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

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Date:   Wed, 21 Dec 2022 07:56:41 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-toolchains@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-m68k@...ts.linux-m68k.org
Subject: Re: [PATCH v2] kbuild: treat char as always unsigned

On Wed, Dec 21, 2022 at 04:29:11PM +0100, Rasmus Villemoes wrote:
> On 21/12/2022 16.05, Geert Uytterhoeven wrote:
> > Hi Günter,
> > 
> > On Wed, Dec 21, 2022 at 3:54 PM Guenter Roeck <linux@...ck-us.net> wrote:
> >> On Wed, Oct 19, 2022 at 02:30:34PM -0600, Jason A. Donenfeld wrote:
> >>> Recently, some compile-time checking I added to the clamp_t family of
> >>> functions triggered a build error when a poorly written driver was
> >>> compiled on ARM, because the driver assumed that the naked `char` type
> >>> is signed, but ARM treats it as unsigned, and the C standard says it's
> >>> architecture-dependent.
> >>>
> >>> I doubt this particular driver is the only instance in which
> >>> unsuspecting authors make assumptions about `char` with no `signed` or
> >>> `unsigned` specifier. We were lucky enough this time that that driver
> >>> used `clamp_t(char, negative_value, positive_value)`, so the new
> >>> checking code found it, and I've sent a patch to fix it, but there are
> >>> likely other places lurking that won't be so easily unearthed.
> >>>
> >>> So let's just eliminate this particular variety of heisensign bugs
> >>> entirely. Set `-funsigned-char` globally, so that gcc makes the type
> >>> unsigned on all architectures.
> >>>
> >>> This will break things in some places and fix things in others, so this
> >>> will likely cause a bit of churn while reconciling the type misuse.
> >>>
> >>
> >> There is an interesting fallout: When running the m68k:q800 qemu emulation,
> >> there are lots of warning backtraces.
> >>
> >> WARNING: CPU: 0 PID: 23 at crypto/testmgr.c:5724 alg_test.part.0+0x7c/0x326
> >> testmgr: alg_test_descs entries in wrong order: 'adiantum(xchacha12,aes)' before 'adiantum(xchacha20,aes)'
> >> ------------[ cut here ]------------
> >> WARNING: CPU: 0 PID: 23 at crypto/testmgr.c:5724 alg_test.part.0+0x7c/0x326
> >> testmgr: alg_test_descs entries in wrong order: 'adiantum(xchacha20,aes)' before 'aegis128'
> >>
> >> and so on for pretty much every entry in the alg_test_descs[] array.
> >>
> >> Bisect points to this patch, and reverting it fixes the problem.
> >>
> >> It looks like the problem is that arch/m68k/include/asm/string.h
> >> uses "char res" to store the result of strcmp(), and char is now
> >> unsigned - meaning strcmp() will now never return a value < 0.
> >> Effectively that means that strcmp() is broken on m68k if
> >> CONFIG_COLDFIRE=n.
> >>
> >> The fix is probably quite simple.
> >>
> >> diff --git a/arch/m68k/include/asm/string.h b/arch/m68k/include/asm/string.h
> >> index f759d944c449..b8f4ae19e8f6 100644
> >> --- a/arch/m68k/include/asm/string.h
> >> +++ b/arch/m68k/include/asm/string.h
> >> @@ -42,7 +42,7 @@ static inline char *strncpy(char *dest, const char *src, size_t n)
> >>  #define __HAVE_ARCH_STRCMP
> >>  static inline int strcmp(const char *cs, const char *ct)
> >>  {
> >> -       char res;
> >> +       signed char res;
> >>
> >>         asm ("\n"
> >>                 "1:     move.b  (%0)+,%2\n"     /* get *cs */
> >>
> >> Does that make sense ? If so I can send a patch.
> > 
> > Thanks, been there, done that
> > https://lore.kernel.org/all/bce014e60d7b1a3d1c60009fc3572e2f72591f21.1671110959.git.geert@linux-m68k.org
> 
> Well, looks like that would still leave strcmp() buggy, you can't
> represent all possible differences between two char values (signed or
> not) in an 8-bit quantity. So any implementation based on returning the
> first non-zero value of *a - *b must store that intermediate value in
> something wider. Otherwise you'll get -128 from strcmp("\x40", "\xc0"),
> but _also_ -128 when you do strcmp("\xc0", "\x40"), which is obviously
> bogus.
> 

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.

Guenter