linux-kernel - RE: [PATCH v2] kbuild: treat char as always unsigned

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Message-ID: <b2144334261246aa8dc5004c5f1a58c9@AcuMS.aculab.com>
Date:   Thu, 22 Dec 2022 10:41:52 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Guenter Roeck <linux@...ck-us.net>
CC:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kbuild@...r.kernel.org" <linux-kbuild@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-toolchains@...r.kernel.org" <linux-toolchains@...r.kernel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-m68k@...ts.linux-m68k.org" <linux-m68k@...ts.linux-m68k.org>
Subject: RE: [PATCH v2] kbuild: treat char as always unsigned

From: Linus Torvalds
> Sent: 21 December 2022 17:07
> 
> On Wed, Dec 21, 2022 at 7:56 AM Guenter Roeck <linux@...ck-us.net> wrote:
> >
> > The above assumes an unsigned char as input to strcmp(). I consider that
> > a hypothetical problem because "comparing" strings with upper bits
> > set doesn't really make sense in practice (How does one compare Günter
> > against Gunter ? And how about Gǖnter ?). On the other side, the problem
> > observed here is real and immediate.
> 
> POSIX does actually specify "Günter" vs "Gunter".
> 
> The way strcmp is supposed to work is to return the sign of the
> difference between the byte values ("unsigned char").
> 
> But that sign has to be computed in 'int', not in 'signed char'.
> 
> So yes, the m68k implementation is broken regardless, but with a
> signed char it just happened to work for the US-ASCII case that the
> crypto case tested.
> 
> I think the real fix is to just remove that broken implementation
> entirely, and rely on the generic one.

I wonder how much slower it is - m68k is likely to be microcoded
and I don't think instruction timings are actually available.
The fastest version probably uses subx (with carry) to generate
0/-1 and leaves +delta for the other result - but getting the
compares and branches in the right order is hard.

I believe some of the other m68k asm functions are also missing
the "memory" 'clobber' and so could get mis-optimised.
While I can write (or rather have written) m68k asm I don't have
a compiler.

I also suspect that any x86 code that uses 'rep scas' is going
to be slow on anything modern.

	David

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