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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Wed, 10 Apr 2019 21:07:56 +0200
Message-ID: <CAK8P3a2n+nEcaNrATQVO8NSGzfrMP5n+SrvjpTYcH4ikCN5LGg@mail.gmail.com>
Subject: Re: [PATCH 10/12] s390: avoid __builtin_return_address(n) on clang
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Ingo Molnar <mingo@redhat.com>,
        clang-built-linux@googlegroups.com,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Apr 10, 2019 at 6:14 PM Steven Rostedt <rostedt@goodmis.org> wrote:
> On Wed, 10 Apr 2019 18:03:57 +0200 Martin Schwidefsky <schwidefsky@de.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))

Right, got it.

Martin, do you want me to send a replacement patch, or can you
commit the patch with

#ifdef CONFIG_CC_IS_CLANG
/* https://bugs.llvm.org/show_bug.cgi?id=41424 */
#define ftrace_return_address(n) 0UL
#else
#define ftrace_return_address(n) __builtin_return_address(n)
#endif

instead?

    Arnd