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

Peter Zijlstra
2020-01-24T08:25:01Z

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?