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

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Date:   Wed, 21 Dec 2022 06:53:32 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     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>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k@...ts.linux-m68k.org
Subject: Re: [PATCH v2] kbuild: treat char as always unsigned

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.

Guenter