[Qemu-devel] [PATCH v4 1/2] gdbstub.c: fix GDB connection segfault caused by empty machines

[Ziyue Yang <skiver.cloud.yzy@gmail.com>]

From: Ziyue Yang <yzylivezh@hotmail.com>

This patch is to fix the segmentation fault caused by attaching
GDB to a QEMU instance initialized with "-M none" option.

The bug can be reproduced by

> ./qemu-system-x86_64 -M none -nographic -S -s

and attach a GDB to it by

> gdb -ex 'target remote :1234

The segmentation fault was originally caused by trying to read
the information about CPU when communicating with GDB. However,
it's impossible for any control flow to exist on an empty machine,
nor can CPU's be hot plugged to an empty machine later by QOM
commands. So I think simply disabling GDB connections on empty
machines makes sense.

Also some updates from fprintf(stderr, ...) to error_report.

Signed-off-by: Ziyue Yang <skiver.cloud.yzy@gmail.com>
---
 gdbstub.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/gdbstub.c b/gdbstub.c
index de62d26..426d55e 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -18,6 +18,7 @@
  */
 #include "qemu/osdep.h"
 #include "qapi/error.h"
+#include "qemu/error-report.h"
 #include "qemu/cutils.h"
 #include "cpu.h"
 #ifdef CONFIG_USER_ONLY
@@ -1731,6 +1732,12 @@ int gdbserver_start(const char *device)
     CharDriverState *mon_chr;
     ChardevCommon common = { 0 };

+    if (!first_cpu) {
+        error_report("gdbstub: meaningless to attach gdb to a "
+                     "machine without any CPU.");
+        return -1;
+    }
+
     if (!device)
         return -1;
     if (strcmp(device, "none") != 0) {
--
2.7.4