On Fri, Aug 7, 2020 at 6:43 PM Arvind Sankar nivedita@alum.mit.edu wrote:
On Fri, Aug 07, 2020 at 02:54:39PM -0700, Nick Desaulniers wrote:
On Fri, Aug 7, 2020 at 2:29 PM Arvind Sankar nivedita@alum.mit.edu wrote:
On Fri, Aug 07, 2020 at 12:41:00PM -0700, Nick Desaulniers wrote:
A recent change 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.
It could also cause kernels compiled with gcc and linked with LLD to fail in the same way, no? The gcc/gas combination will generate the relaxed relocations from I think gas-2.26 onward. Although the only troublesome symbol in the case of gcc/gas is trampoline_32bit_src, referenced from pgtable_64.c (gcc doesn't use a GOTPC reloc for _pgtable etc).
Thanks for taking a look, and the feedback. I appreciate it!
$ gcc --version | head -n 1 gcc (Debian 9.3.0-11) 9.3.0 $ make -j71 clean defconfig bzImage $ llvm-readelf -r arch/x86/boot/compressed/*.o | grep -e R_X86_64_GOTPCRELX -e R_X86_64_REX_GOTPCRELX 0000000000000114 000000120000002a R_X86_64_REX_GOTPCRELX 0000000000000000 trampoline_32bit_src - 4 $ llvm-readelf -r arch/x86/boot/compressed/vmlinux | grep -e R_X86_64_GOTPCRELX -e R_X86_64_REX_GOTPCRELX $
So it looks like yes. I guess then we'd need to add a check for CONFIG_LD_IS_LLD and CONFIG_CC_IS_GCC and binutils version is 2.26+? I don't mind adding support for that combination, but I'd like to skip it in this patch for the sake of backporting something small to stable to get our CI green ASAP, since CONFIG_LD_IS_LLD probably doesn't exist for those stable branches, which will complicate the backport of such a patch. So I'd do it in a follow up patch if we're cool with that?
What if we did it only if we couldn't enable -pie, like the below patch? I think this should cover all the cases without needing LD_IS_LLD checks.
For BFD, the only case that should change is binutils-2.26, which supports relaxations but not -z noreloc-overflow, and will now have relax-relocations disabled. It currently works (with gcc) only because the relaxation of movq foo@GOTPCREL(%rip), %reg to movq $foo, %reg in the non-pie case was only added in 2.27, which is also when -z noreloc-overflow was added, allowing -pie to be enabled. With 2.26, it only gets relaxed to leaq foo(%rip), %reg which is all LLD currently does as well.
Sure, that will work, too. If you'd like to send it along, please add my: Reviewed-by: Nick Desaulniers ndesaulniers@google.com Tested-by: Nick Desaulniers ndesaulniers@google.com
diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile index 8abc30b27ba3..d25bb71f195a 100644 --- a/arch/x86/boot/compressed/Makefile +++ b/arch/x86/boot/compressed/Makefile @@ -60,6 +60,13 @@ else KBUILD_LDFLAGS += $(shell $(LD) --help 2>&1 | grep -q "-z noreloc-overflow" \ && echo "-z noreloc-overflow -pie --no-dynamic-linker") endif
+# Disable relocation relaxation if not building as PIE +ifeq ($(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
LDFLAGS_vmlinux := -T
hostprogs := mkpiggy