X accesses hardware directly (Score 1)

The X Server is not the operating system kernel. It runs in user space. It needs to be root privilige because it accesses and manipulates low level hardware directly. If you have a piece of software that can change register contents and some of those registers change things like bus handling (e.g. DMA, IRQ) then you can do things to make the machine stop working. Try running your system with interrupts disabled. The kernel may be working, but it is getting nothing useful done. FWIW, a properly X Server can cause the kernel to panic, but only via system calls. The other poster that referred to this wasn't clear, but I think they might have intended that the X Server itself can't generate panics. In 1994/1995 one of the most common kernel halt problems reported with Accelerated-X on Linux and FreeBSD was a defective system call for keyboard handling. This has been corrected in both kernels for about four years. However, when it existed, remapping the casp lock key to be a control key would eventually provoke the kernel crash. So, you are right that the X Server is a user level process. It is, however, unusual in UNIX programs in that from user space, it manipulates low level hardware. That does not make intrinisically less safe than running the same code from the kernel. There are many modes of failure of an X Server. Some of them result in system lockup. Some result in the X Server locking up. Some result in incorrect response to a correct request. Only (extensive) testing finds the problems. Only diciplined programming and a respect for the customers data results in the bugs being fixed. Cheers, JeremyC.