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

[Pedro Alves <palves@redhat.com>]

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);

?

Thanks,
Pedro Alves