Messages in this thread | | | From | Nick Desaulniers <> | Date | Thu, 29 Aug 2019 13:57:35 -0700 | 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 1:21 PM Stefan Agner <stefan@agner.ch> wrote: > > On 2019-08-29 21:34, Nathan Chancellor wrote: > > 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 agree with Nathan here, I'd rather prefer the build system to fail > building rather than runtime error. > > If we decide we want to have it building despite it not building a > functional kernel, we should at least add a #warning...
Just to be clear...I was suggesting a build failure, but for __gnu_mcount_nc not having the correct calling convention in older clang releases, which is orthogonal to passing `-meabi gnu`. This patch uses the __gnu_mcount_nc calling convention problem to justify a version check for a flag that while closely related, is not actually the problem, IMO. -- Thanks, ~Nick Desaulniers
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