[systemd-devel] Filter/Parse NETLINK_KOBJECT_UEVENT Messages

[Ryan McClue]

I understand this is slightly off-topic, but I'm completely new to BPF. Analyzing libudev source and Internet I understand the general idea. However, I don't understand how information/what information is passed to the filter from the socket. For example, in my case the socket payload, i.e. buf_str = add@/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2.4/1-2.4:1.0/input/input38/event14
1. How do I pass this string to the sock_filter/sock_fprog structures?
2. Is a correct way of filtering these to implement string parsing to check for '/event' sub-string in EPF bytecode?
I completely agree that using libudev is the way to go (and *a lot* more sane), however this is for my own understanding. Just by investigating this I have found out more about how udev works and now BPF.
‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Monday, September 13th, 2021 at 7:57 PM, Mantas Mikulėnas <grawity at gmail.com> wrote:

> On Mon, Sep 13, 2021 at 12:29 PM Ryan McClue <re.mcclue at protonmail.com> wrote:
>
>> Currently, I'm listening to NETLINK_KOBJECT_UEVENT messages with the following code:
>>
>> union UeventBuffer {
>> struct nlmsghdr netlink_header;
>> char raw[8192];
>> };
>> int sock = socket(PF_NETLINK, SOCK_RAW | SOCK_NONBLOCK, NETLINK_KOBJECT_UEVENT);
>>
>> struct sockaddr_nl addr = {};
>> addr.nl_family = AF_NETLINK;
>> addr.nl_groups = 1 << 0;
>> bind(sock, (struct sockaddr *)&addr, sizeof(addr));
>>
>> UeventBuffer buf = {};
>> struct iovec iov = {};
>> iov.iov_base = &buf;
>> iov.iov_len = sizeof(buf);
>>
>> struct msghdr msg = {};
>> struct sockaddr_nl src_addr = {};
>> msg.msg_name = &src_addr;
>> msg.msg_namelen = sizeof(src_addr);
>> msg.msg_iov = &iov;
>> msg.msg_iovlen = 1;
>>
>> int bytes = recvmsg(sock, &msg, 0);
>> char *buf_str = buf.raw;
>> // parse this buf_str ...
>>
>> I have a few questions to clarify my understanding and to make this more robust:
>> 1. If I add an Xbox One controller, which evtest shows to be /dev/input/event14 I get a whole host of messages, e.g:
>> add@/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2.4/1-2.4:1.0
>> add@/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2.4/1-2.4:1.0/input/input38
>> add@/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2.4/1-2.4:1.0/input/input38/event14
>> add@/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2.4/1-2.4:1.0/input/input38/js0
>> bind@/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2.4/1-2.4:1.0
>> add@/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2.4/1-2.4:1.1
>> add@/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2.4/1-2.4:1.2
>> bind@/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2.4
>> add@/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2.2
>> change@/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2.2
>> Why so many?
>
> It represents the actual device and subsystem layering in Linux – first there's a USB device (on hub 1-2 port 4), then USB interfaces on that device (1.0 to 1.2), then an input-layer device (input38), then two different /dev nodes exposed by it (generic evdev and what I *think* is legacy joydev? Not sure if it's "legacy" or "still current".)
>
> They are necessary; for example, the 1st event that informs systemd-udevd about the low-level USB device is what causes the correct device driver module to be loaded in the first place.
>
>> Can I filter them to just get the ones ending with /input/inputXX/eventXX?
>
> It seems you're supposed to use BPF filters via SO_ATTACH_FILTER; at least that's what libudev does.
>
>> 2. Currently this only picks up input devices. How would I listen to /snd devices, /hid devices, etc.? I assume some change to nl_groups, however what should this be and where is this documented?
>
> There's just one group for all events.
>
> (If I remember correctly, group 1 has all events straight from the kernel, group 2 is the same events augmented and retransmitted by udev. Normally programs want the latter. Both have the same events and include all devices, though.)
>
>> 3. Currently, I'm manually parsing the buf_str to extract the command and device. Are there some supplied macros that parse this information in a more robust way? (as is the case for RTNETLINK)
>
> Well, systemd supplies a whole libudev library, so I would generally recommend using that – *instead of* manually working with netlink... (Or more recently sd-device, but I don't think libudev is likely to be going *poof* anytime soon, especially if you're looking for compatibility with older systems or eudev-using distros.)
>
> Start with udev_monitor_new_from_netlink() and use udev_monitor_filter_add_match_*() to filter by subsystem, devtype, or some arbitrary property, then you'll receive events already parsed. That's 3 lines of code in comparison to your 30.
>
> --
>
> Mantas Mikulėnas
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