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

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Message-ID: <cb6d98dc-49e9-2d3b-1acc-f208e4fd13fc@gmail.com>
Date: Wed, 16 Apr 2025 16:21:07 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Alexander Potapenko <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: Re: [PATCH 6/7] x86: objtool: add support for R_X86_64_REX_GOTPCRELX



On 16. 04. 25 10:54, Alexander Potapenko wrote:
> 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;

These two cases are the same. You probably want:

		case R_X86_64_PC32:
		case R_X86_64_PLT32:
		case R_X86_64_REX_GOTPCRELX:
			val -= (u64)loc;
			if ((s64)val != *(s32 *)&val)
				goto overflow;
			size = 4;
			break;

Uros.