linux-kernel - Re: [PATCH bpf-next v3 1/9] bpf: Implement task local storage

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Message-ID: <20201104221526.dv6qfpfp5lk2t7zw@kafai-mbp.dhcp.thefacebook.com>
Date:   Wed, 4 Nov 2020 14:15:35 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     KP Singh <kpsingh@...omium.org>
CC:     <linux-kernel@...r.kernel.org>, <bpf@...r.kernel.org>,
        Song Liu <songliubraving@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Paul Turner <pjt@...gle.com>, Jann Horn <jannh@...gle.com>,
        Hao Luo <haoluo@...gle.com>
Subject: Re: [PATCH bpf-next v3 1/9] bpf: Implement task local storage

On Wed, Nov 04, 2020 at 05:44:45PM +0100, KP Singh wrote:
[ ... ]

> +static void *bpf_pid_task_storage_lookup_elem(struct bpf_map *map, void *key)
> +{
> +	struct bpf_local_storage_data *sdata;
> +	struct task_struct *task;
> +	unsigned int f_flags;
> +	struct pid *pid;
> +	int fd, err;
> +
> +	fd = *(int *)key;
> +	pid = pidfd_get_pid(fd, &f_flags);
> +	if (IS_ERR(pid))
> +		return ERR_CAST(pid);
> +
> +	/* We should be in an RCU read side critical section, it should be safe
> +	 * to call pid_task.
> +	 */
> +	WARN_ON_ONCE(!rcu_read_lock_held());
> +	task = pid_task(pid, PIDTYPE_PID);
> +	if (!task) {
> +		err = -ENOENT;
> +		goto out;
> +	}
> +
> +	sdata = task_storage_lookup(task, map, true);
> +	put_pid(pid);
> +	return sdata ? sdata->data : NULL;
> +out:
> +	put_pid(pid);
> +	return ERR_PTR(err);
> +}
> +
> +static int bpf_pid_task_storage_update_elem(struct bpf_map *map, void *key,
> +					    void *value, u64 map_flags)
> +{
> +	struct bpf_local_storage_data *sdata;
> +	struct task_struct *task;
> +	unsigned int f_flags;
> +	struct pid *pid;
> +	int fd, err;
> +
> +	fd = *(int *)key;
> +	pid = pidfd_get_pid(fd, &f_flags);
> +	if (IS_ERR(pid))
> +		return PTR_ERR(pid);
> +
> +	/* We should be in an RCU read side critical section, it should be safe
> +	 * to call pid_task.
> +	 */
> +	WARN_ON_ONCE(!rcu_read_lock_held());
> +	task = pid_task(pid, PIDTYPE_PID);
> +	if (!task) {
> +		err = -ENOENT;
> +		goto out;
> +	}
> +
> +	sdata = bpf_local_storage_update(
> +		task, (struct bpf_local_storage_map *)map, value, map_flags);
It seems the task is protected by rcu here and the task may be going away.
Is it ok?

or the following comment in the later "BPF_CALL_4(bpf_task_storage_get, ...)"
is no longer valid?
	/* This helper must only called from where the task is guaranteed
	 * to have a refcount and cannot be freed.
	 */
	 
> +
> +	err = PTR_ERR_OR_ZERO(sdata);
> +out:
> +	put_pid(pid);
> +	return err;
> +}
> +

[ ... ]

> +BPF_CALL_4(bpf_task_storage_get, struct bpf_map *, map, struct task_struct *,
> +	   task, void *, value, u64, flags)
> +{
> +	struct bpf_local_storage_data *sdata;
> +
> +	if (flags & ~(BPF_LOCAL_STORAGE_GET_F_CREATE))
> +		return (unsigned long)NULL;
> +
> +	/* explicitly check that the task_storage_ptr is not
> +	 * NULL as task_storage_lookup returns NULL in this case and
> +	 * bpf_local_storage_update expects the owner to have a
> +	 * valid storage pointer.
> +	 */
> +	if (!task_storage_ptr(task))
> +		return (unsigned long)NULL;
> +
> +	sdata = task_storage_lookup(task, map, true);
> +	if (sdata)
> +		return (unsigned long)sdata->data;
> +
> +	/* This helper must only called from where the task is guaranteed
> +	 * to have a refcount and cannot be freed.
> +	 */
> +	if (flags & BPF_LOCAL_STORAGE_GET_F_CREATE) {
> +		sdata = bpf_local_storage_update(
> +			task, (struct bpf_local_storage_map *)map, value,
> +			BPF_NOEXIST);
> +		return IS_ERR(sdata) ? (unsigned long)NULL :
> +					     (unsigned long)sdata->data;
> +	}
> +
> +	return (unsigned long)NULL;
> +}

[ ... ]

> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 8f50c9c19f1b..f3fe9f53f93c 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -773,7 +773,8 @@ static int map_check_btf(struct bpf_map *map, const struct btf *btf,
>  		    map->map_type != BPF_MAP_TYPE_ARRAY &&
>  		    map->map_type != BPF_MAP_TYPE_CGROUP_STORAGE &&
>  		    map->map_type != BPF_MAP_TYPE_SK_STORAGE &&
> -		    map->map_type != BPF_MAP_TYPE_INODE_STORAGE)
> +		    map->map_type != BPF_MAP_TYPE_INODE_STORAGE &&
> +		    map->map_type != BPF_MAP_TYPE_TASK_STORAGE)
This is to enable spin lock support in a map's value.  Without peeking
patch 5, I was confused a bit here.  It seems patch 5 was missed when
inode storage was added.

>  			return -ENOTSUPP;
>  		if (map->spin_lock_off + sizeof(struct bpf_spin_lock) >
>  		    map->value_size) {