linux-kernel - Re: [PATCH] ARM: Emit __gnu_mcount_nc when using Clang 10.0.0 or newer

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Message-ID: <20190829193432.GA10138@archlinux-threadripper>
Date:   Thu, 29 Aug 2019 12:34:32 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Russell King <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>, Stefan Agner <stefan@...er.ch>,
        Matthias Kaehlcke <mka@...omium.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] ARM: Emit __gnu_mcount_nc when using Clang 10.0.0 or
 newer

On Thu, Aug 29, 2019 at 10:55:28AM -0700, Nick Desaulniers wrote:
> On Wed, Aug 28, 2019 at 11:27 PM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > Currently, multi_v7_defconfig + CONFIG_FUNCTION_TRACER fails to build
> > with clang:
> >
> > arm-linux-gnueabi-ld: kernel/softirq.o: in function `_local_bh_enable':
> > softirq.c:(.text+0x504): undefined reference to `mcount'
> > arm-linux-gnueabi-ld: kernel/softirq.o: in function `__local_bh_enable_ip':
> > softirq.c:(.text+0x58c): undefined reference to `mcount'
> > arm-linux-gnueabi-ld: kernel/softirq.o: in function `do_softirq':
> > softirq.c:(.text+0x6c8): undefined reference to `mcount'
> > arm-linux-gnueabi-ld: kernel/softirq.o: in function `irq_enter':
> > softirq.c:(.text+0x75c): undefined reference to `mcount'
> > arm-linux-gnueabi-ld: kernel/softirq.o: in function `irq_exit':
> > softirq.c:(.text+0x840): undefined reference to `mcount'
> > arm-linux-gnueabi-ld: kernel/softirq.o:softirq.c:(.text+0xa50): more undefined references to `mcount' follow
> >
> > clang can emit a working mcount symbol, __gnu_mcount_nc, when
> > '-meabi gnu' is passed to it. Until r369147 in LLVM, this was
> > broken and caused the kernel not to boot because the calling
> > convention was not correct. Now that it is fixed, add this to
> > the command line when clang is 10.0.0 or newer so everything
> > works properly.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/35
> > Link: https://bugs.llvm.org/show_bug.cgi?id=33845
> > Link: https://github.com/llvm/llvm-project/commit/16fa8b09702378bacfa3d07081afe6b353b99e60
> > Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> > ---
> >  arch/arm/Makefile | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> > index c3624ca6c0bc..7b5a26a866fc 100644
> > --- a/arch/arm/Makefile
> > +++ b/arch/arm/Makefile
> > @@ -112,6 +112,12 @@ ifeq ($(CONFIG_ARM_UNWIND),y)
> >  CFLAGS_ABI     +=-funwind-tables
> >  endif
> >
> > +ifeq ($(CONFIG_CC_IS_CLANG),y)
> > +ifeq ($(shell test $(CONFIG_CLANG_VERSION) -ge 100000; echo $$?),0)
> > +CFLAGS_ABI     +=-meabi gnu
> > +endif
> > +endif
> > +
> 
> Thanks for the patch!  I think this is one of the final issues w/ 32b
> ARM configs when building w/ Clang.
> 
> I'm not super enthused about the version check.  The flag is indeed
> not recognized by GCC, but I think it would actually be more concise
> with $(cc-option) and no compiler or version check.
> 
> Further, I think that the working __gnu_mcount_nc in Clang would
> better be represented as marking the arch/arm/KConfig option for
> CONFIG_FUNCTION_TRACER for dependent on a version of Clang greater
> than or equal to Clang 10, not conditionally adding this flag. (We
> should always add the flag when supported, IMO.  __gnu_mcount_nc's
> calling convention being broken is orthogonal to the choice of
> __gnu_mcount_nc vs mcount, and it's the former's that should be
> checked, not the latter as in this patch.

I will test with or without CONFIG_AEABI like Matthias asked and I will
implement your Kconfig suggestion if it passes all of my tests. The
reason that I did it this way is because I didn't want a user to end up
with a non-booting kernel since -meabi gnu works with older versions of
clang at build time, the issue happens at boot time but the Kconfig
suggestion + cc-option should fix that.

I should have a v2 out this evening.

Cheers,
Nathan