Re: [PATCH] x86: Treat R_386_PLT32 as R_386_PC32

[Nick Desaulniers]

On Wed, Jan 6, 2021 at 4:17 PM Fangrui Song <maskray@xxxxxxxxxx> wrote:
>
> This is similar to commit b21ebf2fb4cde1618915a97cc773e287ff49173e "x86:
> Treat R_X86_64_PLT32 as R_X86_64_PC32", but for i386.  As far as Linux

nit: the format for referring to in tree sha's:

commit b21ebf2fb4cd ("x86: Treat R_X86_64_PLT32 as R_X86_64_PC32")

ie. `commit <first 12 chars of sha> ("<oneline from commit message>")

> kernel is concerned, R_386_PLT32 can be treated the same as R_386_PC32.
>
> R_386_PC32/R_X86_64_PC32 are PC-relative relocation types with the
> requirement that the symbol address is significant.
> R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types without the
> address significance requirement.
>
> 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.
>
> On i386, there are 2 types of PLTs, PIC and non-PIC. Currently the
> convention is to use R_386_PC32 for non-PIC PLT and R_386_PLT32 for PIC
> PLT, but R_386_PLT32 is arguably preferable for -fno-pic code as well:
> this can avoid a "canonical PLT entry" (st_shndx=0, st_value!=0) if the
> symbol turns out to be defined externally. Latest Clang (since
> 961f31d8ad14c66829991522d73e14b5a96ff6d4) can use R_386_PLT32 for

Is https://reviews.llvm.org/rG37f0c8df47d84ba311fc9a2c1884935ba8961e84
related? If so, that should be linked; it would be good to say
"clang-12" rather than "Latest Clang" since in some time "Latest
Clang" will lose meaning.

> compiler produced symbols (if we drop -ffreestanding for CONFIG_X86_32,
> library call optimization can produce newer declarations) and future GCC
> may use R_386_PLT32 as well if the maintainers agree to adopt an option
> like -fdirect-access-external-data to avoid "canonical PLT entry"/copy
> relocations https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112

Punctuation for end of sentence.

>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1210
> Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
> Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>

This fixes a build failure for me with clang-12 (ie. top of tree),
thanks for the patch.

Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

I also see R_386_PC32 referenced in scripts/mod/modpost.c and wonder
if we'd need to potentially handle R_386_PLT32 relocation types there
as well? No current build failures, so maybe YAGNI.

> ---
>  arch/x86/kernel/module.c | 1 +
>  arch/x86/tools/relocs.c  | 2 ++
>  2 files changed, 3 insertions(+)
>
> 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..717e48ca28b6 100644
> --- a/arch/x86/tools/relocs.c
> +++ b/arch/x86/tools/relocs.c
> @@ -867,6 +867,7 @@ 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.
> @@ -910,6 +911,7 @@ 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.
> --
> 2.29.2.729.g45daf8777d-goog
>


-- 
Thanks,
~Nick Desaulniers