https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84908
--- Comment #2 from Jason Vas Dias <jason.vas.dias at gmail dot com> --- Thanks H.J. - RE: > vDSO isn't compiled with -mindirect-branch=thunk-extern in kernel > 4.16-rc5. Why isn't it the case for you? All I know is , when submitting a patched vclock_gettime.c in which the switch in vdso_clock_gettime() had 6 clauses instead of 5, I received in response a mail from kbuild test robot<l...@intel.com>: >Hi > >Thank you for the patch! Yet something to improve: > >[auto build test ERROR on v4.16-rc4] >[also build test ERROR on next-20180315] >[if your patch is applied to the wrong git tree, please drop us a note to help >>improve the system] > >url: >https://github.com/0day-ci/linux/commits/jason-vas-dias-gmail-com/x86-vdso-on-Intel-VDSO-should-handle-CLOCK_MONOTONIC_RAW/20180316-070319 >config: x86_64-rhel (attached as .config) >compiler: gcc-7 (Debian 7.3.0-1) 7.3.0 >reproduce: > # save the attached .config to linux build tree > make ARCH=x86_64 > >All errors (new ones prefixed by >>): > > arch/x86/entry/vdso/vclock_gettime.o: In function `__vdso_clock_gettime': > vclock_gettime.c:(.text+0xf7): undefined reference to > >`__x86_indirect_thunk_rax' > /usr/bin/ld: arch/x86/entry/vdso/vclock_gettime.o: relocation R_X86_64_PC32 > >against undefined symbol `__x86_indirect_thunk_rax' can not be used when > making >a shared object; recompile with -fPIC > /usr/bin/ld: final link failed: Bad value >>> collect2: error: ld returned 1 exit status >-- >>> arch/x86/entry/vdso/vdso32.so.dbg: undefined symbols found >-- >>> objcopy: 'arch/x86/entry/vdso/vdso64.so.dbg': No such file >--- >0-DAY kernel test infrastructure Open Source Technology Center >https://lists.01.org/pipermail/kbuild-all Intel Corporation That github URL does not point to anything anymore. So it was 4.16-rc4 they were compiling against. Maybe it is fixed in 4.16-rc5. I had seen exactly the same problem when compiling the same patch with the RHEL-7 4.8.5-16-el7_4.2.x86_64 compiler, which did not happen when compiling the same code with the previous RHEL-7 4.8.5-16-el7_4.1.x86_64 compiler. So it is caused by -mindirect-branch=thunk-extern ? Aha, I see in kernel Makefile of the RHEL 3.10.0-21.1.el7 kernel: RETPOLINE_CFLAGS += $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register) And it is still in the CFLAGS of the kernel tagged 'v4.16-rc5:' $ grep RETPOLINE linux-4.16-rc5/Makefile RETPOLINE_CFLAGS_GCC := -mindirect-branch=thunk-extern -mindirect-branch-register RETPOLINE_CFLAGS_CLANG := -mretpoline-external-thunk RETPOLINE_CFLAGS := $(call cc-option,$(RETPOLINE_CFLAGS_GCC),$(call cc-option,$(RETPOLINE_CFLAGS_CLANG))) But the compiler I am using for the 4.x builds is 6.4.1 , which does not support -mindirect-branch=thunk-extern / -mindirect-branch-register . I will get round to building one of the latest compilers with retpoline support eventually, but it is not a priority for me at the moment, for my Linux 4.x builds. I just want to be able to build the patched vclock_gettime.c under RHEL, with the new 4.8.5-16 retpoline supporting compiler, and to be able to submit patches that don't break to the 4.16-rc5 branch . YES, I have just tested, removing '-mindirect-branch=thunk-extern -mindirect-branch-register' from the 3.10.0-693.21.1.el7 build DOES allow it to succeed with the 6-clause version of the switch in vclock_gettime.c . But I don't want my patch to have to depend on this being removed - the removal of these flags effectively removes retpoline support, no ? It is a shame to have to do that after it was carefully added. My gripe is really that nothing in GCC should work with a switch with < 6 clauses, and fail with a switch with >= 6 clauses. Just reporting this issue so it can be fixed. Thanks & Best Regards, Jason
