Subject: Re: [RFC net-next v2 2/5] net: psample: add multicast filtering on group_id

From: Aaron Conole <[email-masked]>
Date: Tue, 09 Apr 2024 10:43:08 -0400

Adrian Moreno <[email-masked]> writes:

> On 4/8/24 15:18, Ilya Maximets wrote:
>> [copying my previous reply since this version actually has netdev@ in Cc]
>> On 4/8/24 14:57, Adrian Moreno wrote:
>>> Packet samples can come from several places (e.g: different tc sample
>>> actions), typically using the sample group (PSAMPLE_ATTR_SAMPLE_GROUP)
>>> to differentiate them.
>>>
>>> Likewise, sample consumers that listen on the multicast group may only
>>> be interested on a single group. However, they are currently forced to
>>> receive all samples and discard the ones that are not relevant, causing
>>> unnecessary overhead.
>>>
>>> Allow users to filter on the desired group_id by adding a new command
>>> SAMPLE_FILTER_SET that can be used to pass the desired group id.
>>> Store this filter on the per-socket private pointer and use it for
>>> filtering multicasted samples.
>>>
>>> Signed-off-by: Adrian Moreno <[email-masked]>
>>> ---
>>>   include/uapi/linux/psample.h |   1 +
>>>   net/psample/psample.c        | 127 +++++++++++++++++++++++++++++++++--
>>>   2 files changed, 122 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/include/uapi/linux/psample.h b/include/uapi/linux/psample.h
>>> index e585db5bf2d2..5e0305b1520d 100644
>>> --- a/include/uapi/linux/psample.h
>>> +++ b/include/uapi/linux/psample.h
>>> @@ -28,6 +28,7 @@ enum psample_command {
>>>   	PSAMPLE_CMD_GET_GROUP,
>>>   	PSAMPLE_CMD_NEW_GROUP,
>>>   	PSAMPLE_CMD_DEL_GROUP,
>>> +	PSAMPLE_CMD_SAMPLE_FILTER_SET,
>> Other commands are names as PSAMPLE_CMD_VERB_NOUN, so this new one
>> should be PSAMPLE_CMD_SET_FILTER.  (The SAMPLE part seems unnecessary.)
>> Some functions/structures need to be renamed accordingly.
>> 
>
> Sure, I'll rename it when I sent the next version.
>
>>>   };
>>>     enum psample_tunnel_key_attr {
>>> diff --git a/net/psample/psample.c b/net/psample/psample.c
>>> index a5d9b8446f77..a0cef63dfdec 100644
>>> --- a/net/psample/psample.c
>>> +++ b/net/psample/psample.c
>>> @@ -98,13 +98,84 @@ static int psample_nl_cmd_get_group_dumpit(struct sk_buff *msg,
>>>   	return msg->len;
>>>   }
>>>   -static const struct genl_small_ops psample_nl_ops[] = {
>>> +struct psample_obj_desc {
>>> +	struct rcu_head rcu;
>>> +	u32 group_num;
>>> +	bool group_num_valid;
>>> +};
>>> +
>>> +struct psample_nl_sock_priv {
>>> +	struct psample_obj_desc __rcu *flt;
>> Can we call it 'fileter' ?  I find it hard to read the code with
>> this unnecessary abbreviation.  Same for the lock below.
>> 
>
> Sure.
>
>>> +	spinlock_t flt_lock; /* Protects flt. */
>>> +};
>>> +
>>> +static void psample_nl_sock_priv_init(void *priv)
>>> +{
>>> +	struct psample_nl_sock_priv *sk_priv = priv;
>>> +
>>> +	spin_lock_init(&sk_priv->flt_lock);
>>> +}
>>> +
>>> +static void psample_nl_sock_priv_destroy(void *priv)
>>> +{
>>> +	struct psample_nl_sock_priv *sk_priv = priv;
>>> +	struct psample_obj_desc *flt;
>>> +
>>> +	flt = rcu_dereference_protected(sk_priv->flt, true);
>>> +	kfree_rcu(flt, rcu);
>>> +}
>>> +
>>> +static int psample_nl_sample_filter_set_doit(struct sk_buff *skb,
>>> +					     struct genl_info *info)
>>> +{
>>> +	struct psample_nl_sock_priv *sk_priv;
>>> +	struct nlattr **attrs = info->attrs;
>>> +	struct psample_obj_desc *flt;
>>> +
>>> +	flt = kzalloc(sizeof(*flt), GFP_KERNEL);
>>> +
>>> +	if (attrs[PSAMPLE_ATTR_SAMPLE_GROUP]) {
>>> +		flt->group_num = nla_get_u32(attrs[PSAMPLE_ATTR_SAMPLE_GROUP]);
>>> +		flt->group_num_valid = true;
>>> +	}
>>> +
>>> +	if (!flt->group_num_valid) {
>>> +		kfree(flt);
>> Might be better to not allocate it in the first place.
>> 
>
> Absolutely.
>
>>> +		flt = NULL;
>>> +	}
>>> +
>>> +	sk_priv = genl_sk_priv_get(&psample_nl_family, NETLINK_CB(skb).sk);
>>> +	if (IS_ERR(sk_priv)) {
>>> +		kfree(flt);
>>> +		return PTR_ERR(sk_priv);
>>> +	}
>>> +
>>> +	spin_lock(&sk_priv->flt_lock);
>>> +	flt = rcu_replace_pointer(sk_priv->flt, flt,
>>> +				  lockdep_is_held(&sk_priv->flt_lock));
>>> +	spin_unlock(&sk_priv->flt_lock);
>>> +	kfree_rcu(flt, rcu);
>>> +	return 0;
>>> +}
>>> +
>>> +static const struct nla_policy
>>> +	psample_sample_filter_set_policy[PSAMPLE_ATTR_SAMPLE_GROUP + 1] = {
>>> +	[PSAMPLE_ATTR_SAMPLE_GROUP] = { .type = NLA_U32, },
>> This indentation is confusing, though I'm not sure what's a better
>> way.
>> 
>
> I now! I'll try to move it around see if it improves things.
>
>>> +};
>>> +
>>> +static const struct genl_ops psample_nl_ops[] = {
>>>   	{
>>>   		.cmd = PSAMPLE_CMD_GET_GROUP,
>>>   		.validate = GENL_DONT_VALIDATE_STRICT | GENL_DONT_VALIDATE_DUMP,
>>>   		.dumpit = psample_nl_cmd_get_group_dumpit,
>>>   		/* can be retrieved by unprivileged users */
>>> -	}
>>> +	},
>>> +	{
>>> +		.cmd		= PSAMPLE_CMD_SAMPLE_FILTER_SET,
>>> +		.doit		= psample_nl_sample_filter_set_doit,
>>> +		.policy		= psample_sample_filter_set_policy,
>>> +		.flags		= 0,
>>> +	},
>>>   };
>>>     static struct genl_family psample_nl_family __ro_after_init = {
>>> @@ -114,10 +185,13 @@ static struct genl_family psample_nl_family __ro_after_init = {
>>>   	.netnsok	= true,
>>>   	.module		= THIS_MODULE,
>>>   	.mcgrps		= psample_nl_mcgrps,
>>> -	.small_ops	= psample_nl_ops,
>>> -	.n_small_ops	= ARRAY_SIZE(psample_nl_ops),
>>> +	.ops		= psample_nl_ops,
>>> +	.n_ops		= ARRAY_SIZE(psample_nl_ops),
>>>   	.resv_start_op	= PSAMPLE_CMD_GET_GROUP + 1,
>>>   	.n_mcgrps	= ARRAY_SIZE(psample_nl_mcgrps),
>>> +	.sock_priv_size		= sizeof(struct psample_nl_sock_priv),
>>> +	.sock_priv_init		= psample_nl_sock_priv_init,
>>> +	.sock_priv_destroy	= psample_nl_sock_priv_destroy,
>>>   };
>>>     static void psample_group_notify(struct psample_group *group,
>>> @@ -360,6 +434,42 @@ static int psample_tunnel_meta_len(struct ip_tunnel_info *tun_info)
>>>   }
>>>   #endif
>>>   +static inline void psample_nl_obj_desc_init(struct
>>> psample_obj_desc *desc,
>>> +					    u32 group_num)
>>> +{
>>> +	memset(desc, 0, sizeof(*desc));
>>> +	desc->group_num = group_num;
>>> +	desc->group_num_valid = true;
>>> +}
>>> +
>>> +static bool psample_obj_desc_match(struct psample_obj_desc *desc,
>>> +				   struct psample_obj_desc *flt)
>>> +{
>>> +	if (desc->group_num_valid && flt->group_num_valid &&
>>> +	    desc->group_num != flt->group_num)
>>> +		return false;
>>> +	return true;
>> This fucntion returns 'true' if one of the arguments is not valid.
>> I'd not expect such behavior from a 'match' function.
>> I understand the intention that psample should sample everything
>> to sockets that do not request filters, but that should not be part
>> of the 'match' logic, or more appropriate function name should be
>> chosen.  Also, if the group is not initialized, but the filter is,
>> it should not match, logically.  The validity on filter and the
>> current sample is not symmetric.
>> 
>
> The descriptor should always be initialized but I think double
> checking should be OK as in the context of this particular function,
> it might not be clear it is.
>
>> And I'm not really sure if the 'group_num_valid' is actually needed.
>> Can the NULL pointer be used as an indicator?  If so, then maybe
>> the whole psample_obj_desc structure is not needed as it will
>> contain a single field.
>
> If we only filter on group_id, then yes. However, as I was writing
> this, I thought maybe opening the door to filtering on more fields
> such as the protocol in/out interfaces, etc. Now that I read this I
> understand the current code is confusing: I should have left a comment
> or mention it in the commit message.

If you want to have such filtering options, does it make sense to
instead have the listening program send a set of bpf instructions for
filtering instead?  I think the data should be available at the point
where simple bpf is attached (SO_ATTACH_BPF to the psample socket, and
the filter should run as part of the broadcast message IIRC since it
populates the sk_filter field).

>> 
>>> +}
>>> +
>>> +static int psample_nl_sample_filter(struct sock *dsk, struct sk_buff *skb,
>>> +				    void *data)
>>> +{
>>> +	struct psample_obj_desc *desc = data;
>>> +	struct psample_nl_sock_priv *sk_priv;
>>> +	struct psample_obj_desc *flt;
>>> +	int ret = 0;
>>> +
>>> +	rcu_read_lock();
>>> +	sk_priv = __genl_sk_priv_get(&psample_nl_family, dsk);
>>> +	if (!IS_ERR_OR_NULL(sk_priv)) {
>>> +		flt = rcu_dereference(sk_priv->flt);
>>> +		if (flt)
>>> +			ret = !psample_obj_desc_match(desc, flt);
>>> +	}
>>> +	rcu_read_unlock();
>>> +	return ret;
>>> +}
>>> +
>>>   void psample_sample_packet(struct psample_group *group, struct sk_buff *skb,
>>>   			   u32 sample_rate, const struct psample_metadata *md)
>>>   {
>>> @@ -370,6 +480,7 @@ void psample_sample_packet(struct psample_group *group, struct sk_buff *skb,
>>>   #ifdef CONFIG_INET
>>>   	struct ip_tunnel_info *tun_info;
>>>   #endif
>>> +	struct psample_obj_desc desc;
>>>   	struct sk_buff *nl_skb;
>>>   	int data_len;
>>>   	int meta_len;
>>> @@ -487,8 +598,12 @@ void psample_sample_packet(struct psample_group *group, struct sk_buff *skb,
>>>   #endif
>>>     	genlmsg_end(nl_skb, data);
>>> -	genlmsg_multicast_netns(&psample_nl_family, group->net, nl_skb, 0,
>>> -				PSAMPLE_NL_MCGRP_SAMPLE, GFP_ATOMIC);
>>> +	psample_nl_obj_desc_init(&desc, group->group_num);
>>> +	genlmsg_multicast_netns_filtered(&psample_nl_family,
>>> +					 group->net, nl_skb, 0,
>>> +					 PSAMPLE_NL_MCGRP_SAMPLE,
>>> +					 GFP_ATOMIC, psample_nl_sample_filter,
>>> +					 &desc);
>>>     	return;
>>>   error:
>>