Re: [PATCH] net/tun: expose queue utilization stats via ethtool

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Alexander Shalimov wrote:
> 06.05.2025, 22:32, "Willem de Bruijn" <willemdebruijn.kernel@gmail.com>:
> > Perhaps bpftrace with a kfunc at a suitable function entry point to
> > get access to these ring structures.
> 
> Thank you for your responses!
> 
> Initially, we implemented such monitoring using bpftrace but we were
> not satisfied with the need to double-check the structure definitions
> in tun.c for each new kernel version.
> 
> We attached kprobe to the "tun_net_xmit()" function. This function
> gets a "struct net_device" as an argument, which is then explicitly
> cast to a tun_struct - "struct tun_struct *tun = netdev_priv(dev)".
> However, performing such a cast within bpftrace is difficult because
> tun_struct is defined in tun.c - meaning the structure definition
> cannot be included directly (not a header file). As a result, we were
> forced to add fake "struct tun_struct" and "struct tun_file"
> definitions, whose maintenance across kernel versions became
> cumbersome (see below). The same problems exists even with kfunc and
> btf - we are not able to cast properly netdev to tun_struct.
> 
> That’s why we decided to add this functionality directly to the kernel.

Let's solve this in bpftrace instead. That's no reason to rever to
hardcoded kernel APIs.

It quite possibly already is. I'm no bpftrace expert. Cc:ing bpf@

There seem to be two parts:

The field lookup in struct tun_struct. This should be captured by BTF:

	$ bpftool btf dump file /sys/kernel/btf/vmlinux | grep tun_struct | wc -l
        1

The cast from netdev_priv to struct tun_struct. Note that in recent
kernels netdev_priv is just args->dev->priv. No need for this manual
struct tun_net_device.

> 
> Here is an example of bpftrace:
> 
> #define NET_DEVICE_TUN_OFFSET 0x900
> 
> struct tun_net_device {
>     unsigned char padding[NET_DEVICE_TUN_OFFSET]; #such calculation is pain
>     struct tun_struct tun;
> }
> 
> kprobe:tun_net_xmit {
>     $skb = (struct sk_buff*) arg0;
>     $netdev = $skb->dev;
>     $tun_dev = (struct tun_net_device *)arg1;
>     $tun = $tun_dev->tun;
>    ....
> }
> 
> Could you please recommend the right way to implement such bpftrace script?
> Either better place in kernel for the patch.