[Qemu-devel] [PATCH] Fix segfault after migration completes

[Luiz Capitulino <lcapitulino@redhat.com>]

To reproduce:

1. Start the source VM with:

 # qemu [...] -S

2. Start the destination VM with:

 # qemu <source VM cmd-line> -incoming tcp:0:4444

3. In the source VM:

  (qemu) migrate -d tcp:0:4444

3. The source VM will segfault as soon as migration completes (might not
   happen in the first try)

Here's the backtrace:

#0  0x0000000000516f39 in qemu_file_get_error (f=0x0) at /home/lcapitulino/src/qmp-unstable/savevm.c:431
431	    return f->last_error;

#0  0x0000000000516f39 in qemu_file_get_error (f=0x0) at /home/lcapitulino/src/qmp-unstable/savevm.c:431
#1  0x00000000004e7a9a in migrate_fd_put_notify (opaque=0x987640) at /home/lcapitulino/src/qmp-unstable/migration.c:255
#2  0x000000000046d59a in qemu_iohandler_poll (readfds=0x7fff45ccfe50, writefds=0x7fff45ccfdd0, xfds=0x7fff45ccfd50, ret=1)
    at /home/lcapitulino/src/qmp-unstable/iohandler.c:124
#3  0x00000000004e6033 in main_loop_wait (nonblocking=0) at /home/lcapitulino/src/qmp-unstable/main-loop.c:463
#4  0x00000000004db5b0 in main_loop () at /home/lcapitulino/src/qmp-unstable/vl.c:1478
#5  0x00000000004dffed in main (argc=16, argv=0x7fff45cd0318, envp=0x7fff45cd03a0) at /home/lcapitulino/src/qmp-unstable/vl.c:3449

So, 's->file' is NULL in migrate_fd_put_notify(). The interesting thing
is that it's valid in the qemu_file_put_notify() call, which makes me
think that either: there's a race somewhere or qemu_file_put_notify() is
itself clearing 's->file'. In both cases the fix below could just be hiding
the real issue, but let's get started...

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---
 migration.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/migration.c b/migration.c
index bdca72e..f6e6208 100644
--- a/migration.c
+++ b/migration.c
@@ -252,7 +252,7 @@ static void migrate_fd_put_notify(void *opaque)
 
     qemu_set_fd_handler2(s->fd, NULL, NULL, NULL, NULL);
     qemu_file_put_notify(s->file);
-    if (qemu_file_get_error(s->file)) {
+    if (s->file && qemu_file_get_error(s->file)) {
         migrate_fd_error(s);
     }
 }
-- 
1.7.7.1.488.ge8e1c.dirty