIVE;
+        gdbserver_user_state.fork_peer_pid = pid;
+        gdbserver_user_state.fork_peer_tid = pid;
+
+        if (!gdbserver_state.allow_stop_reply) {
+            goto fail;
+        }
+        g_string_printf(gdbserver_state.str_buf,
+                        "T%02xfork:p%02x.%02x;thread:p%02x.%02x;",
+                        gdb_target_signal_to_gdb(gdb_target_sigtrap()),
+                        pid, pid, (int)getpid(), qemu_get_thread_id());
+        gdb_put_strbuf();
+    }
+
+    gdbserver_state.state = RS_IDLE;
+    gdbserver_state.allow_stop_reply = false;
+    gdbserver_user_state.running_state = 0;
+    for (;;) {
+        switch (gdbserver_user_state.fork_state) {
+        case GDB_FORK_ENABLED:
+            if (gdbserver_user_state.running_state) {
+                return;
+            }
+            QEMU_FALLTHROUGH;
+        case GDB_FORK_ACTIVE:
+            if (read(gdbserver_user_state.fd, &b, 1) != 1) {
+                goto fail;
+            }
+            gdb_read_byte(b);
+            break;
+        case GDB_FORK_DEACTIVATING:
+            b = GDB_FORK_ACTIVATE;
+            if (write(fd, &b, 1) != 1) {
+                goto fail;
+            }
+            gdbserver_user_state.fork_state = GDB_FORK_INACTIVE;
+            break;
+        case GDB_FORK_INACTIVE:
+            if (read(fd, &b, 1) != 1) {
+                goto fail;
+            }
+            switch (b) {
+            case GDB_FORK_ACTIVATE:
+                gdbserver_user_state.fork_state = GDB_FORK_ACTIVE;
+                break;
+            case GDB_FORK_ENABLE:
+                close(fd);
+                gdbserver_user_state.fork_state = GDB_FORK_ENABLED;
+                break;
+            case GDB_FORK_DISABLE:
+                gdbserver_user_state.fork_state = GDB_FORK_DISABLED;
+                break;
+            default:
+                g_assert_not_reached();
+            }
+            break;
+        case GDB_FORK_ENABLING:
+            b = GDB_FORK_DISABLE;
+            if (write(fd, &b, 1) != 1) {
+                goto fail;
+            }
+            close(fd);
+            gdbserver_user_state.fork_state = GDB_FORK_ENABLED;
+            break;
+        case GDB_FORK_DISABLING:
+            b = GDB_FORK_ENABLE;
+            if (write(fd, &b, 1) != 1) {
+                goto fail;
+            }
+            gdbserver_user_state.fork_state = GDB_FORK_DISABLED;
+            break;
+        case GDB_FORK_DISABLED:
+            close(fd);
+            disable_gdbstub(cpu);
+            return;
+        default:
+            g_assert_not_reached();
+        }
+    }
+
+fail:
+    close(fd);
+    if (pid == 0) {
+        disable_gdbstub(cpu);
+    }
 }
 
 void gdb_handle_query_supported_user(const char *gdb_supported)
 {
+    if (strstr(gdb_supported, "fork-events+")) {
+        gdbserver_user_state.fork_events = true;
+    }
+    g_string_append(gdbserver_state.str_buf, ";fork-events+");
 }
 
 bool gdb_handle_set_thread_user(uint32_t pid, uint32_t tid)
 {
+    if (gdbserver_user_state.fork_state == GDB_FORK_ACTIVE &&
+            pid == gdbserver_user_state.fork_peer_pid &&
+            tid == gdbserver_user_state.fork_peer_tid) {
+        gdbserver_user_state.fork_state = GDB_FORK_DEACTIVATING;
+        gdb_put_packet("OK");
+        return true;
+    }
     return false;
 }
 
 bool gdb_handle_detach_user(uint32_t pid)
 {
+    bool enable;
+
+    if (gdbserver_user_state.fork_state == GDB_FORK_ACTIVE) {
+        enable = pid == gdbserver_user_state.fork_peer_pid;
+        if (enable || pid == getpid()) {
+            gdbserver_user_state.fork_state = enable ? GDB_FORK_ENABLING :
+                                                       GDB_FORK_DISABLING;
+            gdb_put_packet("OK");
+            return true;
+        }
+    }
     return false;
 }