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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 22 Oct 2022 11:16:33 -0700
Message-ID: <CAHk-=wgaeTa9nAeJ8DP1cBWrs8fZvJ7k1-L8-kjxEOxpLf+XNA@mail.gmail.com>
Subject: Re: [PATCH] kbuild: treat char as always signed
To:     Gabriel Paubert <paubert@iram.es>
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 Fri, Oct 21, 2022 at 11:06 PM Gabriel Paubert <paubert@iram.es> wrote:
>
> Ok, I=C2=B4ve just tried it, except that I had something slightly differe=
nt in
> mind, but perhaps should have been clearer in my first post.
>
> I have change your code to the following:

I actually tested that, but using a slightly different version, and my
non-union test case ended up like

   size_t strlen(const char *p)
  {
        return __builtin_strlen(p);
  }

and then gcc actually complains about

    warning: infinite recursion detected

and I (incorrectly) thought this was unworkable. But your version
seems to work fine.

So yeah, for the kernel I think we could do something like this. It's
ugly, but it gets rid of the crazy warning.

Practically speaking this might be a bit painful, because we've got
several different variations of this all due to all the things like
our debugging versions (see <linux/fortify-string.h> for example), so
some of our code is this crazy jungle of "with this config, use this
wrapper".

But if somebody wants to deal with the '-Wpointer-sign' warnings,
there does seem to be a way out. Maybe with another set of helper
macros, creating those odd __transparent_union__ wrappers might even
end up reasonable.

It's not like we don't have crazy macros for function wrappers
elsewhere (the SYSCALL macros come to mind - shudder). The macros
themselves may be a nasty horror, but when done right the _use_ point
of said macros can be nice and clean.

                  Linus