linux-kernel - Re: [PATCH v2 1/2] bpf: Add kfuncs for storing struct task

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Date:   Mon, 3 Oct 2022 12:20:57 -0700
From:   Martin KaFai Lau <martin.lau@...ux.dev>
To:     David Vernet <void@...ifault.com>, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org
Cc:     kernel-team@...com, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, yhs@...com, song@...nel.org,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org, tj@...nel.org,
        memxor@...il.com
Subject: Re: [PATCH v2 1/2] bpf: Add kfuncs for storing struct task_struct *
 as a kptr

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 ?