Re: [PATCH, PR 57748] Check for out of bounds access, Part 2

[Martin Jambor <mjambor at suse dot cz>]

Hi,

On Tue, Sep 24, 2013 at 12:02:17PM +0200, Richard Biener wrote:
> On Tue, Sep 24, 2013 at 4:52 AM, Bernd Edlinger
> <bernd.edlinger@hotmail.de> wrote:
> > Hi,
> >
> > with the attached patch the read-side of the out of bounds accesses are fixed.
> > There is a single new test case pr57748-3.c that is derived from Martin's test case.
> > The test case does only test the read access and does not depend on part 1 of the
> > patch.
> >
> > This patch was boot-strapped and regression tested on x86_64-unknown-linux-gnu.
> >
> > Additionally I generated eCos and an eCos-application (on ARMv5 using packed
> > structures) with an arm-eabi cross compiler, and looked for differences in the
> > disassembled code with and without this patch, but there were none.
> >
> > OK for trunk?
> 
> Index: gcc/expr.c
> ===================================================================
> --- gcc/expr.c  (revision 202778)
> +++ gcc/expr.c  (working copy)
> @@ -9878,7 +9878,7 @@
>                           && modifier != EXPAND_STACK_PARM
>                           ? target : NULL_RTX),
>                          VOIDmode,
> -                        modifier == EXPAND_SUM ? EXPAND_NORMAL : modifier);
> +                        EXPAND_MEMORY);
> 
>         /* If the bitfield is volatile, we want to access it in the
>            field's mode, not the computed mode.
> 
> context suggests that we may arrive with EXPAND_STACK_PARM here
> which is a correctness modifier (see its docs).  But I'm not too familiar
> with the details of the various expand modifiers, Eric may be though.
> 
> That said, I still believe that fixing the misalign path in expand_assignment
> would be better than trying to avoid it.  For this testcase the issue is
> again that expand_assignment passes the wrong mode/target to the
> movmisalign optab.

Perhaps I'm stating the obvious, but this is supposed to address a
separate bug in the expansion of the RHS (as opposed to the first bug
which was in the expansion of the LHS), and so we would have to make
expand_assignment to also examine potential misalignments in the RHS,
which it so far does not do, despite its complexity.

Having said that, I also dislike the patch, but I am quite convinced
that if we allow non-BLK structures with zero sized arrays, the fix
will be ugly - but I'll be glad to be shown I've been wrong.

Martin