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 <graw...@gmail.com> wrote:
> On Mon, Sep 13, 2021 at 12:29 PM Ryan McClue <re.mcc...@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