Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 3 Sep 2019 11:48:16 -0700
From:   Matthias Kaehlcke <mka@chromium.org>
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Russell King <linux@armlinux.org.uk>,
        Arnd Bergmann <arnd@arndb.de>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        clang-built-linux@googlegroups.com,
        Nick Desaulniers <ndesaulniers@google.com>,
        Stefan Agner <stefan@agner.ch>
Subject: Re: [PATCH v2] ARM: Emit __gnu_mcount_nc when using Clang 10.0.0 or
 newer
Message-ID: <20190903184816.GF70797@google.com>
References: <20190829062635.45609-1-natechancellor@gmail.com>
 <20190831060530.43082-1-natechancellor@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20190831060530.43082-1-natechancellor@gmail.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Aug 30, 2019 at 11:05:31PM -0700, Nathan Chancellor 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 with '-pg' because the
> calling convention was not correct. Always build with '-meabi gnu'
> when using clang but ensure that '-pg' (which is added with
> CONFIG_FUNCTION_TRACER and its prereq CONFIG_HAVE_FUNCTION_TRACER)
> cannot be added with it unless this is fixed (which means using
> clang 10.0.0 and newer).
> 
> 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
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> Reviewed-by: Stefan Agner <stefan@agner.ch>
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

Reviewed-by: Matthias Kaehlcke <mka@chromium.org>