[PATCH] Forbid watchpoint on a constant value
Sergio Durigan Junior
sergiodj@redhat.com
Fri May 21 21:43:00 GMT 2010
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
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