Re: [PATCH 00/19] Enable -Wshadow=local for kernel/sched

From: Matthew Wilcox
Date: Tue Apr 16 2024 - 20:52:48 EST


On Tue, Apr 16, 2024 at 05:29:02PM -0700, Linus Torvalds wrote:
> On Tue, 16 Apr 2024 at 14:15, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >
> > I was looking at -Wshadow=local again, and remembered this series. It
> > sounded like things were close, but a tweak was needed. What would be
> > next to get this working?
>
> So what is the solution to
>
> #define MAX(a,b) ({ \
> typeof(a) __a = (a); \
> typeof(b) __b = (b); \
> __a > __b ? __a : __b; \
> })

#define __MAX(a, __a, b, __b) ({ \
typeof(a) __a = (a); \
typeof(b) __b = (b); \
__a > __b ? __a : __b; \
})

#define MAX(a, b) __MAX(a, UNIQUE_ID(a), b, UNIQUE_ID(b))

At least, I think that was the plan. This was two years ago and I've
mostly forgotten.

> int test(int a, int b, int c)
> {
> return MAX(a, MAX(b,c));
> }
>
> where -Wshadow=all causes insane warnings that are bogus garbage?
>
> Honestly, Willy's patch-series is a hack to avoid this kind of very
> natural nested macro pattern.
>
> But it's a horrible hack, and it does it by making the code actively worse.
>
> Here's the deal: if we can't handle somethng like the above without
> warning, -Wshadow isn't getting enabled.
>
> Because we don't write worse code because of bad warnings.
>
> IOW, what is the sane way to just say "this variable can shadow the
> use site, and it's fine"?
>
> Without that kind of out, I don't think -Wshadow=local is workable.
>
> Linus