Re: [Qemu-devel] [PATCH v2] Fix segfault on migration completion

[Markus Armbruster <armbru@redhat.com>]

Luiz Capitulino <lcapitulino@redhat.com> writes:

> A simple migration reproduces it:
>
> 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
>
> 4. The source VM will segfault as soon as migration completes (might not
>    happen in the first try)
>
> What is happening here is that qemu_file_put_notify() can end up closing
> 's->file' (in which case it's also set to NULL). The call stack is rather
> complex, but Eduardo helped tracking it to:
>
> select loop -> migrate_fd_put_notify() -> qemu_file_put_notify() ->
> buffered_put_buffer() -> migrate_fd_put_ready() ->
> migrate_fd_completed() -> migrate_fd_cleanup().
>
> To be honest, it's not completely clear to me in which cases 's->file'
> is not closed (on error maybe)? But I doubt this fix will make anything
> worse.
>
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> Acked-by: Eduardo Habkost <ehabkost@redhat.com>
> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> ---
>
> V2: better commit log
>
>  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);
>      }
>  }

I wonder whether we can lose the error in s->file by closing s->file
before we get here.  But even if we can, we still report more errors
than before the series this patch fixes.