On Tue, Aug 11, 2020 at 10:36 AM Arvind Sankar <nivedita@xxxxxxxxxxxx> 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. > > [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-ndesaulniers@xxxxxxxxxx/ > [2] https://github.com/ClangBuiltLinux/linux/issues/1121 > [3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0 > [4] https://lore.kernel.org/lkml/20200731202738.2577854-1-nivedita@xxxxxxxxxxxx/ > > Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx> > Reported-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > Cc: stable@xxxxxxxxxxxxxxx # 4.19.x Thanks Arvind, good write up. Just curious about this stable tag, how come you picked 4.19? I can see boot failures in our CI for x86+LLD back to 4.9. Can we amend that tag to use `# 4.9`? I'd be happy to help submit backports should they fail to apply cleanly. https://travis-ci.com/github/ClangBuiltLinux/continuous-integration/builds/179237488 > --- > arch/x86/boot/compressed/Makefile | 6 ++++++ > 1 file changed, 6 insertions(+) > > diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile > index 3962f592633d..c5449bea58ec 100644 > --- a/arch/x86/boot/compressed/Makefile > +++ b/arch/x86/boot/compressed/Makefile > @@ -62,6 +62,12 @@ KBUILD_LDFLAGS += $(shell $(LD) --help 2>&1 | grep -q "\-z noreloc-overflow" \ > endif > LDFLAGS_vmlinux := -T > > +# Disable relocation relaxation if not linking as PIE > +ifeq ($(filter -pie,$(KBUILD_LDFLAGS)),) > +KBUILD_CFLAGS += $(call as-option, -Wa$(comma)-mrelax-relocations=no) > +KBUILD_AFLAGS += $(call as-option, -Wa$(comma)-mrelax-relocations=no) > +endif > + > hostprogs := mkpiggy > HOST_EXTRACFLAGS += -I$(srctree)/tools/include > > -- > 2.26.2 > -- Thanks, ~Nick Desaulniers
