linux-kernel - [PATCH 6/7] x86: objtool: add support for R_X86_64_REX

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Message-ID: <20250416085446.480069-7-glider@google.com>
Date: Wed, 16 Apr 2025 10:54:44 +0200
From: Alexander Potapenko <glider@...gle.com>
To: glider@...gle.com
Cc: quic_jiangenj@...cinc.com, linux-kernel@...r.kernel.org, 
	kasan-dev@...glegroups.com, x86@...nel.org, 
	Aleksandr Nogikh <nogikh@...gle.com>, Andrey Konovalov <andreyknvl@...il.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Ingo Molnar <mingo@...hat.com>, Josh Poimboeuf <jpoimboe@...nel.org>, Marco Elver <elver@...gle.com>, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 6/7] x86: objtool: add support for R_X86_64_REX_GOTPCRELX

When compiling modules with -fsanitize-coverage=trace-pc-guard, Clang
will emit R_X86_64_REX_GOTPCRELX relocations for the
__start___sancov_guards and __stop___sancov_guards symbols. Although
these relocations can be resolved within the same binary, they are left
over by the linker because of the --emit-relocs flag.

This patch makes it possible to resolve the R_X86_64_REX_GOTPCRELX
relocations at runtime, as doing so does not require a .got section.
In addition, add a missing overflow check to R_X86_64_PC32/R_X86_64_PLT32.

Cc: x86@...nel.org
Signed-off-by: Alexander Potapenko <glider@...gle.com>
---
 arch/x86/include/asm/elf.h      | 1 +
 arch/x86/kernel/module.c        | 8 ++++++++
 arch/x86/um/asm/elf.h           | 1 +
 tools/objtool/arch/x86/decode.c | 1 +
 4 files changed, 11 insertions(+)

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 1fb83d47711f9..15d0438467e94 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -63,6 +63,7 @@ typedef struct user_i387_struct elf_fpregset_t;
 #define R_X86_64_8		14	/* Direct 8 bit sign extended  */
 #define R_X86_64_PC8		15	/* 8 bit sign extended pc relative */
 #define R_X86_64_PC64		24	/* Place relative 64-bit signed */
+#define R_X86_64_REX_GOTPCRELX	42	/* R_X86_64_GOTPCREL with optimizations */
 
 /*
  * These are used to set parameters in the core dumps.
diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index 8984abd91c001..6c8b524bfbe3b 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -133,6 +133,14 @@ static int __write_relocate_add(Elf64_Shdr *sechdrs,
 		case R_X86_64_PC32:
 		case R_X86_64_PLT32:
 			val -= (u64)loc;
+			if ((s64)val != *(s32 *)&val)
+				goto overflow;
+			size = 4;
+			break;
+		case R_X86_64_REX_GOTPCRELX:
+			val -= (u64)loc;
+			if ((s64)val != *(s32 *)&val)
+				goto overflow;
 			size = 4;
 			break;
 		case R_X86_64_PC64:
diff --git a/arch/x86/um/asm/elf.h b/arch/x86/um/asm/elf.h
index 62ed5d68a9788..f314478ce9bc3 100644
--- a/arch/x86/um/asm/elf.h
+++ b/arch/x86/um/asm/elf.h
@@ -119,6 +119,7 @@ do {								\
 #define R_X86_64_8		14	/* Direct 8 bit sign extended  */
 #define R_X86_64_PC8		15	/* 8 bit sign extended pc relative */
 #define R_X86_64_PC64		24	/* Place relative 64-bit signed */
+#define R_X86_64_REX_GOTPCRELX	42	/* R_X86_64_GOTPCREL with optimizations */
 
 /*
  * This is used to ensure we don't load something for the wrong architecture.
diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index fe1362c345647..8736524d60344 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -93,6 +93,7 @@ bool arch_pc_relative_reloc(struct reloc *reloc)
 	case R_X86_64_PLT32:
 	case R_X86_64_GOTPC32:
 	case R_X86_64_GOTPCREL:
+	case R_X86_64_REX_GOTPCRELX:
 		return true;
 
 	default:
-- 
2.49.0.604.gff1f9ca942-goog