Linux Kernel – Using devtmpfs for /dev

The thing here is that /dev/sr0 is linked to a kernel device driver. That device driver will allow access to a physical CDROM if available through that node; VMWare and VirtualBox emulate hardware as you mention and hence the kernel and device driver think they're communicating with hardware.

The /dev/sr0 doesn't point to a certain buffer directly, it provides an interface to the block device interface that allows userspace processes to acces the contents of the hardware device.

If you want to make an image available as a block device, then your only choice (besides virtualization and emulating hardware) is to use loop devices with losetup... or to write your own replacement device driver, but I expect that's not a viable option for now.

If you want to make that image available as /dev/sr0 (are we talking about faking out some software that demands access to a CDROM at that location?) then you could move that file to e.g. /dev/sr0.moved and then symlink the appropriate /dev/loopX to /dev/sr0. Of course, if the software in question tries special commands that only apply to CDROM devices, then this won't work. Otherwise it shouldn't be a problem.

The kernel knows device numbers because it decides device numbers. Each driver registers the device numbers that it manages. The numbers are either hard-coded in the source code, or, in some cases, allocated dynamically. The sysfs filesystem allows applications such as udev to discover the devices supported by the kernel. See How does udev get device numbers for the devices it creates? for more details.

The driver's initialization code probes for the hardware, and registers devices based on what hardware it found during probing. Some types of hardware don't support probing; for example, the ISA bus (a largely outdated bus on PC-type computers) provides no way to list connected hardware, so the driver can only try to communicate and pray that there isn't a different peripheral attached at the same address. On some platforms, the bootloader includes a device tree describing the available peripherals and where they are mapped, and the Linux kernel activates drivers based on this information. There are three ways a driver may be loaded for a peripheral.

• The driver may be included as part of the kernel image.
• The driver may be compiled as a module, and loaded explicitly (e.g. by including it in /etc/modules or in an initramfs).
• There is a mechanism for automatically loading certain drivers based on information reported by bus types that can list connected peripherals together with a universal identification number, such as PCI (the main bus in modern PC) and USB. The kernel runs modprobe and passes it a symbolic name that encodes the peripheral's identification, which is an alias for the “real” name of the driver module. See Are driver modules loaded and unloaded automatically?