Date:   Wed, 16 Feb 2022 22:52:24 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Daniel Borkmann <daniel@...earbox.net>
CC:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>, <kernel-team@...com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v4 net-next 5/8] bpf: Keep the (rcv) timestamp behavior
 for the existing tc-bpf@...ress

On Wed, Feb 16, 2022 at 05:50:43PM -0800, Martin KaFai Lau wrote:
> On Thu, Feb 17, 2022 at 01:03:21AM +0100, Daniel Borkmann wrote:
> > On 2/16/22 6:51 AM, Martin KaFai Lau wrote:
> > > On Wed, Feb 16, 2022 at 12:30:53AM +0100, Daniel Borkmann wrote:
> > > > On 2/11/22 8:13 AM, Martin KaFai Lau wrote:
> > > > > The current tc-bpf@...ress reads and writes the __sk_buff->tstamp
> > > > > as a (rcv) timestamp.  This patch is to backward compatible with the
> > > > > (rcv) timestamp expectation when the skb->tstamp has a mono delivery_time.
> > > > > 
> > > > > If needed, the patch first saves the mono delivery_time.  Depending on
> > > > > the static key "netstamp_needed_key", it then resets the skb->tstamp to
> > > > > either 0 or ktime_get_real() before running the tc-bpf@...ress.  After
> > > > > the tc-bpf prog returns, if the (rcv) timestamp in skb->tstamp has not
> > > > > been changed, it will restore the earlier saved mono delivery_time.
> > > > > 
> > > > > The current logic to run tc-bpf@...ress is refactored to a new
> > > > > bpf_prog_run_at_ingress() function and shared between cls_bpf and act_bpf.
> > > > > The above new delivery_time save/restore logic is also done together in
> > > > > this function.
> > > > > 
> > > > > Signed-off-by: Martin KaFai Lau <kafai@...com>
> > > > > ---
> > > > >    include/linux/filter.h | 28 ++++++++++++++++++++++++++++
> > > > >    net/sched/act_bpf.c    |  5 +----
> > > > >    net/sched/cls_bpf.c    |  6 +-----
> > > > >    3 files changed, 30 insertions(+), 9 deletions(-)
> > > > > 
> > > > > diff --git a/include/linux/filter.h b/include/linux/filter.h
> > > > > index d23e999dc032..e43e1701a80e 100644
> > > > > --- a/include/linux/filter.h
> > > > > +++ b/include/linux/filter.h
> > > > > @@ -699,6 +699,34 @@ static inline void bpf_compute_data_pointers(struct sk_buff *skb)
> > > > >    	cb->data_end  = skb->data + skb_headlen(skb);
> > > > >    }
> > > > > +static __always_inline u32 bpf_prog_run_at_ingress(const struct bpf_prog *prog,
> > > > > +						   struct sk_buff *skb)
> > > > > +{
> > > > > +	ktime_t tstamp, saved_mono_dtime = 0;
> > > > > +	int filter_res;
> > > > > +
> > > > > +	if (unlikely(skb->mono_delivery_time)) {
> > > > > +		saved_mono_dtime = skb->tstamp;
> > > > > +		skb->mono_delivery_time = 0;
> > > > > +		if (static_branch_unlikely(&netstamp_needed_key))
> > > > > +			skb->tstamp = tstamp = ktime_get_real();
> > > > > +		else
> > > > > +			skb->tstamp = tstamp = 0;
> > > > > +	}
> > > > > +
> > > > > +	/* It is safe to push/pull even if skb_shared() */
> > > > > +	__skb_push(skb, skb->mac_len);
> > > > > +	bpf_compute_data_pointers(skb);
> > > > > +	filter_res = bpf_prog_run(prog, skb);
> > > > > +	__skb_pull(skb, skb->mac_len);
> > > > > +
> > > > > +	/* __sk_buff->tstamp was not changed, restore the delivery_time */
> > > > > +	if (unlikely(saved_mono_dtime) && skb_tstamp(skb) == tstamp)
> > > > > +		skb_set_delivery_time(skb, saved_mono_dtime, true);
> > > > 
> > > > So above detour is for skb->tstamp backwards compatibility so users will see real time.
> > > > I don't see why we special case {cls,act}_bpf-only, given this will also be the case
> > > > for other subsystems (e.g. netfilter) when they read access plain skb->tstamp and get
> > > > the egress one instead of ktime_get_real() upon deferred skb_clear_delivery_time().
> > > > 
> > > > If we would generally ignore it, then the above bpf_prog_run_at_ingress() save/restore
> > > > detour is not needed (so patch 5/6 should be dropped). (Meaning, if we need to special
> > > > case {cls,act}_bpf only, we could also have gone for simpler bpf-only solution..)
> > > The limitation here is there is only one skb->tstamp field.  I don't see
> > > a bpf-only solution or not will make a difference here.
> > 
> > A BPF-only solution would probably just treat the skb->tstamp as (semi-)opaque,
> > meaning, there're no further bits on clock type needed in skb, but given the
> > environment is controlled by an orchestrator it can decide which tstamps to
> > retain or which to reset (e.g. by looking at skb->sk). (The other approach is
> > exposing info on clock base as done here to some degree for mono/real.)
> hmm... I think we may be talking about different things.
> 
> Using a bit or not still does not change the fact that
> there is only one skb->tstamp field which may have a delivery
> time or rcv tstamp.  If the delivery time is reset before
> forwarding to ingress or the delivery time was never there, then
> it will be stamped with the rcv timestamp at ingress.
> The bpf needs a way to distinguish between them.
> skb->sk can at most tell the clock base if skb->tstamp
> does indeed have the delivery_time.
> 
> > 
> > > Regarding the netfilter (good point!), I only see it is used in nfnetlink_log.c
> > > and nfnetlink_queue.c.  Like the tapping cases (earlier than the bpf run-point)
> > > and in general other ingress cases, it cannot assume the rcv timestamp is
> > > always there, so they can be changed like af_packet in patch 3
> > > which is a straight forward change.  I can make the change in v5.
> > > 
> > > Going back to the cls_bpf at ingress.  If the concern is on code cleanliness,
> > > how about removing this dance for now while the current rcv tstamp usage is
> > > unclear at ingress.  Meaning keep the delivery_time (if any) in skb->tstamp.
> > > This dance could be brought in later when there was breakage and legit usecase
> > > reported.  The new bpf prog will have to use the __sk_buff->delivery_time_type
> > > regardless if it wants to use skb->tstamp as the delivery_time, so they won't
> > > assume delivery_time is always in skb->tstamp and it will be fine even this
> > > dance would be brought back in later.
> > 
> > Yes, imho, this is still better than the bpf_prog_run_at_ingress() workaround.
> ic. so it is ok to remove the mono dtime save/restore logic here and only brought
> back in if there was legit breakage reported?
Another idea on this which I think is a better middle ground solution
to remove the dance here.

When reading __sk_buff->tstamp, the verifier can do a rewrite if needed.
The rewrite will depend on whether the __sk_buff->delivery_time_type
has been read or not.

If delivery_time_type is not read, it will rewrite to this:
/* BPF_READ: __u64 a = __sk_buff->tstamp; */
if (!skb->tc_at_ingress || !skb->mono_delivery_time)
	a = skb->tstamp;
else
	a = 0;

That will be consistent with other kernel ingress path
expectation (either 0 or rcv tstamp).

If __sk_buff->delivery_time_type is read, no rewrite is needed
and skb->tstamp will be read as is.

> btw, did you look at patch 7 which added the __sk_buff->delivery_time_type
> and bpf_set_delivery_time()?
> 
> > Ideally, we know when we call helpers like ktime_get_ns() that the clock will
> > be mono. We could track that on verifier side in the register type, and when we
> > end up writing to skb->tstamp, we could implicitly also set the clock base bits
> > in skb for the ctx rewrite telling that it's of type 'mono'. Same for reading,
> > we could add __sk_buff->tstamp_type which program can access (imo tstamp_type
> > is more generic than a __sk_buff->delivery_time_type). If someone needs
> > ktime_get_clocktai_ns() for sch_etf in future, it could be similar tracking
> > mechanism. Also setting skb->tstamp to 0 ...
> hmm... I think it is talking about a way to automatically
> update the __sk_buff->delivery_time_type (mono_delivery_time bit) and
> also avoid adding the new bpf_set_delivery_time() helper in patch 7?
> 
> It may have case that time is not always from helper ktime_get_ns() and
> cannot be decided statically. e.g. what if we want to set the current
> skb->tstamp based on when the previous skb was sent in a cgroup.  There
> will be cases coming up that require runtime decision.
> 
> Also, imo, it may be a surprise behavior for the user who only
> changed __skb_buff->tstamp but then figured out
> __sk_buff->delivery_time_type is also changed in
> the background.
> 
> Beside, not sure if the compiler will optimize the 2nd read on
> __sk_buff->delivery_time_type.  The bpf may need a
> READ_ONCE(__sk_buff->delivery_time_type) after writing __skb_buff->tstamp.
> We can add volatile to the delivery_time_type in the UAPI but I think
> it is overkill.
> 
> It is better to treat tstamp and delivery_time_type separately
> for direct access, and have the skb_set_delivery_time() to change
> both of them together.  Also, more checks can be done in
> skb_set_delivery_time() which is more flexible to return
> errors.
> 
> For TCP, it will be already in mono, so skb_set_delivery_time()
> is usually not needed.
> 
> Regarding the name delivery_time_type vs tstamp_type, I thought about
> that and finally picked the delivery_time_type because I want
> a clear separation from rcv tstamp for now until there is more
> clarity on how rcv tstamp is used in tc-bpf.
> 
> > > Regarding patch 6, it is unrelated.  It needs to clear the
> > > mono_delivery_time bit if the bpf writes 0 to the skb->tstamp.
> > 
> > ... doesn't need to be done as code after bpf_prog_run(), but should be brought
> > closer to when we write to the ctx where verifier generates the relevant insns.
> > Imo, that's better than having this outside in bpf_prog_run() which is then
> > checked no matter what program was doing or even accessing tstamp.
> This will also change the mono_delivery_time bit in the background
> and will be similar to the above points.

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