| From bb73d07148c405c293e576b40af37737faf23a6a Mon Sep 17 00:00:00 2001 |
| From: Fangrui Song <maskray@google.com> |
| Date: Wed, 27 Jan 2021 12:56:00 -0800 |
| Subject: x86/build: Treat R_386_PLT32 relocation as R_386_PC32 |
| |
| From: Fangrui Song <maskray@google.com> |
| |
| commit bb73d07148c405c293e576b40af37737faf23a6a upstream. |
| |
| This is similar to commit |
| |
| b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as R_X86_64_PC32") |
| |
| but for i386. As far as the kernel is concerned, R_386_PLT32 can be |
| treated the same as R_386_PC32. |
| |
| R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types which |
| can only be used by branches. If the referenced symbol is defined |
| externally, a PLT will be used. |
| |
| R_386_PC32/R_X86_64_PC32 are PC-relative relocation types which can be |
| used by address taking operations and branches. If the referenced symbol |
| is defined externally, a copy relocation/canonical PLT entry will be |
| created in the executable. |
| |
| On x86-64, there is no PIC vs non-PIC PLT distinction and an |
| R_X86_64_PLT32 relocation is produced for both `call/jmp foo` and |
| `call/jmp foo@PLT` with newer (2018) GNU as/LLVM integrated assembler. |
| This avoids canonical PLT entries (st_shndx=0, st_value!=0). |
| |
| On i386, there are 2 types of PLTs, PIC and non-PIC. Currently, |
| the GCC/GNU as convention is to use R_386_PC32 for non-PIC PLT and |
| R_386_PLT32 for PIC PLT. Copy relocations/canonical PLT entries |
| are possible ABI issues but GCC/GNU as will likely keep the status |
| quo because (1) the ABI is legacy (2) the change will drop a GNU |
| ld diagnostic for non-default visibility ifunc in shared objects. |
| |
| clang-12 -fno-pic (since [1]) can emit R_386_PLT32 for compiler |
| generated function declarations, because preventing canonical PLT |
| entries is weighed over the rare ifunc diagnostic. |
| |
| Further info for the more interested: |
| |
| https://github.com/ClangBuiltLinux/linux/issues/1210 |
| https://sourceware.org/bugzilla/show_bug.cgi?id=27169 |
| https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6 [1] |
| |
| [ bp: Massage commit message. ] |
| |
| Reported-by: Arnd Bergmann <arnd@arndb.de> |
| Signed-off-by: Fangrui Song <maskray@google.com> |
| Signed-off-by: Borislav Petkov <bp@suse.de> |
| Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> |
| Reviewed-by: Nathan Chancellor <natechancellor@gmail.com> |
| Tested-by: Nick Desaulniers <ndesaulniers@google.com> |
| Tested-by: Nathan Chancellor <natechancellor@gmail.com> |
| Tested-by: Sedat Dilek <sedat.dilek@gmail.com> |
| Link: https://lkml.kernel.org/r/20210127205600.1227437-1-maskray@google.com |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| --- |
| arch/x86/kernel/module.c | 1 + |
| arch/x86/tools/relocs.c | 12 ++++++++---- |
| 2 files changed, 9 insertions(+), 4 deletions(-) |
| |
| --- a/arch/x86/kernel/module.c |
| +++ b/arch/x86/kernel/module.c |
| @@ -114,6 +114,7 @@ int apply_relocate(Elf32_Shdr *sechdrs, |
| *location += sym->st_value; |
| break; |
| case R_386_PC32: |
| + case R_386_PLT32: |
| /* Add the value, subtract its position */ |
| *location += sym->st_value - (uint32_t)location; |
| break; |
| --- a/arch/x86/tools/relocs.c |
| +++ b/arch/x86/tools/relocs.c |
| @@ -867,9 +867,11 @@ static int do_reloc32(struct section *se |
| case R_386_PC32: |
| case R_386_PC16: |
| case R_386_PC8: |
| + case R_386_PLT32: |
| /* |
| - * NONE can be ignored and PC relative relocations don't |
| - * need to be adjusted. |
| + * NONE can be ignored and PC relative relocations don't need |
| + * to be adjusted. Because sym must be defined, R_386_PLT32 can |
| + * be treated the same way as R_386_PC32. |
| */ |
| break; |
| |
| @@ -910,9 +912,11 @@ static int do_reloc_real(struct section |
| case R_386_PC32: |
| case R_386_PC16: |
| case R_386_PC8: |
| + case R_386_PLT32: |
| /* |
| - * NONE can be ignored and PC relative relocations don't |
| - * need to be adjusted. |
| + * NONE can be ignored and PC relative relocations don't need |
| + * to be adjusted. Because sym must be defined, R_386_PLT32 can |
| + * be treated the same way as R_386_PC32. |
| */ |
| break; |
| |