Re: [PATCH v2 1/2] bpf: Add kfuncs for storing struct task_struct * as a kptr

From: Martin KaFai Lau <martin.lau@linux.dev>
To: David Vernet <void@manifault.com>,
	ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org
Cc: kernel-team@fb.com, bpf@vger.kernel.org,
	linux-kernel@vger.kernel.org, yhs@fb.com, song@kernel.org,
	john.fastabend@gmail.com, kpsingh@kernel.org, sdf@google.com,
	haoluo@google.com, jolsa@kernel.org, tj@kernel.org,
	memxor@gmail.com
Subject: Re: [PATCH v2 1/2] bpf: Add kfuncs for storing struct task_struct * as a kptr
Date: Mon, 3 Oct 2022 12:20:57 -0700	[thread overview]
Message-ID: <b27deaa1-7ab2-a969-3475-4fa78db34f86@linux.dev> (raw)
In-Reply-To: <20221001144716.3403120-2-void@manifault.com>

On 10/1/22 7:47 AM, David Vernet wrote:
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index b069517a3da0..36cbe1b8f8b1 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -1700,20 +1700,93 @@ bpf_base_func_proto(enum bpf_func_id func_id)
>   	}
>   }
>   
> -BTF_SET8_START(tracing_btf_ids)
> +__diag_push();
> +__diag_ignore_all("-Wmissing-prototypes",
> +		  "Global functions as their definitions will be in vmlinux BTF");
> +
> +/**
> + * bpf_task_acquire - Acquire a reference to a task. A task acquired by this
> + * kfunc which is not stored in a map as a kptr, must be released by calling
> + * bpf_task_release().
> + * @p: The task on which a reference is being acquired.
> + */
> +__used noinline
> +struct task_struct *bpf_task_acquire(struct task_struct *p)
> +{
> +	refcount_inc(&p->rcu_users);

This probably needs to be refcount_inc_not_zero() also for the cases like during 
the task free tracepoint ?