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

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Date:   Fri, 16 Dec 2022 14:44:58 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     David Laight <David.Laight@...LAB.COM>,
        "'Leizhen (ThunderTown)'" <thunder.leizhen@...wei.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

From: David Laight
> Sent: 16 December 2022 13:30
> 
> From: Leizhen (ThunderTown)
> > Sent: 16 December 2022 12:02
> >
> ..
> > > 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 */
> 
> Adding 'volatile' there shouldn't make any real difference.
> 
> I'd double-check the asm constraints for the two pointers.
> They are modified by the asm, but the caller's variables
> must not be changed.
> 
> I think that means they need to be normal 'input' parameters
> and the result must be in different register (early clobber?).
> Currently the pointers are "+r" - which I think means they
> are input-output and any caller-supplied variable is
> likely to get changed.

Definitely badly broken.
'cs' and 'ct' should be input parameters.
'res' needs to be an early-clobber output parameter "=&r".

Since it is actually a 'static inline' (not just a #define)
then letting cs/ct be changed probably doesn't matter.

But the lack of 'early clobber' will cause grief.

	David

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