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on the epc instruction. The epc instruction, given the permissions of the page in which the epc is located, allows the privilege level to be increased with little or no overhead. The previous privilege level is recorded in the current frame marker and is restored by a regular (function) return. Since the epc instruction has to live in a page with non-standard properties, we hardwire a "gateway" page in the address space. The address of the gateway page is exported to userland in ar.k7. This allows us to rewire the page without breaking the ABI. The syscall stubs in libc are regular function calls that slightly differ from the normal runtime. The difference is mostly to simplify the stubs themselves by by moving some of the logic to the kernel. The libc stubs call into the gateway page (offset 0), from where the kernel trampolines to the code that sets up a minimal trapframe and arranges to execute from the kernel stack. The way back is basicly the same. The kernel returns to the gateway page, whereby privilege is dropped, and jumps back to the syscall stub. Only the special registers are saved in the trapframe. None of the scratch registers are preserved and since the kernel follows the same runtime model, none of the preserved registers are saved. Future enhancements can include the implementation of lightweight syscalls, where kernel functions are performed without setting up a trapframe. Good candidates are the *context syscalls for example. Now that there's a gateway page from which code can be executed in a non-privileged context, we also have the ideal place to put the signal trampolines. By moving the signal trampolines from the user stack to the gateway page, we open up the doors to unexecutable stacks. The gateway page contains signal trampolines for both the "legacy" break-based syscall code and the new and improved epc- based syscall code. Approved: re@ (blanket)
This is the top level of the FreeBSD source directory. This file was last revised on: $FreeBSD$ For copyright information, please see the file COPYRIGHT in this directory (additional copyright information also exists for some sources in this tree - please see the specific source directories for more information). The Makefile in this directory supports a number of targets for building components (or all) of the FreeBSD source tree, the most commonly used one being ``world'', which rebuilds and installs everything in the FreeBSD system from the source tree except the kernel, the kernel-modules and the contents of /etc. The ``buildkernel'' and ``installkernel'' targets build and install the kernel and the modules (see below). Please see the top of the Makefile in this directory for more information on the standard build targets and compile-time flags. Building a kernel is a somewhat more involved process, documentation for which can be found at: http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/kernelconfig.html And in the config(8) man page. Note: If you want to build and install the kernel with the ``buildkernel'' and ``installkernel'' targets, you might need to build world before. More information is available in the handbook. The sample kernel configuration files reside in the sys/<arch>/conf sub-directory (assuming that you've installed the kernel sources), the file named GENERIC being the one used to build your initial installation kernel. The file NOTES contains entries and documentation for all possible devices, not just those commonly used. It is the successor of the ancient LINT file, but in contrast to LINT, it is not buildable as a kernel but a pure reference and documentation file. Source Roadmap: --------------- bin System/user commands. contrib Packages contributed by 3rd parties. crypto Cryptography stuff (see crypto/README). etc Template files for /etc. games Amusements. gnu Various commands and libraries under the GNU Public License. Please see gnu/COPYING* for more information. include System include files. kerberos5 Kerberos5 (Heimdal) package. lib System libraries. libexec System daemons. release Release building Makefile & associated tools. sbin System commands. secure Cryptographic libraries and commands. share Shared resources. sys Kernel sources. tools Utilities for regression testing and miscellaneous tasks. usr.bin User commands. usr.sbin System administration commands. For information on synchronizing your source tree with one or more of the FreeBSD Project's development branches, please see: http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/synching.html