On Wed, Jan 27, 2021 at 12:56 PM Fangrui Song <[email protected]> wrote: > > This is similar to commit b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as > R_X86_64_PC32"), but for i386. As far as Linux 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. > https://sourceware.org/bugzilla/show_bug.cgi?id=27169 > > clang-12 -fno-pic (since > https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6) > can emit R_386_PLT32 for compiler generated function declarations, > because preventing canonical PLT entries is weighed over the rare ifunc > diagnostic.
Boris, my CI is red since https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6 landed (Dec 5) for i386. If you need a shorter (or less toolchain verbiage) commit message, please consider simply: ``` This is similar to commit b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as R_X86_64_PC32"), but for i386. From that commit message: As far as the Linux kernel is concerned, R_386_PLT32 can be treated the same as R_386_PC32. clang-12 -fno-pic (since https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6) can emit R_386_PLT32 for compiler generated function declarations. ``` It would help me abuse <strikethrough>alcohol</strikethrough>coffee less to have one less fire. > > Link: https://github.com/ClangBuiltLinux/linux/issues/1210 > Reported-by: Arnd Bergmann <[email protected]> > Signed-off-by: Fangrui Song <[email protected]> > Reviewed-by: Nick Desaulniers <[email protected]> > Reviewed-by: Nathan Chancellor <[email protected]> > Tested-by: Nick Desaulniers <[email protected]> > Tested-by: Nathan Chancellor <[email protected]> > > --- > Change in v2: > * Improve commit message > --- > Change in v3: > * Change the GCC link to the more relevant GNU as link. > * Fix the relevant llvm-project commit. > --- > Change in v4: > * Improve comments and commit message > --- > 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 34b153cbd4ac..5e9a34b5bd74 100644 > --- 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; > diff --git a/arch/x86/tools/relocs.c b/arch/x86/tools/relocs.c > index ce7188cbdae5..1c3a1962cade 100644 > --- a/arch/x86/tools/relocs.c > +++ b/arch/x86/tools/relocs.c > @@ -867,9 +867,11 @@ 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; > > @@ -910,9 +912,11 @@ 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; > > -- > 2.30.0.280.ga3ce27912f-goog > -- Thanks, ~Nick Desaulniers
