Subject: Re: [PATCH bpf-next V2 1/2] bpf: BPF-helper for MTU checking add length input

From: Jesper Dangaard Brouer <[email-masked]>
Date: Sat, 27 Feb 2021 11:37:41 +0100

On Sat, 27 Feb 2021 00:36:02 +0100
Daniel Borkmann <[email-masked]> wrote:

> On 2/18/21 12:49 PM, Jesper Dangaard Brouer wrote:
> > The FIB lookup example[1] show how the IP-header field tot_len
> > (iph->tot_len) is used as input to perform the MTU check.
> > 
> > This patch extend the BPF-helper bpf_check_mtu() with the same ability
> > to provide the length as user parameter input, via mtu_len parameter.
> > 
> > [1] samples/bpf/xdp_fwd_kern.c
> > 
> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > Acked-by: John Fastabend <[email-masked]>
> > ---
> >   include/uapi/linux/bpf.h |   17 +++++++++++------
> >   net/core/filter.c        |   12 ++++++++++--
> >   2 files changed, 21 insertions(+), 8 deletions(-)
> > 
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 4c24daa43bac..4ba4ef0ff63a 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -3850,8 +3850,7 @@ union bpf_attr {
> >    *
> >    * long bpf_check_mtu(void *ctx, u32 ifindex, u32 *mtu_len, s32 len_diff, u64 flags)
> >    *	Description
> > -
> > - *		Check ctx packet size against exceeding MTU of net device (based
> > + *		Check packet size against exceeding MTU of net device (based
> >    *		on *ifindex*).  This helper will likely be used in combination
> >    *		with helpers that adjust/change the packet size.
> >    *
> > @@ -3868,6 +3867,14 @@ union bpf_attr {
> >    *		against the current net device.  This is practical if this isn't
> >    *		used prior to redirect.
> >    *
> > + *		On input *mtu_len* must be a valid pointer, else verifier will
> > + *		reject BPF program.  If the value *mtu_len* is initialized to
> > + *		zero then the ctx packet size is use.  When value *mtu_len* is
> > + *		provided as input this specify the L3 length that the MTU check
> > + *		is done against. Remember XDP and TC length operate at L2, but
> > + *		this value is L3 as this correlate to MTU and IP-header tot_len
> > + *		values which are L3 (similar behavior as bpf_fib_lookup).
> > + *
> >    *		The Linux kernel route table can configure MTUs on a more
> >    *		specific per route level, which is not provided by this helper.
> >    *		For route level MTU checks use the **bpf_fib_lookup**\ ()
> > @@ -3892,11 +3899,9 @@ union bpf_attr {
> >    *
> >    *		On return *mtu_len* pointer contains the MTU value of the net
> >    *		device.  Remember the net device configured MTU is the L3 size,
> > - *		which is returned here and XDP and TX length operate at L2.
> > + *		which is returned here and XDP and TC length operate at L2.
> >    *		Helper take this into account for you, but remember when using
> > - *		MTU value in your BPF-code.  On input *mtu_len* must be a valid
> > - *		pointer and be initialized (to zero), else verifier will reject
> > - *		BPF program.
> > + *		MTU value in your BPF-code.
> >    *
> >    *	Return
> >    *		* 0 on success, and populate MTU value in *mtu_len* pointer.
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 7059cf604d94..fcc3bda85960 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -5660,7 +5660,7 @@ BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
> >   	if (unlikely(flags & ~(BPF_MTU_CHK_SEGS)))
> >   		return -EINVAL;
> >   
> > -	if (unlikely(flags & BPF_MTU_CHK_SEGS && len_diff))
> > +	if (unlikely(flags & BPF_MTU_CHK_SEGS && (len_diff || *mtu_len)))
> >   		return -EINVAL;
> >   
> >   	dev = __dev_via_ifindex(dev, ifindex);
> > @@ -5670,7 +5670,11 @@ BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
> >   	mtu = READ_ONCE(dev->mtu);
> >   
> >   	dev_len = mtu + dev->hard_header_len;
> > -	skb_len = skb->len + len_diff; /* minus result pass check */
> > +
> > +	/* If set use *mtu_len as input, L3 as iph->tot_len (like fib_lookup) */
> > +	skb_len = *mtu_len ? *mtu_len + dev->hard_header_len : skb->len;
> > +
> > +	skb_len += len_diff; /* minus result pass check */
> >   	if (skb_len <= dev_len) {
> >   		ret = BPF_MTU_CHK_RET_SUCCESS;
> >   		goto out;
> > @@ -5715,6 +5719,10 @@ BPF_CALL_5(bpf_xdp_check_mtu, struct xdp_buff *, xdp,
> >   	/* Add L2-header as dev MTU is L3 size */
> >   	dev_len = mtu + dev->hard_header_len;
> >   
> > +	/* Use *mtu_len as input, L3 as iph->tot_len (like fib_lookup) */
> > +	if (*mtu_len)
> > +		xdp_len = *mtu_len + dev->hard_header_len;
> > +
> >   	xdp_len += len_diff; /* minus result pass check */
> >   	if (xdp_len > dev_len)
> >   		ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
> >   
> 
> Btw, one more note on the whole bpf_*_check_mtu() helper... Last week I implemented
> PMTU discovery support for clients for Cilium's XDP stand-alone LB in DSR mode, so I
> was briefly considering whether to use the bpf_xdp_check_mtu() helper for retrieving
> the device MTU, but then I thought to myself why having an unnecessary per-packet cost
> for an extra helper call if I could just pass it in via constant instead. So I went
> with the latter instead of the helper with the tradeoff to restart the Cilium agent
> if someone actually changes MTU in prod which is a rare event anyway.
> 
> Looking at what bpf_xdp_check_mtu() for example really offers is retrieval of dev->mtu
> as well as dev->hard_header_len and the rest can all be done inside the BPF prog itself
> w/o the helper overhead. Why am I mentioning this.. because the above change is a similar
> case of what could have been done /inside/ the BPF prog anyway (especially on XDP where
> extra overhead should be cut where possible).

The XDP case looks super simple now, but I thinking ahead.  When
Lorenzo adds multi-buff support, then we can/must update this helper to
use another XDP length value, based on the multi-buff jumbo-frame len.

Maybe we need another helper or what you propose below. BUT we could
also allow this helper (via flag?) to ALSO check if dev support
multi-buff XDP transmit (besides MTU limit with multi-buff len).  Then
the BPF-programmer can know this packet cannot be redirected to the
device.

> I think it got lost somewhere in the many versions of the original set where it was
> mentioned before, but allowing to retrieve the dev object into BPF context and then
> exposing it similarly to how we handle the case of struct bpf_tcp_sock would have been
> much cleaner approach, e.g. the prog from XDP and tc context would be able to do:
> 
>    struct bpf_dev *dev = ctx->dev;
> 
> And we expose initially, for example:
> 
>    struct bpf_dev {
>      __u32 mtu;
>      __u32 hard_header_len;
>      __u32 ifindex;
>      __u32 rx_queues;
>      __u32 tx_queues;
>    };
> 
> And we could also have a BPF helper for XDP and tc that would fetch a /different/ dev
> given we're under RCU context anyway, like ...
> 
> BPF_CALL_2(bpf_get_dev, struct xdp_buff *, xdp, u32, ifindex)
> {
> 	return dev_get_by_index_rcu(dev_net(xdp->rxq->dev), index);
> }
> 
> ... returning a new dev_or_null type. With this flexibility everything else can be done
> inside the prog, and later on it easily allows to expose more from dev side. Actually,
> I'm inclined to code it up ...

I love the idea to retrieve the dev object into BPF context.  It is
orthogonal, and doesn't replace the MTU helpers as the packet ctx
objects (SKB and xdp_buff) are more complex, and the helper allows us
to extend them without users have to update their BPF-code (as desc
above).

I do think it makes a lot of sense to expose/retrieve dev object into
BPF context.  As I hinted about, when we implement XDP multi-buff, then
the bpf_redirect BPF-helper cannot check if the remote device support
multi-buff transmit (as it don't have packet ctx).  If we have the dev
object, the we could expose XDP features that allow us (BPF-programmer)
to check this prior to doing the redirect.

To be clear:
 * I still think *this* patch is relevant and should be applied.

I'm also on-board with retrieve the dev object into BPF context, as it
have other use-cases.
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer