linux-kernel - [PATCH v2] x86: Treat R_386_PLT32 as R_386

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Date:   Thu,  7 Jan 2021 10:55:55 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Fangrui Song <maskray@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: [PATCH v2] x86: Treat R_386_PLT32 as R_386_PC32

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@...` 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.

clang-12 -fno-pic (since
https://github.com/llvm/llvm-project/commit/961f31d8ad14c66829991522d73e14b5a96ff6d4)
can emit 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 -fno-pic may emit R_386_PLT32 as well
if an option like -fno-direct-access-external-data is adopted to avoid
canonical PLT entry/copy relocations.

Link: https://github.com/ClangBuiltLinux/linux/issues/1210
Link: https://github.com/llvm/llvm-project/commit/961f31d8ad14c66829991522d73e14b5a96ff6d4
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112
Reported-by: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Fangrui Song <maskray@...gle.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Tested-by: Nathan Chancellor <natechancellor@...il.com>

---
Change in v2:
* Improve commit message
---
 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