Re: [Qemu-devel] [PATCH] monitor: fix dangling CPU pointer

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 16 Oct 2017 13:24:20 +0200
Greg Kurz <groug@kaod.org> wrote:

> On Mon, 16 Oct 2017 10:41:39 +0200
> Igor Mammedov <imammedo@redhat.com> wrote:
> 
> > On Fri, 13 Oct 2017 13:47:51 +0200
> > Greg Kurz <groug@kaod.org> wrote:
> > 
> > [...]
> >   
> > > Note that the resolution should really return a CPU object, otherwise
> > > we have a bug. This is achieved by relying on object_resolve_path()
> > > and CPU() instead of calling object_resolve_path_type(path, TYPE_CPU).    
> > I'm not sure what you are trying to say here, could you explain
> > a bit more why CPU(object_resolve_path()) is chosen vs
> > object_resolve_path_type(path, TYPE_CPU)
> >   
> 
> IIUC, if we use object_resolve_path_type(path, TYPE_CPU) and path doesn't
> point to a CPU object, it will return NULL (see object_resolve_abs_path())
> just like if the CPU got hot-unplugged.
> 
> My point is that the path we got from object_get_canonical_path() did
> point to a CPU: if later on this path resolves to an object that isn't
> a CPU, then some code somewhere used the same QOM path for some unrelated
> object. I tend to think this is a bug in QEMU and we shouldn't silently
> ignore it.
sometimes QOM path to a cpu object could be:
 1. arbitrary if set explicitly by board 
 2. /unattached/device[X] or /peripheral-anon/device[X]
       potentially might end-up with another device at the same path
       in case of hot-remove + migration
 3. /peripheral
       hot-remove named (i.e. non null 'id') cpu and then plug a non cpu device with the same 'id',
       which is legitimate thing to do

I'd use object_resolve_path_type(path, TYPE_CPU) and
gracefully error out in case it returns NULL.

> Makes sense ?
> 
> >   
> > > Suggested-by: Igor Mammedov <imammedo@redhat.com>
> > > Signed-off-by: Greg Kurz <groug@kaod.org>
> > > ---
> > >  monitor.c |   25 ++++++++++++++++++++-----
> > >  1 file changed, 20 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/monitor.c b/monitor.c
> > > index fe0d1bdbb461..8489b2ad99c0 100644
> > > --- a/monitor.c
> > > +++ b/monitor.c
> > > @@ -200,7 +200,7 @@ struct Monitor {
> > >  
> > >      ReadLineState *rs;
> > >      MonitorQMP qmp;
> > > -    CPUState *mon_cpu;
> > > +    gchar *mon_cpu_path;
> > >      BlockCompletionFunc *password_completion_cb;
> > >      void *password_opaque;
> > >      mon_cmd_t *cmd_table;
> > > @@ -579,6 +579,7 @@ static void monitor_data_init(Monitor *mon)
> > >  
> > >  static void monitor_data_destroy(Monitor *mon)
> > >  {
> > > +    g_free(mon->mon_cpu_path);
> > >      qemu_chr_fe_deinit(&mon->chr, false);
> > >      if (monitor_is_qmp(mon)) {
> > >          json_message_parser_destroy(&mon->qmp.parser);
> > > @@ -1047,20 +1048,34 @@ int monitor_set_cpu(int cpu_index)
> > >      if (cpu == NULL) {
> > >          return -1;
> > >      }
> > > -    cur_mon->mon_cpu = cpu;
> > > +    g_free(cur_mon->mon_cpu_path);
> > > +    cur_mon->mon_cpu_path = object_get_canonical_path(OBJECT(cpu));
> > >      return 0;
> > >  }
> > >  
> > >  CPUState *mon_get_cpu(void)
> > >  {
> > > -    if (!cur_mon->mon_cpu) {
> > > +    CPUState *cpu;
> > > +
> > > +    if (cur_mon->mon_cpu_path) {
> > > +        Object *obj = object_resolve_path(cur_mon->mon_cpu_path, NULL);
> > > +
> > > +        if (!obj) {
> > > +            g_free(cur_mon->mon_cpu_path);
> > > +            cur_mon->mon_cpu_path = NULL;
> > > +        } else {
> > > +            cpu = CPU(obj);    
> > this potentially might abort if obj couldn't be cast to TYPE_CPU
> >   
> 
> This is deliberate... is there a case where cur_mon->mon_cpu_path would
> legitimately point to something that isn't of type TYPE_CPU ?
> 
> >   
> > > +        }
> > > +    }
> > > +    if (!cur_mon->mon_cpu_path) {
> > >          if (!first_cpu) {
> > >              return NULL;
> > >          }
> > >          monitor_set_cpu(first_cpu->cpu_index);
> > > +        cpu = first_cpu;
> > >      }
> > > -    cpu_synchronize_state(cur_mon->mon_cpu);
> > > -    return cur_mon->mon_cpu;
> > > +    cpu_synchronize_state(cpu);
> > > +    return cpu;
> > >  }
> > >  
> > >  CPUArchState *mon_get_cpu_env(void)
> > > 
> > >     
> >   
> 
>