Re: [PATCH v2 6/9] arm64: cfi: Use CONFIG_ARM64_FUNCTION_PREFIX_NOPS for the type hash offset

[Bill Wendling]

On Thu, Sep 24, 2026 at 3:37 PM Jose Fernandez (Anthropic)
<jose.fernandez@xxxxxxxxx> wrote:
>
> Make cfi_get_offset(), SYM_TYPED_FUNC_START and the BPF JIT use
> CONFIG_ARM64_FUNCTION_PREFIX_NOPS instead of hardcoding four bytes. kCFI
> kernels with CALL_OPS need three NOPs, or five with ThinLTO and BTI.
>
> CONFIG_ARM64_FUNCTION_PREFIX_NOPS is 0 on every CFI kernel today, so no
> functional change.
>
> Reviewed-by: Ben Cressey <ben@xxxxxxxxxxx>
> Reviewed-by: Florent Revest (Anthropic) <florent.revest@xxxxxxxxx>
> Assisted-by: LLM
> Signed-off-by: Jose Fernandez (Anthropic) <jose.fernandez@xxxxxxxxx>
> ---
>  arch/arm64/include/asm/cfi.h     |  8 ++++++++
>  arch/arm64/include/asm/linkage.h | 11 +++++++++++
>  arch/arm64/net/bpf_jit_comp.c    |  8 ++++++--
>  3 files changed, 25 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/cfi.h b/arch/arm64/include/asm/cfi.h
> index ab90f0351b7ae..03f02d4614c23 100644
> --- a/arch/arm64/include/asm/cfi.h
> +++ b/arch/arm64/include/asm/cfi.h
> @@ -4,4 +4,12 @@
>
>  #define __bpfcall
>
> +#ifdef CONFIG_CFI
> +static inline int cfi_get_offset(void)
> +{
> +       return 4 + 4 * CONFIG_ARM64_FUNCTION_PREFIX_NOPS;

*drive by review* Could you explain the two fours in this equation
with a comment? (Feel free to ignore this if it's obvious to
developers.)

-bw

> +}
> +#define cfi_get_offset cfi_get_offset
> +#endif /* CONFIG_CFI */
> +
>  #endif /* _ASM_ARM64_CFI_H */
> diff --git a/arch/arm64/include/asm/linkage.h b/arch/arm64/include/asm/linkage.h
> index d1f7a16729d25..14bc3c67c38b3 100644
> --- a/arch/arm64/include/asm/linkage.h
> +++ b/arch/arm64/include/asm/linkage.h
> @@ -39,6 +39,17 @@
>         SYM_START(name, SYM_L_WEAK, SYM_A_NONE)         \
>         bti c ;
>
> +/*
> + * The compiler emits CONFIG_ARM64_FUNCTION_PREFIX_NOPS NOPs between a C
> + * function's kCFI type hash and its entry point. Callers check the hash
> + * at that offset.
> + */
> +#define __CFI_TYPE(name)                               \
> +       .4byte __kcfi_typeid_##name ASM_NL              \
> +       .rept CONFIG_ARM64_FUNCTION_PREFIX_NOPS ASM_NL  \
> +       nop ASM_NL                                      \
> +       .endr
> +
>  #define SYM_TYPED_FUNC_START(name)                             \
>         SYM_TYPED_START(name, SYM_L_GLOBAL, SYM_A_ALIGN)        \
>         bti c ;
> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index c18e005a41dbe..7ad14d9847138 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -185,8 +185,12 @@ static inline void emit_bti(u32 insn, struct jit_ctx *ctx)
>
>  static inline void emit_kcfi(u32 hash, struct jit_ctx *ctx)
>  {
> -       if (IS_ENABLED(CONFIG_CFI))
> -               emit_u32_data(hash, ctx);
> +       if (!IS_ENABLED(CONFIG_CFI))
> +               return;
> +
> +       emit_u32_data(hash, ctx);
> +       for (int i = 0; i < CONFIG_ARM64_FUNCTION_PREFIX_NOPS; i++)
> +               emit(A64_NOP, ctx);
>  }
>
>  /*
>
> --
> 2.52.0
>