Hi Ingo, I saw you picked up Arvind's other series into x86/boot. Would you mind please including this, as well? Our CI is quite red for x86...
EOM On Sat, Aug 15, 2020 at 8:49 AM Sedat Dilek <sedat.di...@gmail.com> wrote: > > On Wed, Aug 12, 2020 at 2:43 AM Arvind Sankar <nived...@alum.mit.edu> wrote: > > > > The x86-64 psABI [0] specifies special relocation types > > (R_X86_64_[REX_]GOTPCRELX) for indirection through the Global Offset > > Table, semantically equivalent to R_X86_64_GOTPCREL, which the linker > > can take advantage of for optimization (relaxation) at link time. This > > is supported by LLD and binutils versions 2.26 onwards. > > > > The compressed kernel is position-independent code, however, when using > > LLD or binutils versions before 2.27, it must be linked without the -pie > > option. In this case, the linker may optimize certain instructions into > > a non-position-independent form, by converting foo@GOTPCREL(%rip) to $foo. > > > > This potential issue has been present with LLD and binutils-2.26 for a > > long time, but it has never manifested itself before now: > > - LLD and binutils-2.26 only relax > > movq foo@GOTPCREL(%rip), %reg > > to > > leaq foo(%rip), %reg > > which is still position-independent, rather than > > mov $foo, %reg > > which is permitted by the psABI when -pie is not enabled. > > - gcc happens to only generate GOTPCREL relocations on mov instructions. > > - clang does generate GOTPCREL relocations on non-mov instructions, but > > when building the compressed kernel, it uses its integrated assembler > > (due to the redefinition of KBUILD_CFLAGS dropping -no-integrated-as), > > which has so far defaulted to not generating the GOTPCRELX > > relocations. > > > > Nick Desaulniers reports [1,2]: > > A recent change [3] to a default value of configuration variable > > (ENABLE_X86_RELAX_RELOCATIONS OFF -> ON) in LLVM now causes Clang's > > integrated assembler to emit R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX > > relocations. LLD will relax instructions with these relocations based > > on whether the image is being linked as position independent or not. > > When not, then LLD will relax these instructions to use absolute > > addressing mode (R_RELAX_GOT_PC_NOPIC). This causes kernels built with > > Clang and linked with LLD to fail to boot. > > > > Patch series [4] is a solution to allow the compressed kernel to be > > linked with -pie unconditionally, but even if merged is unlikely to be > > backported. As a simple solution that can be applied to stable as well, > > prevent the assembler from generating the relaxed relocation types using > > the -mrelax-relocations=no option. For ease of backporting, do this > > unconditionally. > > > > [0] > > https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/linker-optimization.tex#L65 > > [1] > > https://lore.kernel.org/lkml/20200807194100.3570838-1-ndesaulni...@google.com/ > > [2] https://github.com/ClangBuiltLinux/linux/issues/1121 > > [3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0 > > [4] > > https://lore.kernel.org/lkml/20200731202738.2577854-1-nived...@alum.mit.edu/ > > > > Signed-off-by: Arvind Sankar <nived...@alum.mit.edu> > > Reported-by: Nick Desaulniers <ndesaulni...@google.com> > > Reviewed-by: Nick Desaulniers <ndesaulni...@google.com> > > Tested-by: Nick Desaulniers <ndesaulni...@google.com> > > Cc: sta...@vger.kernel.org > > Signed-off-by: Arvind Sankar <nived...@alum.mit.edu> > > Thanks for the patch. > > Tested-by: Sedat Dilek <sedat.di...@gmail.com> > > - Sedat - > > [1] > https://github.com/ClangBuiltLinux/linux/issues/1120#issuecomment-674409705 > > > --- > > arch/x86/boot/compressed/Makefile | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/arch/x86/boot/compressed/Makefile > > b/arch/x86/boot/compressed/Makefile > > index 3962f592633d..ff7894f39e0e 100644 > > --- a/arch/x86/boot/compressed/Makefile > > +++ b/arch/x86/boot/compressed/Makefile > > @@ -43,6 +43,8 @@ KBUILD_CFLAGS += -Wno-pointer-sign > > KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=) > > KBUILD_CFLAGS += -fno-asynchronous-unwind-tables > > KBUILD_CFLAGS += -D__DISABLE_EXPORTS > > +# Disable relocation relaxation in case the link is not PIE. > > +KBUILD_CFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no) > > > > KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__ > > GCOV_PROFILE := n > > -- > > 2.26.2 > > > > -- > > You received this message because you are subscribed to the Google Groups > > "Clang Built Linux" group. > > To unsubscribe from this group and stop receiving emails from it, send an > > email to clang-built-linux+unsubscr...@googlegroups.com. > > To view this discussion on the web visit > > https://groups.google.com/d/msgid/clang-built-linux/20200812004308.1448603-1-nivedita%40alum.mit.edu. -- Thanks, ~Nick Desaulniers
