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

[Guenter Roeck <linux@roeck-us.net>]

On Wed, Dec 21, 2022 at 04:05:45PM +0100, Geert Uytterhoeven wrote:
> 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.
> 

So anything non-Coldfire is and will remain broken on m68k for the time
being ? Wouldn't it be better to fix the acute problem now and address
the long-standing problem(s) separately ? 

Thanks,
Guenter