Re: [PATCH v2] ubsan: missed -fsanitize=bounds for compound ops [PR108060]

[Marek Polacek <polacek@redhat.com>]

On Fri, Mar 10, 2023 at 07:07:36PM +0100, Jakub Jelinek wrote:
> On Thu, Mar 09, 2023 at 07:44:53PM -0500, Marek Polacek wrote:
> > On Thu, Mar 09, 2023 at 09:44:49AM +0100, Jakub Jelinek wrote:
> > > On Thu, Mar 09, 2023 at 08:12:47AM +0000, Richard Biener wrote:
> > > > I think this is a reasonable way to address the regression, so OK.
> > > 
> > > It is true that both C and C++ (including c++14_down and c++17 and later
> > > where the latter have different ordering rules) evaluate the lhs of
> > > MODIFY_EXPR after rhs, so conceptually this patch makes sense.
> > 
> > Thank you both for taking a look.
> > 
> > > But I wonder why we do in ubsan_maybe_instrument_array_ref:
> > >       if (e != NULL_TREE)
> > >         {
> > >           tree t = copy_node (*expr_p);
> > >           TREE_OPERAND (t, 1) = build2 (COMPOUND_EXPR, TREE_TYPE (op1),
> > >                                         e, op1);
> > >           *expr_p = t;
> > >         }
> > > rather than modification of the ARRAY_REF's operand in place.  If we
> > > did that, we wouldn't really care about the order, shared tree would
> > > be instrumented once, with SAVE_EXPR in there making sure we don't
> > > compute that multiple times.  Is that because the 2 copies could
> > > have side-effects and we do want to evaluate those multiple times?
> > 
> > I'd assumed that that was the point of the copy_node.  But now that
> > I'm actually experimenting with it, I can't trigger any problems
> > without the copy_node.  So maybe we can use the following patch, which
> > also adds a new test, bounds-21.c, to check that side-effects are
> > evaluated correctly.  I didn't bother writing a description for this
> > patch yet because I sort of think we should apply both patches at the
> > same time.  
> 
> Perhaps it would be safer to apply for GCC 13 just your first patch
> and maybe the testsuite coverage from this one and defer this change
> for GCC 14?

That sounds good, I'll push the original patch with the new test now.
Thanks.
 
> > Regtested on x86_64-pc-linux-gnu.
> > 
> > -- >8 --
> > 	PR sanitizer/108060
> > 	PR sanitizer/109050
> > 
> > gcc/c-family/ChangeLog:
> > 
> > 	* c-ubsan.cc (ubsan_maybe_instrument_array_ref): Don't copy_node.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* c-c++-common/ubsan/bounds-17.c: New test.
> > 	* c-c++-common/ubsan/bounds-18.c: New test.
> > 	* c-c++-common/ubsan/bounds-19.c: New test.
> > 	* c-c++-common/ubsan/bounds-20.c: New test.
> > 	* c-c++-common/ubsan/bounds-21.c: New test.
> 
> 	Jakub
> 

Marek