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

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Rasmus,

On Wed, Dec 21, 2022 at 4:29 PM Rasmus Villemoes
<rasmus.villemoes@prevas.dk> wrote:
> On 21/12/2022 16.05, Geert Uytterhoeven wrote:
> > On Wed, Dec 21, 2022 at 3:54 PM Guenter Roeck <linux@roeck-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.

So we have https://lore.kernel.org/all/87bko3ia88.fsf@igel.home ;-)

And the other issue is m68k strcmp() calls being dropped by the
optimizer, cfr. the discussion in
https://lore.kernel.org/all/b673f98db7d14d53a6e1a1957ef81741@AcuMS.aculab.com

> I recently fixed that long-standing bug in U-Boot's strcmp() and a
> similar one in nolibc in the linux tree. I wonder how many more
> instances exist.

Thanks, commit fb63362c63c7aeac ("lib: fix buggy strcmp and strncmp") in
v2023.01-rc1, which is not yet in a released version.
(and in plain C, not in asm ;-)

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds