Re: [PATCH] Forbid watchpoint on a constant value

[Sergio Durigan Junior <sergiodj@redhat.com>]

On Friday 21 May 2010 04:05:00, Jan Kratochvil wrote:
> On Fri, 21 May 2010 01:13:08 +0200, Joel Brobecker wrote:
> > > 	(gdb) watch 5
> > > 	Cannot watch constant value 5.
> [...]
> > For myself, I can see how a warning might be useful, but forbidding it might
> > be viewed as a little excessive,
> 
> `watch 5' can never trigger.  I cannot agree with creating a watchpoint which
> will never trigger.

`watch var = 5' will never trigger as well, but we've decided to accept it
anyway.  But I understand your point.

> The more problematic is the part
> 	const i = 5;
> 	(gdb) watch i
> +       case OP_VAR_VALUE:
> +           if (TYPE_CODE (SYMBOL_TYPE (s)) != TYPE_CODE_FUNC
> +               && !TYPE_CONST (SYMBOL_TYPE (s)))
> +             return 0;
> as you are right that a symbol can be tagged by buggy compiler as
> DW_TAG_const_type despite its value changes in the compiler output.  Another
> possibility is a memory corruption. On both -O0 -g and -O2 -g output it can
> change during:
> 	$ echo 'const int v;main(){*(int*)&v=1;}'|gcc -o 1 -g -x c -;gdb -nx -ex 'watch v' -ex r ./1
> 	Hardware watchpoint 1: v
> 	Old value = 0
> 	New value = 1

I can make the function return a different code in the case of OP_VAR_VALUE,
so that watch_command_1 will know if we're dealing with a _possible_ constant
value (and then asks the user if she really wants to put a watchpoint in that
variable, as Eli suggested).  WDYT?

> This case
> 	$ echo 'main(){const int v;*(int*)&v=1;}'|gcc -o 1 -O2 -g -x c -;gdb -nx -ex start -ex 'watch v' -ex c ./1
> gets compiled as:
> 	 <2><4b>: Abbrev Number: 3 (DW_TAG_variable)
> 	    <4c>   DW_AT_name        : v        
> 	    <54>   DW_AT_const_value : 1        
> which would be a GDB internal error if it would ever trigger.
> 
> A safer patch would be to check SYMBOL_CLASS for LOC_CONST/etc. of the
> variable instead of relying on compiler's DW_TAG_const_type correctness.

Ok, I will change my patch to use SYMBOL_CLASS and will resubmit it.

Thanks,

-- 
Sergio Durigan Junior
Red Hat