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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 19 Oct 2022 10:14:20 -0700
Message-ID: <CAHk-=wiMWk2t8FHn0iqVVe1mn62OTAD6ffL5rn9Eeu021H9d1Q@mail.gmail.com>
Subject: Re: [PATCH] kbuild: treat char as always signed
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     "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 9:57 AM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> This is an ABI change.  It is also hugely detrimental to generated
> code quality on architectures that make the saner choice (that is, have
> most instructions zero-extend byte quantities).

Yeah, I agree. We should just accept the standard wording, and be
aware that 'char' has indeterminate signedness.

But:

> Instead, don't actively disable the compiler warnings that catch such
> cases?  So start with removing footguns like
>
>   # disable pointer signed / unsigned warnings in gcc 4.0
>   KBUILD_CFLAGS += -Wno-pointer-sign

Nope, that won't fly.

The pointer-sign thing doesn't actually help (ie it won't find places
where you actually compare a char), and it causes untold damage in
doing completely insane things.

For example, it suddenly starts warning if  you *are* being careful,
and explicitly use 'unsigned char array[]' things to avoid any sign
issues, and then want to do simple and straightforward things with
said array (like doing a 'strcmp()' on it).

Seriously, -Wpointer-sign is not just useless, it's actively _evil_.
The fact that you suggest that clearly means that you've never used
it.

                      Linus