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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Wed, 19 Oct 2022 11:20:58 -0700
Message-ID: <CAKwvOdn4iocWHY_-sXMqE7F1XrV669QsyQDzh7vPFg6+7368Cg@mail.gmail.com>
Subject: Re: [PATCH] kbuild: treat char as always signed
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Segher Boessenkool <segher@kernel.crashing.org>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        linux-kernel@vger.kernel.org, linux-kbuild@vger.kernel.org,
        linux-arch@vger.kernel.org, linux-toolchains@vger.kernel.org,
        Masahiro Yamada <masahiroy@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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On Wed, Oct 19, 2022 at 11:11 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> But while sparse does a lot of basic optimizations, it still left
> enough "look, you're doing sign-extensions on a 'char'" on the table
> that it warned about perfectly valid stuff.
>
> And maybe that's fundamentally hard.
>
> The "-Wpointer-sign" thing could probably be fairly easily improved,
> by just recognizing that things like 'strlen()' and friends do not
> care about the sign of 'char', and neither does a 'strcmp()' that only
> checks for equality (but if you check the *sign* of strcmp, it does
> matter).
>
> It's been some time since I last tried it, but at least from memory,
> it really was mostly the standard C string functions that caused
> almost all problems.  Your *own* functions you can just make sure the
> signedness is right, but it's really really annoying when you try to
> be careful about the byte signs, and the compiler starts complaining
> just because you want to use the bog-standard 'strlen()' function.
>
> And no, something like 'ustrlen()' with a hidden cast is just noise
> for a warning that really shouldn't exist.
>
> So some way to say 'this function really doesn't care about the sign
> of this pointer' (and having the compiler know that for the string
> functions it already knows about anyway) would probably make almost
> all problems with -Wsign-warning go away.
>
> Put another way: 'char *' is so fundamental and inherent in C, that
> you can't just warn when people use it in contexts where sign really
> doesn't matter.

A few times in the past, we've split a warning flag into a group so
that we could be more specific about distinct cases. Perhaps if
-Wpointer-sign was a group that implied -Wpointer-char-sign, then the
kernel could use -Wpointer-sign -Wno-pointer-char-sign.

I don't know if that's the right granularity though.
-- 
Thanks,
~Nick Desaulniers