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

Geert Uytterhoeven
2022-12-21T15:06:06Z

Hi Günter,

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

Note that we detected other issues with the m68k strcmp(), so
probably that patch wouldn't go in as-is.

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