linux-kernel - Re: [PATCH] x86: Treat R_386_PLT32 as R_386

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Date:   Wed, 6 Jan 2021 19:31:45 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] x86: Treat R_386_PLT32 as R_386_PC32

On Wed, Jan 06, 2021 at 04:17:39PM -0800, Fangrui Song 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
> 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@...` 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
> 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
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1210
> Reported-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Fangrui Song <maskray@...gle.com>

I agree with Nick's comments about the commit message. With those
addressed:

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Tested-by: Nathan Chancellor <natechancellor@...il.com>

> ---
>  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
>