Re: [PATCH v2 02/10] netfilter: Avoid assigning 'const' pointer to non-const pointer

Nick Desaulniers
2020-01-24T17:20:26Z

On Fri, Jan 24, 2020 at 12:24 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Jan 23, 2020 at 11:07:59AM -0800, Nick Desaulniers wrote:
>
> > Good thing it's the variable being modified was not declared const; I
> > get spooked when I see -Wdiscarded-qualifiers because of Section
> > 6.7.3.6 of the ISO C11 draft spec:
> >
> > ```
> > If an attempt is made to modify an object defined with a
> > const-qualified type through use
> > of an lvalue with non-const-qualified type, the behavior is undefined.
> > If an attempt is
> > made to refer to an object defined with a volatile-qualified type
> > through use of an lvalue
> > with non-volatile-qualified type, the behavior is undefined.133)
> >
> > 133) This applies to those objects that behave as if they were defined
> > with qualified types, even if they are
> > never actually defined as objects in the program (such as an object at
> > a memory-mapped input/output
> > address).
> > ```
> >
> > Which is about the modification of a const-declared variable (explicit
> > UB which Clang actively exploits),
>
> Just for curiosity's sake. What does clang actually do in that case?
>

https://bugs.llvm.org/show_bug.cgi?id=42763
I was playing around with this in godbolt but couldn't quickly come up
with a simple reproducer. IIRC, I've fixed maybe 3 instances of this
recently in code though.
--
Thanks,
~Nick Desaulniers