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Date:   Mon, 21 Nov 2022 11:41:09 -0800
MIME-Version: 1.0
Subject: Re: [PATCH bpf-next v7 3/4] bpf: Add kfunc bpf_rcu_read_lock/unlock()
Content-Language: en-US
To:     Yonghong Song <yhs@fb.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, kernel-team@fb.com,
        Martin KaFai Lau <martin.lau@kernel.org>, bpf@vger.kernel.org
References: <20221121170515.1193967-1-yhs@fb.com>
 <20221121170530.1196341-1-yhs@fb.com>
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From:   Martin KaFai Lau <martin.lau@linux.dev>
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On 11/21/22 9:05 AM, Yonghong Song wrote:
> @@ -4704,6 +4715,15 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
>   		return -EACCES;
>   	}
>   
> +	/* Access rcu protected memory */
> +	if ((reg->type & MEM_RCU) && env->prog->aux->sleepable &&
> +	    !env->cur_state->active_rcu_lock) {
> +		verbose(env,
> +			"R%d is ptr_%s access rcu-protected memory with off=%d, not rcu protected\n",
> +			regno, tname, off);
> +		return -EACCES;
> +	}
> +
>   	if (env->ops->btf_struct_access && !type_is_alloc(reg->type)) {
>   		if (!btf_is_kernel(reg->btf)) {
>   			verbose(env, "verifier internal error: reg->btf must be kernel btf\n");
> @@ -4731,12 +4751,27 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
>   	if (ret < 0)
>   		return ret;
>   
> +	/* The value is a rcu pointer. The load needs to be in a rcu lock region,
> +	 * similar to rcu_dereference().
> +	 */
> +	if ((flag & MEM_RCU) && env->prog->aux->sleepable && !env->cur_state->active_rcu_lock) {
> +		verbose(env,
> +			"R%d is rcu dereference ptr_%s with off=%d, not in rcu_read_lock region\n",
> +			regno, tname, off);
> +		return -EACCES;
> +	}

Would this make the existing rdonly use case fail?

SEC("fentry.s/" SYS_PREFIX "sys_getpgid")
int task_real_parent(void *ctx)
{
	struct task_struct *task, *real_parent;

	task = bpf_get_current_task_btf();
         real_parent = task->real_parent;
         bpf_printk("pid %u\n", real_parent->pid);
         return 0;
}