On Thu, Aug 30, 2018 at 05:10:53PM -0400, Alan Stern wrote: > On Thu, 30 Aug 2018, Faisal Mehmood wrote: > > > Based on my (limited) understanding if I were to disable udev, the > > userspace should not be able to enumerate/interact with any newly > > connected device since udev handles uevents generated by kernel. > > (right?) > > That's not quite right. Yes, udev does handle uevents. But a newly > connected device may be usable from userspace, to some extent, without > any handling of uevents. > > > So as a test, I disabled systemd-udevd and then plugged in a flash > > drive. I disabled support for usb mass storage in the kernel. So 'lsblk' > > didn't show the device, just as expected. However, 'lsusb' still > > dynamically updates the list as I repeatedly plugin and remove a flash > > drive. > > An example of what I described above. > > > Then I studied libusb/lsusb code for a bit. It seems that lsusb makes use > > of 'libusb_get_device_list()' to find the connected usb devices. But I > > couldn't find any detail regarding implementation of > > libusb_get_device_list. > > That's an internal aspect of libusb, so it might not be described in > the documentation. You can always check the source code if you really > want to know how it works. I think it may look at the files under > /sys/bus/usb/devices/ -- that's certainly one possibility which would > always work even without udev. > > > I am sure I'm missing something. Could someone please help me understand > > what is happening behind the scenes? > > udev does things like loading drivers into the kernel (modprobe'ing > them) and creating nodes under /dev. But a driver that is already > present in the kernel doesn't need to be modprobe'd, and the files > under /sys are created automatically by the kernel rather than by udev. Also, udev has not created /dev nodes for probably a decade now. That's what devtmpfs does. udev can create symlinks to existing dev nodes, and some "special-case" device nodes where the kernel name is not what userspace wants to use. But really, why would anyone _want_ to remove udev, it's such a nice program and solves so many problems :) thanks, greg k-h
