Function Tracing with clang on ARM32

[mka@chromium.org (Matthias Kaehlcke)]

Hi Stefan,

A similar/related issue was fixed for arm64 by
https://reviews.llvm.org/rL266039

The corresponding LLVM bug is
https://bugs.llvm.org//show_bug.cgi?id=27311

Apparently for arm32 the use of '-meabi gnu' was chosen as solution:

http://llvm.org/viewvc/llvm-project?view=revision&revision=265888

Cheers

Matthias

On Sat, Aug 11, 2018 at 02:29:24PM +0200, Stefan Agner wrote:
> Hi,
> 
> I am currently looking into function tracing support when compiling an
> ARM32 kernel with clang.
> 
> With v4.18-rc1 and:
> CONFIG_FTRACE=y                                                         
>                                                                    
> CONFIG_FUNCTION_TRACER=y
> 
> Building fails:
> 
>   LD      vmlinux.o                                                     
>                                                                   
>   MODPOST vmlinux.o
> kernel/softirq.o: In function `_local_bh_enable':
> /home/ags/projects/toradex/imx/linux-vanilla-llvm/kernel/softirq.c:152:
> undefined reference to `mcount'
> kernel/softirq.o: In function `__local_bh_enable_ip':
> /home/ags/projects/toradex/imx/linux-vanilla-llvm/kernel/softirq.c:159:
> undefined reference to `mcount'
> kernel/softirq.o: In function `do_softirq':
> /home/ags/projects/toradex/imx/linux-vanilla-llvm/kernel/softirq.c:316:
> undefined reference to `mcount'
> kernel/softirq.o: In function `irq_enter':
> /home/ags/projects/toradex/imx/linux-vanilla-llvm/kernel/softirq.c:337:
> undefined reference to `mcount'
> kernel/softirq.o: In function `irq_exit':
> /home/ags/projects/toradex/imx/linux-vanilla-llvm/kernel/softirq.c:395:
> undefined reference to `mcount'
> 
> It seems that clang/LLVM uses a different mcount interface. However,
> there is an option to use the gcc compatible symbol __gnu_mcount_nc when
> using -meabi gnu. So adding
> CFLAGS_ABI     +=-meabi gnu
> 
> Builds the kernel successfully. However, the kernel crashes immediately
> on boot. Even with earlyprintk enabled, I only get:
> 
> Uncompressing Linux... done, booting the kernel.
> 
> I poked a bit around in arch/arm/kernel/entry-ftrace.S. Not calling
> __mcount in __gnu_mcount_nc produces a bootable kernel. Removing the
> call into the ftrace logic by only using mcount_enter/exit in the
> __mcount macro does not boot...
> 
> I guess the mcount ABI is slightly different in LLVM/clang still then?
> Before I dive deeper, maybe anyone has an idea?
>