Re: [PATCH bpf-next v1 10/14] bpf, powerpc: JIT arena kfunc argument rebasing

> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> index fc9db691e8203..fc235fbfbeb97 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -624,8 +624,24 @@ static int prepare_for_kfunc_call(const struct bpf_prog *fp, u32 *image,
>  	for (i = 0; i < m->nr_args; i++) {
>  		/* Note that BPF ABI only allows up to 5 args for kfuncs */
>  		u32 reg = bpf_to_ppc(BPF_REG_1 + i), size = m->arg_size[i];
> +		u8 flags = m->arg_flags[i];
> 
> -		if (!(m->arg_flags[i] & BTF_FMODEL_SIGNED_ARG)) {
> +		if (flags & BTF_FMODEL_ARENA_ARG) {
> +			if (WARN_ON_ONCE(!ctx->arena_vm_start))
> +				return -1;
> +
> +			/* rN = kern_vm_start + (u32)rN */
> +			if (zero_extend(image, ctx, reg, reg, 4))
> +				return -1;
> +			if (flags & BTF_FMODEL_NULLABLE_ARG) {
> +				EMIT(PPC_RAW_CMPLDI(reg, 0));
> +				PPC_BCC_CONST_SHORT(COND_EQ, 8);
> +			}
> +			EMIT(PPC_RAW_ADD(reg, reg, bpf_to_ppc(ARENA_VM_START)));
> +			continue;
> +		}

This isn't a bug, but the comment above prepare_for_kfunc_call lists:

/*
 * Handle powerpc ABI expectations from caller:
 *   - Unsigned arguments are zero-extended.
 *   - Signed arguments are sign-extended.
 */

After this change the function also rebases __arena arguments onto the
arena kernel base. Would it be worth adding that as a third bullet?


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/32538351576