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From:   =?UTF-8?B?RsSBbmctcnXDrCBTw7JuZw==?= <maskray@google.com>
Date:   Fri, 22 Jan 2021 00:49:43 -0800
Message-ID: <CAFP8O3JBkxYfQ+shsmu=8e1QE4Wza6vt69PaZGNegLKQS9b1DA@mail.gmail.com>
Subject: Re: [PATCH v3] x86: Treat R_386_PLT32 as R_386_PC32
To:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        X86 ML <x86@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>
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On Thu, Jan 14, 2021 at 2:48 PM Fangrui Song <maskray@google.com> 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_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.
>
> clang-12 -fno-pic (since
> https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)
> can emit R_386_PLT32 for compiler generated function declarations as
> well to avoid a canonical PLT entry (st_shndx=0, st_value!=0) if the
> symbol turns out to be defined externally. GCC/GNU as will likely keep
> using R_386_PC32 because (1) the ABI is legacy (2) the change will drop
> a GNU ld non-default visibility ifunc for shared objects.
> https://sourceware.org/bugzilla/show_bug.cgi?id=27169
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1210
> Reported-by: Arnd Bergmann <arnd@arndb.de>
> Signed-off-by: Fangrui Song <maskray@google.com>
> 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>
>
> ---
> 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 id.
> ---
>  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.
> --
> 2.30.0.296.g2bfb1c46d8-goog
>

Ping:)