Re: [PATCH 6/7] x86: objtool: add support for R_X86_64_REX_GOTPCRELX

[Uros Bizjak <ubizjak@gmail.com>]

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@kernel.org
> Signed-off-by: Alexander Potapenko <glider@google.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.