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

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Date:   Fri, 16 Dec 2022 19:40:53 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     David Laight <David.Laight@...LAB.COM>,
        '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" <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



On 2022/12/16 18:40, David Laight wrote:
> 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"

Compilation failed. I tried "return (int)(signed char)res;", it's still failed.

> 
> 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.

Currently, only ASCCIs. So it won't be the reason.

> So it is possible that gcc is making assumptions and
> skipping the sign extend under some circumstances.

Wow, because compare_symbol_name() works properly during the previous binary
search, the compiler must have done something bad. So I add '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 */

> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
> 

-- 
Regards,
  Zhen Lei