Re: [PATCH] infcmd.c: Don't attempt to record a NULL value after a finish command

[Kevin Buettner <kevinb@redhat.com>]

On Wed, 30 Sep 2015 11:27:25 +0100
Pedro Alves <palves@redhat.com> wrote:

> On 09/30/2015 05:43 AM, Kevin Buettner wrote:
> > Architectures which use RETURN_VALUE_STRUCT_CONVENTION will have a
> > NULL return value after executing a finish command.  See get_return_value()
> > in infcmd.c.
> > 
> > This patch avoids an eventual SIGSEV (caused by attempting to
> > derefrence a NULL pointer) by adding a suitable test to
> > finish_command_fsm_should_stop().
> > 
> > I encountered this problem while testing msp430:
> > 
> > (gdb) PASS: gdb.base/structs.exp: zed L<n> for finish; return 1 structs-tc
> > finish
> > Run till exit from #0  fun1 () at /ironwood1/sourceware-git/msp430-elf/../binutils-gdb/gdb/testsuite/gdb.base/structs.c:125
> > ERROR: Process no longer exists
> > 
> > gdb/ChangeLog:
> >     	* infcmd.c (finish_command_fsm_should_stop): Don't attempt to
> >     	record a NULL value.
> > ---
> >  gdb/infcmd.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/gdb/infcmd.c b/gdb/infcmd.c
> > index c4d7d8b..6be95e4 100644
> > --- a/gdb/infcmd.c
> > +++ b/gdb/infcmd.c
> > @@ -1788,7 +1788,7 @@ finish_command_fsm_should_stop (struct thread_fsm *self)
> >  	internal_error (__FILE__, __LINE__,
> >  			_("finish_command: function has no target type"));
> >  
> > -      if (TYPE_CODE (rv->type) != TYPE_CODE_VOID)
> > +      if (rv->value != NULL && TYPE_CODE (rv->type) != TYPE_CODE_VOID)
> >  	{
> >  	  struct value *func;
> 
> It's this else block that below sets rv->value in the first place:
> 
>       if (TYPE_CODE (rv->type) != TYPE_CODE_VOID)
> 	{
> 	  struct value *func;
> 
> 	  func = read_var_value (f->function, NULL, get_current_frame ());
> 	  rv->value = get_return_value (func, rv->type);
> 	  rv->value_history_index = record_latest_value (rv->value);
> 	}
> 
> So seems like that patch would break !RETURN_VALUE_STRUCT_CONVENTION
> targets?
> 
> I assume that it's record_latest_value that crashes?  Isn't the right
> fix instead:
> 
> 	  rv->value = get_return_value (func, rv->type);
> -	  rv->value_history_index = record_latest_value (rv->value);
> +          if (rv->value != NULL)
> +  	    rv->value_history_index = record_latest_value (rv->value);
> 

You're right.  (I honestly don't know what I was thinking.)

Please commit your patch instead.

Thanks,

Kevin