Re: [PATCH bpf-next v2 1/2] bpf, riscv: Add support for indirect jumps

[Pu Lehui <pulehui@huawei.com>]

On 2026/9/28 10:21, Chen Pei wrote:
> Implement JIT support for the indirect jump instruction (BPF_JMP |
> BPF_JA | BPF_X), a.k.a. gotox, which lets a BPF program jump through a
> BPF_MAP_TYPE_INSN_ARRAY jump table.
> 
> Emit "jalr zero, rd, 0" and hand the xlated to jitted offsets to
> bpf_prog_update_insn_ptrs(), which is what fills in the jump table
> entries; without that call the load fails with -EFAULT in
> bpf_insn_array_ready(). ctx->offset[] holds the offset of the insn
> *following* insn i, as bpf_prog_fill_jited_linfo() expects, so it is
> shifted by one and offset[0] comes from the prologue length. build_body()
> now records both halves of a multi-insn record, so no slot keeps a
> fabricated offset.
> 
> Only the RV64 JIT is covered; RV32 keeps failing to load as before.
> 
> Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
> Reviewed-by: Björn Töpel <bjorn@kernel.org>
> Acked-by: Björn Töpel <bjorn@kernel.org>
> ---
> 
> Changes since v1:
> - Reword the code comment to say "shift ctx->offset[] by one".
> 
>   arch/riscv/net/bpf_jit_comp64.c |  5 +++++
>   arch/riscv/net/bpf_jit_core.c   | 16 ++++++++++++++--
>   2 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index ed0a6f871dea..9de3749fb268 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -1691,6 +1691,11 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>   			emit_zextw(rd, rd, ctx);
>   		break;
>   
> +	/* JUMP reg */
> +	case BPF_JMP | BPF_JA | BPF_X:
> +		emit_jalr(RV_REG_ZERO, rd, 0, ctx);
> +		break;
> +
>   	/* JUMP off */
>   	case BPF_JMP | BPF_JA:
>   	case BPF_JMP32 | BPF_JA:
> diff --git a/arch/riscv/net/bpf_jit_core.c b/arch/riscv/net/bpf_jit_core.c
> index 470a6ace5662..5265dd5bd39e 100644
> --- a/arch/riscv/net/bpf_jit_core.c
> +++ b/arch/riscv/net/bpf_jit_core.c
> @@ -26,10 +26,13 @@ static int build_body(struct rv_jit_context *ctx, bool extra_pass, int *offset)
>   		int ret;
>   
>   		ret = bpf_jit_emit_insn(insn, ctx, extra_pass);
> -		if (ret > 0)
> -			i++; /* skip the next instruction */
>   		if (offset)
>   			offset[i] = ctx->ninsns;
> +		if (ret > 0) {
> +			i++; /* skip the next instruction */
> +			if (offset)
> +				offset[i] = ctx->ninsns;
> +		}
>   		if (ret < 0)
>   			return ret;
>   	}
> @@ -176,6 +179,15 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_pr
>   		for (i = 0; i < prog->len; i++)
>   			ctx->offset[i] = ninsns_rvoff(ctx->offset[i]);
>   		bpf_prog_fill_jited_linfo(prog, ctx->offset);
> +
> +		/*
> +		 * bpf_prog_update_insn_ptrs() wants the start of each insn, so
> +		 * shift ctx->offset[] by one and get insn 0 from the prologue.
> +		 */
> +		for (i = prog->len - 1; i > 0; i--)
> +			ctx->offset[i] = ctx->offset[i - 1];
> +		ctx->offset[0] = ninsns_rvoff(ctx->prologue_len);
> +		bpf_prog_update_insn_ptrs(prog, ctx->offset, jit_data->ro_image);
>   out_offset:
>   		kvfree(ctx->offset);
>   		kfree(jit_data);

Reviewed-by: Pu Lehui <pulehui@huawei.com>