From: Fangrui Song maskray@google.com
[ Upstream commit bb73d07148c405c293e576b40af37737faf23a6a ]
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/a084c0388e2a59b9556f2de008333323... [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: Sasha Levin sashal@kernel.org --- arch/x86/kernel/module.c | 1 + arch/x86/tools/relocs.c | 12 ++++++++---- 2 files changed, 9 insertions(+), 4 deletions(-)
diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c index 19977d2f97fb7..3c09ca384199d 100644 --- a/arch/x86/kernel/module.c +++ b/arch/x86/kernel/module.c @@ -125,6 +125,7 @@ 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; diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c index 5b6c8486a0bec..d1c3f82c78826 100644 --- a/arch/x86/tools/relocs.c +++ b/arch/x86/tools/relocs.c @@ -839,9 +839,11 @@ static int do_reloc32(struct section *sec, Elf_Rel *rel, Elf_Sym *sym, 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;
@@ -882,9 +884,11 @@ static int do_reloc_real(struct section *sec, Elf_Rel *rel, Elf_Sym *sym, 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;