Re: [PATCH v2 2/5] migration: Fix race on qemu_file_shutdown()

[Juan Quintela]

Peter Xu <peterx@redhat.com> wrote:
> In qemu_file_shutdown(), there's a possible race if with current order of
> operation.  There're two major things to do:
>
>   (1) Do real shutdown() (e.g. shutdown() syscall on socket)
>   (2) Update qemufile's last_error
>
> We must do (2) before (1) otherwise there can be a race condition like:
>
>       page receiver                     other thread
>       -------------                     ------------
>       qemu_get_buffer()
>                                         do shutdown()
>         returns 0 (buffer all zero)
>         (meanwhile we didn't check this retcode)
>       try to detect IO error
>         last_error==NULL, IO okay
>       install ALL-ZERO page
>                                         set last_error
>       --> guest crash!
>
> To fix this, we can also check retval of qemu_get_buffer(), but not all
> APIs can be properly checked and ultimately we still need to go back to
> qemu_file_get_error().  E.g. qemu_get_byte() doesn't return error.
>
> Maybe some day a rework of qemufile API is really needed, but for now keep
> using qemu_file_get_error() and fix it by not allowing that race condition
> to happen.  Here shutdown() is indeed special because the last_error was
> emulated.  For real -EIO errors it'll always be set when e.g. sendmsg()
> error triggers so we won't miss those ones, only shutdown() is a bit tricky
> here.
>
> Cc: Daniel P. Berrange <berrange@redhat.com>
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>