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

Leizhen (ThunderTown)
2022-12-16T11:41:05Z

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