linux-kernel - RE: [PATCH v9] kallsyms: Add self-test facility

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Date:   Fri, 16 Dec 2022 10:40:04 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Geert Uytterhoeven' <geert@...ux-m68k.org>,
        "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
CC:     Josh Poimboeuf <jpoimboe@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        "live-patching@...r.kernel.org" <live-patching@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Luis Chamberlain" <mcgrof@...nel.org>,
        "linux-modules@...r.kernel.org" <linux-modules@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Ingo Molnar" <mingo@...hat.com>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: RE: [PATCH v9] kallsyms: Add self-test facility

From: Geert Uytterhoeven 
> Sent: 15 December 2022 13:25
...
> Looks like commit 3bc753c06dd02a35 ("kbuild: treat char as always
> unsigned") is to blame.
> 
> Changing:
> 
>     --- 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 */
> 
> fixes strcmp, but the test still fails:

Try 'int res;' and an explicit sign extend (I think):
	"3: extb %2\n"

The strcmp() is still wrong if either input string
has characters with the top bit set.
The result needs to be based of the carry flag not
the sign of the byte subtract.

It is too long since I've written m68k asm.
I've checked, all byte operations leave the high 24bits
unchanged.
So it is possible that gcc is making assumptions and
skipping the sign extend under some circumstances.

	David

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