2018-04-19 13:16 UTC+0200 ~ Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> On 04/17/2018 04:34 PM, Quentin Monnet wrote:
>> Add documentation for eBPF helper functions to bpf.h user header file.
>> This documentation can be parsed with the Python script provided in
>> another commit of the patch series, in order to provide a RST document
>> that can later be converted into a man page.
>>
>> The objective is to make the documentation easily understandable and
>> accessible to all eBPF developers, including beginners.
>>
>> This patch contains descriptions for the following helper functions, all
>> written by Daniel:
>>
>> - bpf_get_prandom_u32()
>> - bpf_get_smp_processor_id()
>> - bpf_get_cgroup_classid()
>> - bpf_get_route_realm()
>> - bpf_skb_load_bytes()
>> - bpf_csum_diff()
>> - bpf_skb_get_tunnel_opt()
>> - bpf_skb_set_tunnel_opt()
>> - bpf_skb_change_proto()
>> - bpf_skb_change_type()
>>
>> v3:
>> - bpf_get_prandom_u32(): Fix helper name :(. Add description, including
>> a note on the internal random state.
>> - bpf_get_smp_processor_id(): Add description, including a note on the
>> processor id remaining stable during program run.
>> - bpf_get_cgroup_classid(): State that CONFIG_CGROUP_NET_CLASSID is
>> required to use the helper. Add a reference to related documentation.
>> State that placing a task in net_cls controller disables cgroup-bpf.
>> - bpf_get_route_realm(): State that CONFIG_CGROUP_NET_CLASSID is
>> required to use this helper.
>> - bpf_skb_load_bytes(): Fix comment on current use cases for the helper.
>>
>> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
>> Signed-off-by: Quentin Monnet <quentin.monnet@xxxxxxxxxxxxx>
>> ---
>> include/uapi/linux/bpf.h | 152 +++++++++++++++++++++++++++++++++++++++++++++++
>> 1 file changed, 152 insertions(+)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index c59bf5b28164..d748f65a8f58 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
[...]
>> @@ -615,6 +632,27 @@ union bpf_attr {
>> * Return
>> * 0 on success, or a negative error in case of failure.
>> *
>> + * u32 bpf_get_cgroup_classid(struct sk_buff *skb)
>> + * Description
>> + * Retrieve the classid for the current task, i.e. for the
>> + * net_cls (network classifier) cgroup to which *skb* belongs.
>> + *
>> + * This helper is only available is the kernel was compiled with
>> + * the **CONFIG_CGROUP_NET_CLASSID** configuration option set to
>> + * "**y**" or to "**m**".
>> + *
>> + * Note that placing a task into the net_cls controller completely
>> + * disables the execution of eBPF programs with the cgroup.
>
> I'm not sure I follow the above sentence, what do you mean by that?
Please see comment below.
> I would definitely also add here that this helper is limited to cgroups v1
> only, and that it works on clsact TC egress hook but not the ingress one.
>
>> + * Also note that, in the above description, the "network
>> + * classifier" cgroup does not designate a generic classifier, but
>> + * a particular mechanism that provides an interface to tag
>> + * network packets with a specific class identifier. See also the
>
> The "generic classifier" part is a bit strange to parse. I would probably
> leave the first part out and explain that this provides a means to tag
> packets based on a user-provided ID for all traffic coming from the tasks
> belonging to the related cgroup.
In this paragraph and in the one above, I am trying to address Alexei
comments to the RFC v2:
please add that kernel should be configured with
CONFIG_NET_CLS_CGROUP=y|m
and mention Documentation/cgroup-v1/net_cls.txt
Otherwise 'network classifier' is way too generic.
I'd also mention that placing a task into net_cls controller
disables all of cgroup-bpf.
But I must admit I am struggling a bit on this helper. If I understand
correctly, "network classifier" is too generic and could be confused
with TC classifiers? Hence the attempt to avoid that in the second
paragraph... As for "placing a task into net_cls controller disables all
of cgroup-bpf", again if I understand correctly, I think it refers to
cgroup_sk_alloc_disable() being called in write_classid() in file
net/core/netclassid_cgroup.c. But I am not familiar with it and cannot
find a nice way to explain that in the doc :/.
>> + * related kernel documentation, available from the Linux sources
>> + * in file *Documentation/cgroup-v1/net_cls.txt*.
>> + * Return
>> + * The classid, or 0 for the default unconfigured classid.
>> + *
>> * int bpf_skb_vlan_push(struct sk_buff *skb, __be16 vlan_proto, u16 vlan_tci)
>> * Description
>> * Push a *vlan_tci* (VLAN tag control information) of protocol
>> @@ -734,6 +772,16 @@ union bpf_attr {
>> * are **TC_ACT_REDIRECT** on success or **TC_ACT_SHOT** on
>> * error.
>> *
>> + * u32 bpf_get_route_realm(struct sk_buff *skb)
>> + * Description
>> + * Retrieve the realm or the route, that is to say the
>> + * **tclassid** field of the destination for the *skb*. This
>> + * helper is available only if the kernel was compiled with
>> + * **CONFIG_IP_ROUTE_CLASSID** configuration option.
>
> Could mention that this is a similar user provided tag like in the net_cls
> case with cgroups only that this applies to routes here (dst entries) which
> hold this tag.
>
> Also, should say that this works with clsact TC egress hook or alternatively
> conventional classful egress qdiscs, but not on TC ingress. In case of clsact
> TC egress hook this has the advantage that the dst entry has not been dropped
> yet in the xmit path. Therefore, the dst entry does not need to be artificially
> held via netif_keep_dst() for a classful qdisc until the skb is freed.
Here I am not sure to follow, the advantage is for clsact, but this is
only for a classful qdisc that we can avoid holding the dst entry? How
does holding the entry with netif_keep_dst() translate, from a user
space point of view?
Thanks a lot for the exhaustive review!
Quentin
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