Published on Feb 12, 2016
When we talk about free software, we usually refer to the free software licenses. We also need relief from software patents, so our freedom is not restricted by them. But there is a third type of freedom we need, and that's user freedom.
Expert users don't take a system as it is. They like to change the configuration, and they want to run the software that works best for them. That includes window managers as well as your favourite text editor. But even on a GNU/Linux system consisting only of free software, you can not easily use the filesystem format, network protocol or binary format you want without special privileges. In traditional Unix systems, user freedom is severly restricted by the system administrator.
Description of Hurd
The Hurd is built on top of CMU's Mach 3.0 kernel and uses Mach's virtual memory management and message-passing facilities. The GNU C Library will provide the Unix system call interface, and will call the Hurd for needed services it can't provide itself. The design and implementation of the Hurd is being lead by Michael Bushnell, with assistance from Richard Stallman, Roland McGrath, Jan Brittenson, and others.
Multiprocessor issues for TLB
Maintaining TLB consistency on a shared-memory multiprocessor is a much more complex problem. Although all processors share the main memory, each has its own TLB. Problems arise when one processor changes an entry in a page table that may be active on another processor. The latter may have a copy of that entry in its TLB and hence may continue to use obsolete mapping. It is essential to propagate the change to the TLBs of any processor that may be using the page table.
Many translator settings remain constant for a long time. It would be very lame to always repeat the same couple of dozens settrans calls manually or at boot time. So the Hurd provides a filesystem extension that allows to store translator settings inside the filesystem and let the filesystem servers do the work to start those servers on demand. Such translator settings are called "passive translators". A passive translator is really just a command line string stored in an inode of the filesystem. If during a pathname resolution a server encounters such a passive translator, and no active translator does exist already (for this node), it will use this string to start up a new translator for this inode, and then let the C library continue with the path resolution as described above. Passive translators are installed with settrans using the -p option.
The Authentication Server
One of the Hurd's more central servers is the authentication server. Any user could write a program which implements the authentication protocol; this does not violate the system's security. When a service needs to authenticate a user, it communicates with its trusted authentication server. If that user is using a different authentication server, the transaction will fail and the server can refuse to communicate further. Because, in effect, this forces all programs on the system to use the same authentication server, we have designed its interface to make any safe operation possible, and to include no extraneous operations.