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

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Date:   Fri, 16 Dec 2022 20:01:38 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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>, <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>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        David Laight <David.Laight@...lab.com>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH v9] kallsyms: Add self-test facility



On 2022/12/16 19:28, Geert Uytterhoeven wrote:
> Hi Zhen,
> 
> On Fri, Dec 16, 2022 at 10:43 AM Leizhen (ThunderTown)
> <thunder.leizhen@...wei.com> wrote:
>> On 2022/12/16 15:42, Leizhen (ThunderTown) wrote:
>>> On 2022/12/15 22:51, Geert Uytterhoeven wrote:
>>>> On 30f3bb09778de64 with your debug patch v2:
>>> I've set up the qemu environment, and I'll try to solve it by tomorrow at the latest.
>>
>> It seems that the problem is still strcmp(). After I commented strcmp() in
>> arch/m68k/include/asm/string.h, and force it to use the one in lib/string.c,
>> it works well.
> 
> I can confirm that.
> 
> One difference is that the one in lib/string.c always return -1/0/1,
> while the m68k version can return other negative or positive numbers.
> 
> However, adding:
> 
>        if (res < 0) return -1;
>        if (res > 0) return 1;
> 
> to the m68k version doesn't make a difference.
> 
> Renaming the m68k version (changed to -1/0/1) to m68k_strcmp(), and
> the generic version to lib_strcmp(), and adding a wrapper that calls
> and compares both, shows that both functions do return the same value,
> and the test succeeds.
> 
> Moving the m68k version inside lib/string.c makes the test pass, too.
> So it must be related to the function being inline, and gcc making
> (incorrect) assumptions...

Yes, it's the compiler's fault. I just replied David Laight:

I added 'volatile' to prevent compiler optimizations, and it's OK now.

diff --git a/arch/m68k/include/asm/string.h b/arch/m68k/include/asm/string.h
index f759d944c449940..3db81e5a783c72a 100644
--- a/arch/m68k/include/asm/string.h
+++ b/arch/m68k/include/asm/string.h
@@ -42,9 +42,9 @@ 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"
+       asm volatile ("\n"
                "1:     move.b  (%0)+,%2\n"     /* get *cs */
                "       cmp.b   (%1)+,%2\n"     /* compare a byte */
                "       jne     2f\n"           /* not equal, break out */

> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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
> 
> .
> 

-- 
Regards,
  Zhen Lei