linux-kernel - Re: [PATCH 10/12] s390: avoid __builtin_return

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Date:   Wed, 10 Apr 2019 12:14:21 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-s390@...r.kernel.org, Vasily Gorbik <gor@...ux.ibm.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/12] s390: avoid __builtin_return_address(n) on clang

On Wed, 10 Apr 2019 18:03:57 +0200
Martin Schwidefsky <schwidefsky@...ibm.com> wrote:

> > --- a/arch/s390/include/asm/ftrace.h
> > +++ b/arch/s390/include/asm/ftrace.h
> > @@ -13,7 +13,12 @@
> > 
> >  #ifndef __ASSEMBLY__
> > 
> > +#ifdef CONFIG_CC_IS_CLANG
> > +/* https://bugs.llvm.org/show_bug.cgi?id=41424 */
> > +#define ftrace_return_address(n) __builtin_return_address(0)
> > +#else
> >  #define ftrace_return_address(n) __builtin_return_address(n)
> > +#endif
> > 
> >  void _mcount(void);
> >  void ftrace_caller(void);  
> 
> I can say I like this one. If the compiler can not do __builtin_return_address(n)
> it feels wrong to just use __builtin_return_address(0).

I agree. The proper return value is 0UL, see include/linux/ftrace.h

/* Archs may use other ways for ADDR1 and beyond */
#ifndef ftrace_return_address
# ifdef CONFIG_FRAME_POINTER
#  define ftrace_return_address(n) __builtin_return_address(n)
# else
#  define ftrace_return_address(n) 0UL
# endif
#endif

This is why we treat zero differently:

#define CALLER_ADDR0 ((unsigned long)ftrace_return_address0)
#define CALLER_ADDR1 ((unsigned long)ftrace_return_address(1))
#define CALLER_ADDR2 ((unsigned long)ftrace_return_address(2))
#define CALLER_ADDR3 ((unsigned long)ftrace_return_address(3))
#define CALLER_ADDR4 ((unsigned long)ftrace_return_address(4))
#define CALLER_ADDR5 ((unsigned long)ftrace_return_address(5))
#define CALLER_ADDR6 ((unsigned long)ftrace_return_address(6))


-- Steve