Re: [PATCH] LoongArch: BPF: Sign extend struct ops return values properly

[Jinyang He <hejinyang@xxxxxxxxxxx>]

On 2025-08-20 18:39, Hengqi Chen wrote:

> The ns_bpf_qdisc selftest triggers a kernel panic:
[...]
>   
> +/*
> + * Sign-extend the register if necessary
> + */
> +static int sign_extend(struct jit_ctx *ctx, int r, u8 size)
> +{
> +	switch (size) {
> +	case 1:
> +		emit_insn(ctx, sllid, r, r, 56);
> +		emit_insn(ctx, sraid, r, r, 56);
> +		return 0;
> +	case 2:
> +		emit_insn(ctx, sllid, r, r, 48);
> +		emit_insn(ctx, sraid, r, r, 48);
> +		return 0;
Hi, Hengqi,

For sign-extend char or short, we can use `ext.w.b` or `ext.w.h`.


> +	case 4:
> +		emit_insn(ctx, addiw, r, r, 0);
> +		return 0;
> +	case 8:
> +		return 0;
> +	default:
> +		return -1;
> +	}
> +}
> +
>   static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_image *im,
>   					 const struct btf_func_model *m, struct bpf_tramp_links *tlinks,
>   					 void *func_addr, u32 flags)
> @@ -1602,8 +1628,8 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
>   	}
>   
>   	for (i = 0; i < fentry->nr_links; i++) {
> -		ret = invoke_bpf_prog(ctx, fentry->links[i], args_off, retval_off,
> -				      run_ctx_off, flags & BPF_TRAMP_F_RET_FENTRY_RET);
> +		ret = invoke_bpf_prog(ctx, fentry->links[i], m, args_off, retval_off,
> +			      run_ctx_off, flags & BPF_TRAMP_F_RET_FENTRY_RET);
>   		if (ret)
>   			return ret;
>   	}
> @@ -1612,7 +1638,7 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
>   		if (!branches)
>   			return -ENOMEM;
>   
> -		invoke_bpf_mod_ret(ctx, fmod_ret, args_off, retval_off, run_ctx_off, branches);
> +		invoke_bpf_mod_ret(ctx, fmod_ret, m, args_off, retval_off, run_ctx_off, branches);
>   	}
>   
>   	if (flags & BPF_TRAMP_F_CALL_ORIG) {
> @@ -1638,7 +1664,8 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
>   	}
>   
>   	for (i = 0; i < fexit->nr_links; i++) {
> -		ret = invoke_bpf_prog(ctx, fexit->links[i], args_off, retval_off, run_ctx_off, false);
> +		ret = invoke_bpf_prog(ctx, fexit->links[i], m, args_off,
> +				      retval_off, run_ctx_off, false);
>   		if (ret)
>   			goto out;
>   	}
> @@ -1657,6 +1684,12 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
>   	if (save_ret) {
>   		emit_insn(ctx, ldd, LOONGARCH_GPR_A0, LOONGARCH_GPR_FP, -retval_off);
>   		emit_insn(ctx, ldd, regmap[BPF_REG_0], LOONGARCH_GPR_FP, -(retval_off - 8));
> +		if (is_struct_ops) {
> +			move_reg(ctx, LOONGARCH_GPR_A0, regmap[BPF_REG_0]);
> +			ret = sign_extend(ctx, LOONGARCH_GPR_A0, m->ret_size);
> +			if (ret)
> +				goto out;
> +		}
>   	}
>   
>   	emit_insn(ctx, ldd, LOONGARCH_GPR_S1, LOONGARCH_GPR_FP, -sreg_off);