[PATCH 11/18] gdbstub: Try unlinking the unix socket before binding

["Alex Bennée" <alex.bennee@linaro.org>]

From: Ilya Leoshkevich <iii@linux.ibm.com>

In case an emulated process execve()s another emulated process, bind()
will fail, because the socket already exists. So try deleting it. Use
the existing unix_listen() function which does this. Link qemu-user
with qemu-sockets.c and add the monitor_get_fd() stub.

Note that it is not possible to handle this in do_execv(): deleting
gdbserver_user_state.socket_path before safe_execve() is not correct,
because the latter may fail, and afterwards we may lose control.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250117001542.8290-3-iii@linux.ibm.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 gdbstub/user.c     | 29 +++--------------------------
 stubs/monitor-fd.c |  9 +++++++++
 stubs/meson.build  |  2 ++
 util/meson.build   |  2 ++
 4 files changed, 16 insertions(+), 26 deletions(-)
 create mode 100644 stubs/monitor-fd.c

diff --git a/gdbstub/user.c b/gdbstub/user.c
index fd29d595f4..8225b70280 100644
--- a/gdbstub/user.c
+++ b/gdbstub/user.c
@@ -315,12 +315,10 @@ static bool gdb_accept_socket(int gdb_fd)
     return true;
 }
 
-static int gdbserver_open_socket(const char *path)
+static int gdbserver_open_socket(const char *path, Error **errp)
 {
     g_autoptr(GString) buf = g_string_new("");
-    struct sockaddr_un sockaddr = {};
     char *pid_placeholder;
-    int fd, ret;
 
     pid_placeholder = strstr(path, "%d");
     if (pid_placeholder != NULL) {
@@ -330,28 +328,7 @@ static int gdbserver_open_socket(const char *path)
         path = buf->str;
     }
 
-    fd = socket(AF_UNIX, SOCK_STREAM, 0);
-    if (fd < 0) {
-        perror("create socket");
-        return -1;
-    }
-
-    sockaddr.sun_family = AF_UNIX;
-    pstrcpy(sockaddr.sun_path, sizeof(sockaddr.sun_path) - 1, path);
-    ret = bind(fd, (struct sockaddr *)&sockaddr, sizeof(sockaddr));
-    if (ret < 0) {
-        perror("bind socket");
-        close(fd);
-        return -1;
-    }
-    ret = listen(fd, 1);
-    if (ret < 0) {
-        perror("listen socket");
-        close(fd);
-        return -1;
-    }
-
-    return fd;
+    return unix_listen(path, errp);
 }
 
 static bool gdb_accept_tcp(int gdb_fd)
@@ -424,7 +401,7 @@ bool gdbserver_start(const char *port_or_path, Error **errp)
     if (port > 0) {
         gdb_fd = gdbserver_open_port(port, errp);
     } else {
-        gdb_fd = gdbserver_open_socket(port_or_path);
+        gdb_fd = gdbserver_open_socket(port_or_path, errp);
     }
 
     if (gdb_fd < 0) {
diff --git a/stubs/monitor-fd.c b/stubs/monitor-fd.c
new file mode 100644
index 0000000000..9bb6749885
--- /dev/null
+++ b/stubs/monitor-fd.c
@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+#include "qemu/osdep.h"
+#include "monitor/monitor.h"
+
+int monitor_get_fd(Monitor *mon, const char *fdname, Error **errp)
+{
+    abort();
+}
diff --git a/stubs/meson.build b/stubs/meson.build
index a8b3aeb564..b0fee37e05 100644
--- a/stubs/meson.build
+++ b/stubs/meson.build
@@ -61,6 +61,8 @@ if have_user
   if not have_system
     stub_ss.add(files('qdev.c'))
   endif
+
+  stub_ss.add(files('monitor-fd.c'))
 endif
 
 if have_system
diff --git a/util/meson.build b/util/meson.build
index 5d8bef9891..780b5977a8 100644
--- a/util/meson.build
+++ b/util/meson.build
@@ -84,6 +84,8 @@ if have_block or have_ga
   util_ss.add(files('qemu-coroutine.c', 'qemu-coroutine-lock.c', 'qemu-coroutine-io.c'))
   util_ss.add(files(f'coroutine-@coroutine_backend@.c'))
   util_ss.add(files('thread-pool.c', 'qemu-timer.c'))
+endif
+if have_block or have_ga or have_user
   util_ss.add(files('qemu-sockets.c'))
 endif
 if have_block
-- 
2.39.5