Date:   Tue, 1 Mar 2022 10:47:14 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
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Subject: Re: [PATCH 2/6] treewide: remove using list iterator after loop body
 as a ptr

On Tue, Mar 1, 2022 at 10:14 AM Kees Cook <keescook@...omium.org> wrote:
>
> The first big glitch with -Wshadow was with shadowed global variables.
> GCC 4.8 fixed that, but it still yells about shadowed functions. What
> _almost_ works is -Wshadow=local.

Heh. Yeah, I just have long memories of "-Wshadow was a disaster". You
looked into the details.

> Another way to try to catch misused shadow variables is
> -Wunused-but-set-varible, but it, too, has tons of false positives.

That on the face of it should be an easy warning to get technically
right for a compiler.

So I assume the "false positives" are simply because we end up having
various variables that really don't end up being used - and
"intentionally" so).

Or rather, they might only be used under some config option - perhaps
the use is even syntactically there and parsed, but the compiler
notices that it's turned off under some

        if (IS_ENABLED(..))

option? Because yeah, we have a lot of those.

I think that's a common theme with a lot of compiler warnings: on the
face of it they sound "obviously sane" and nobody should ever write
code like that.

A conditional that is always true? Sounds idiotic, and sounds like a
reasonable thing for a compiler to warn about, since why would you
have a conditional in the first place for that?

But then you realize that maybe the conditional is a build config
option, and "always true" suddenly makes sense. Or it's a test for
something that is always true on _that_architecture_ but not in some
general sense (ie testing "sizeof()"). Or it's a purely syntactic
conditional, like "do { } while (0)".

It's why I'm often so down on a lot of the odd warnings that are
hiding under W=1 and friends. They all may make sense in the trivial
case ("That is insane") but then in the end they happen for sane code.

And yeah, -Wshadow has had tons of history with macro nesting, and
just being badly done in the first place (eg "strlen" can be a
perfectly fine local variable).

That said, maybe people could ask the gcc and clan people for a way to
_mark_ the places where we expect to validly see shadowing. For
example, that "local variable in a macro expression statement" thing
is absolutely horrendous to fix with preprocessor tricks to try to
make for unique identifiers.

But I think it would be much more syntactically reasonable to add (for
example) a "shadow" attribute to such a variable exactly to tell the
compiler "yeah, yeah, I know this identifier could shadow an outer
one" and turn it off that way.

               Linus

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