Messages in this thread | | | Date | Thu, 29 Aug 2019 12:34:32 -0700 | From | Nathan Chancellor <> | Subject | Re: [PATCH] ARM: Emit __gnu_mcount_nc when using Clang 10.0.0 or newer |
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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@gmail.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@gmail.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
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