Re: [PATCH v2 2/9] x86/extable: switch to using FIELD_GET_SIGNED()

[Peter Zijlstra]

On Mon, Apr 27, 2026 at 05:41:19PM -0400, Yury Norov wrote:
> The EX_DATA register is laid out such that EX_DATA_IMM occupied MSB.
> It's done to make sure that FIELD_GET() will sign-extend the IMM
> field during extraction.
> 
> To enforce that, all EX_DATA masks are made signed integers. This
> works, but relies on the particular implementation of FIELD_GET(),
> i.e. masking then shifting, not vice versa; and the particular
> placement of the fields in the register.
> 
> Switch to using the dedicated FIELD_GET_SIGNED(), and relax those
> limitations.
> 
> Signed-off-by: Yury Norov <ynorov@xxxxxxxxxx>

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

> ---
>  arch/x86/include/asm/extable_fixup_types.h | 13 ++++---------
>  arch/x86/mm/extable.c                      |  2 +-
>  2 files changed, 5 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/include/asm/extable_fixup_types.h b/arch/x86/include/asm/extable_fixup_types.h
> index 906b0d5541e8..fd0cfb472103 100644
> --- a/arch/x86/include/asm/extable_fixup_types.h
> +++ b/arch/x86/include/asm/extable_fixup_types.h
> @@ -2,15 +2,10 @@
>  #ifndef _ASM_X86_EXTABLE_FIXUP_TYPES_H
>  #define _ASM_X86_EXTABLE_FIXUP_TYPES_H
>  
> -/*
> - * Our IMM is signed, as such it must live at the top end of the word. Also,
> - * since C99 hex constants are of ambiguous type, force cast the mask to 'int'
> - * so that FIELD_GET() will DTRT and sign extend the value when it extracts it.
> - */
> -#define EX_DATA_TYPE_MASK		((int)0x000000FF)
> -#define EX_DATA_REG_MASK		((int)0x00000F00)
> -#define EX_DATA_FLAG_MASK		((int)0x0000F000)
> -#define EX_DATA_IMM_MASK		((int)0xFFFF0000)
> +#define EX_DATA_TYPE_MASK		(0x000000FF)
> +#define EX_DATA_REG_MASK		(0x00000F00)
> +#define EX_DATA_FLAG_MASK		(0x0000F000)
> +#define EX_DATA_IMM_MASK		(0xFFFF0000)
>  
>  #define EX_DATA_REG_SHIFT		8
>  #define EX_DATA_FLAG_SHIFT		12
> diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
> index 6b9ff1c6cafa..ceb8d03191ab 100644
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -322,7 +322,7 @@ int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
>  
>  	type = FIELD_GET(EX_DATA_TYPE_MASK, e->data);
>  	reg  = FIELD_GET(EX_DATA_REG_MASK,  e->data);
> -	imm  = FIELD_GET(EX_DATA_IMM_MASK,  e->data);
> +	imm  = FIELD_GET_SIGNED(EX_DATA_IMM_MASK, e->data);
>  
>  	switch (type) {
>  	case EX_TYPE_DEFAULT:
> -- 
> 2.51.0
>