Re: [PATCH net-next v2 00/12] net-timestamp: bpf extension to equip applications transparently

On Sun, Oct 13, 2024 at 1:48 AM Willem de Bruijn
[off-list ref] wrote:

Jason Xing wrote:

quoted

From: Jason Xing <kernelxing@tencent.com>

A few weeks ago, I planned to extend SO_TIMESTMAMPING feature by using
tracepoint to print information (say, tstamp) so that we can
transparently equip applications with this feature and require no
modification in user side.

Later, we discussed at netconf and agreed that we can use bpf for better
extension, which is mainly suggested by John Fastabend and Willem de
Bruijn. Many thanks here! So I post this series to see if we have a
better solution to extend. My feeling is BPF is a good place to provide
a way to add timestamping by administrators, without having to rebuild
applications.

This approach mostly relies on existing SO_TIMESTAMPING feature, users
only needs to pass certain flags through bpf_setsocktop() to a separate
tsflags. For TX timestamps, they will be printed during generation
phase. For RX timestamps, we will wait for the moment when recvmsg() is
called.

After this series, we could step by step implement more advanced
functions/flags already in SO_TIMESTAMPING feature for bpf extension.

In this series, I only support TCP protocol which is widely used in
SO_TIMESTAMPING feature.

---
V2
Link: https://lore.kernel.org/all/20241008095109.99918-1-kerneljasonxing@gmail.com/ (local)
1. Introduce tsflag requestors so that we are able to extend more in the
future. Besides, it enables TX flags for bpf extension feature separately
without breaking users. It is suggested by Vadim Fedorenko.
2. introduce a static key to control the whole feature. (Willem)
3. Open the gate of bpf_setsockopt for the SO_TIMESTAMPING feature in
some TX/RX cases, not all the cases.

Note:
The main concern we've discussion in V1 thread is how to deal with the
applications using SO_TIMESTAMPING feature? In this series, I allow both
cases to happen at the same time, which indicates that even one
applications setting SO_TIMESTAMPING can still be traced through BPF
program. Please see patch [04/12].

This revision does not address the main concern.

An administrator installed BPF program can affect results of a process
using SO_TIMESTAMPING in ways that break it.

Sorry, I didn't get it. How the following code snippet would break users?

void __skb_tstamp_tx(struct sk_buff *orig_skb,
                     const struct sk_buff *ack_skb,
                     struct skb_shared_hwtstamps *hwtstamps,
                     struct sock *sk, int tstype)
{
        if (!sk)
                return;

        if (static_branch_unlikely(&bpf_tstamp_control))
                bpf_skb_tstamp_tx_output(sk, orig_skb, tstype, hwtstamps);

        skb_tstamp_tx_output(orig_skb, ack_skb, hwtstamps, sk,
tstype);
}

You can see, the application shipped with SO_TIMESTAMPING still prints
timestamps even when the application stays in the attached cgroup
directory.

Thanks,
Jason