On 2020-05-20, Ard Biesheuvel wrote:
On Wed, 20 May 2020 at 17:53, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
On Wed, May 20, 2020 at 10:28:13AM +0800, kbuild test robot wrote:
> tree: https://git.kernel.org/pub/scm/linux/kernel/git/efi/efi.git next
> head: e5fbadd1179cdadeaa44d71a9ec068c5bc100524
> commit: ce5e3f909fc0ed67e58367b9c56a54c20a51080b [22/40] efi/printf: Add 64-bit and 8-bit integer support
> config: x86_64-defconfig (attached as .config)
> compiler: clang version 11.0.0 (https://github.com/llvm/llvm-project 135b877874fae96b4372c8a3fbfaa8ff44ff86e3)
> reproduce:
> wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> chmod +x ~/bin/make.cross
> # install x86_64 cross compiling tool for clang build
> # apt-get install binutils-x86-64-linux-gnu
> git checkout ce5e3f909fc0ed67e58367b9c56a54c20a51080b
> # save the attached .config to linux build tree
> COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=x86_64
>
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kbuild test robot <lkp@xxxxxxxxx>
>
> All warnings (new ones prefixed by >>, old ones prefixed by <<):
>
> >> ld.lld: warning: vsprintf.c:(.discard.unreachable+0x0): has non-ABS relocation R_X86_64_PC32 against symbol ''
>
> ---
> 0-DAY CI Kernel Test Service, Intel Corporation
> https://lists.01.org/hyperkitty/list/kbuild-all@xxxxxxxxxxxx
Clang folk, is anything actually wrong here? This seems to result from
use of unreachable() annotation, which generates these
.discard.unreachable sections apparently to help objtool.
This came up before, and IIRC, it has already been fixed, only this
branch is based on v5.7-rc2.
LLD reports the diagnostic because a PC-relative relocation referencing a
SHF_ALLOC section from a non-SHF_ALLOC section (.discard.unreachable) cannot be
reasonably represented. A non-SHF_ALLOC section is not part of the memory image, thus conceptually
the distance between a non-SHF_ALLOC and a SHF_ALLOC is not a constant which can be resolved at linktime.
This is actually a nice diagnostic which has caught a number of issues.
% readelf -Wr drivers/firmware/efi/libstub/vsprintf.stub.o
..
Relocation section '.rela.discard.unreachable' at offset 0xe38 contains 1 entry:
Offset Info Type Symbol's Value Symbol's Name + Addend
0000000000000000 0000000600000002 R_X86_64_PC32 0000000000000000 .text + ac0
---
The linker command line is:
% ld.lld -m elf_x86_64 -T arch/x86/boot/compressed/vmlinux.lds arch/x86/boot/compressed/kernel_info.o arch/x86/boot/compressed/head_64.o arch/x86/boot/compressed/misc.o arch/x86/boot/compressed/string.o arch/x86/boot/compressed/cmdline.o arch/x86/boot/compressed/error.o arch/x86/boot/compressed/piggy.o arch/x86/boot/compressed/cpuflags.o arch/x86/boot/compressed/early_serial_console.o arch/x86/boot/compressed/kaslr.o arch/x86/boot/compressed/kaslr_64.o arch/x86/boot/compressed/mem_encrypt.o arch/x86/boot/compressed/pgtable_64.o arch/x86/boot/compressed/acpi.o drivers/firmware/efi/libstub/lib.a arch/x86/boot/compressed/efi_thunk_64.o -o arch/x86/boot/compressed/vmlinux
ld.lld: warning: vsprintf.c:(.discard.unreachable+0x0): has non-ABS relocation R_X86_64_PC32 against symbol ''
arch/x86/boot/compressed/vmlinux has an output section .discard.unreachable , which is likely desired.
Note that arch/x86/kernel/vmlinux.lds.S has an output section description /DISCARD/ : { ... *(.discard.*) ... },
but arch/x86/boot/compressed/vmlinux doesn't.
We can let arch/x86/boot/compressed/vmlinux reuse the /DISCARD/ to fix the issue.
