Messages in this thread | | | Date | Fri, 19 Apr 2024 15:06:39 -0700 | From | Kees Cook <> | Subject | Re: [PATCH 00/19] Enable -Wshadow=local for kernel/sched |
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On Wed, Apr 17, 2024 at 01:52:28AM +0100, Matthew Wilcox wrote: > On Tue, Apr 16, 2024 at 05:29:02PM -0700, Linus Torvalds wrote: > > On Tue, 16 Apr 2024 at 14:15, Kees Cook <keescook@chromium.org> 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))
Yup, this is what we've had for a long time now. See include/linux/minmax.h
> 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.
This isn't a hill I want to die on, but it's just another case where we've fought bugs more than once that would have stood out immediately if we had -Wshadow=local enabled, but there is basically only 1 user. In my bug-fighting calculus, it makes sense to deal with fixing the 1 user so we can gain the coverage everywhere else.
But there are much worse bug sources, so if Willy's series isn't workable, I'll drop this again for now. :)
-Kees
-- Kees Cook
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