Re: [PATCH][ubsan] Add VLA bound instrumentation

[Jason Merrill <jason@redhat.com>]

On 10/30/2013 12:15 PM, Marek Polacek wrote:
> On Wed, Oct 30, 2013 at 11:56:25AM -0400, Jason Merrill wrote:
>> Saving 'size' here doesn't help since it's already been used above.
>> Could you use itype instead of size here?
>
> I already experimented with that and I think I can't, since we call
> the finish_expr_stmt too soon, which results in:
>
>      int x = 1;
>      int a[0:(sizetype) SAVE_EXPR <D.2143>];
>
>      <<cleanup_point   int x = 1;>>;
>      <<cleanup_point <<< Unknown tree: expr_stmt
>      if (SAVE_EXPR <D.2143> <= 0)
>        {
>          __builtin___ubsan_handle_vla_bound_not_positive (&*.Lubsan_data0, (unsigned long) SAVE_EXPR <D.2143>);
>        }
>      else
>        {
>          0
>        }, (void) SAVE_EXPR <D.2143>; >>>>>;
>        ssizetype D.2143;
>      <<cleanup_point <<< Unknown tree: expr_stmt
>      (void) (D.2143 = (ssizetype)  ++x + -1) >>>>>;

Ah, looks like you're getting an unfortunate interaction with 
stabilize_vla_size, which is replacing the contents of the SAVE_EXPR 
with a reference to a variable that isn't initialized yet.  Perhaps we 
should move the stabilize_vla_size call into compute_array_index_type, too.

> and that ICEs in gimplify_var_or_parm_decl, presumably because the
> if (SAVE_EXPR <D.2143> <= 0) { ... } should be emitted *after* that
> cleanup_point.  When we generated the C++1y check in cp_finish_decl,
> we emitted the check after the cleanup_point, and everything was OK.
> I admit I don't understand the cleanup_points very much and I don't
> know exactly where they are coming from, because normally I don't see
> them coming out of C FE. :)

You can ignore the cleanup_points; they just wrap every full-expression.

Jason