Common Firmware Environment

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About

The Common Firmware Environment (CFE) is the firmware developed by Broadcom for the BCM947xx SoC platform (among others). It is the first code that runs when the router boots and performs functions similar to Apple's Open Firmware:

  • Initializes the system
  • Sets up a basic environment in which code can run
  • Optionally provides a command line interface non-standard usage
  • Loads and executes a kernel image (expecting to be jettisoned shortly thereafter)

So, in normal operation, a user will not see CFE working at all; it will load the Linksys kernel and send it on its merry way without hesitation. For us, however, CFE is crucial, because it provides us with the ability to load an image over the network using TFTP.

We have access to two major documents covering CFE, the reference manual, and the functional specification. Much of the content in these two documents overlaps.

Getting into CFE

To get into CFE, it will be very helpful to enable "boot wait" from the Administration Tab under the router's Web GUI. This will cause the router to wait on startup for a signal to stop booting into the firmware and enter CFE.

Once you have that set up and you've connected to the router, just type "reboot" (assuming OpenWRT is installed, it may be different for other firmwares) to reboot the router. This can also be done by power-cycling the router. As it's booting up, send a continuous stream of Ctrl+C characters to cancel booting and you'll be entered right into CFE.

From there, you can prod around CFE's features or load your own kernel using the command line interface.

Command Line Interface

The CFE Command Line Interface (CLI) is a very simple "shell-like" command prompt. It has a few environmental variables and minimal functionality. However, it is complex enough power to load a boot image over the network and begin executing code. The User interface is described on page 19 of 145 in the CFE documentation.

To get the to the CLI, you can use either the power-on method or load OpenWRT and type reboot. The CFE boot screen looks like:

CFE version 1.0.37 for BCM947XX (32bit,SP,LE)
Build Date: Fri Sep 23 17:46:42 CST 2005 (root@localhost.localdomain)
Copyright (C) 2000,2001,2002,2003 Broadcom Corporation.

Initializing Arena
Initializing Devices.

No DPN
et0: Broadcom BCM47xx 10/100 Mbps Ethernet Controller 3.90.37.0
CPU type 0x29008: 200MHz
Total memory: 16384 KBytes

Total memory used by CFE:  0x80300000 - 0x803A39C0 (670144)
Initialized Data:          0x803398D0 - 0x8033BFE0 (10000)
BSS Area:                  0x8033BFE0 - 0x8033D9C0 (6624)
Local Heap:                0x8033D9C0 - 0x803A19C0 (409600)
Stack Area:                0x803A19C0 - 0x803A39C0 (8192)
Text (code) segment:       0x80300000 - 0x803398D0 (235728)
Boot area (physical):      0x003A4000 - 0x003E4000
Relocation Factor:         I:00000000 - D:00000000

Boot version: v3.7
The boot is CFE

mac_init(): Find mac [00:16:B6:28:7D:4F] in location 0
Nothing...

eou_key_init(): Find key pair in location 0
The eou device id is same
The eou public key is same
The eou private key is same
Device eth0:  hwaddr 00-16-B6-28-7D-4F, ipaddr 192.168.1.1, mask 255.255.255.0
        gateway not set, nameserver not set
Reading :: Failed.: Interrupted
CFE> ^C
CFE> 

Of course, items like the hwaddr will be different from router to router.

Once you have a command prompt, you can type help and get a listing of commands available:

CFE> help
Available commands:

rndis               Broadcom USB RNDIS utility.
et                  Broadcom Ethernet utility.
modify              Modify flash data.
nvram               NVRAM utility.
reboot              Reboot.
flash               Update a flash memory device
memtest             Test memory.
f                   Fill contents of memory.
e                   Modify contents of memory.
d                   Dump memory.
u                   Disassemble instructions.
autoboot            Automatic system bootstrap.
batch               Load a batch file into memory and execute it
go                  Verify and boot OS image.
boot                Load an executable file into memory and execute it
load                Load an executable file into memory without executing it
save                Save a region of memory to a remote file via TFTP
ping                Ping a remote IP host.
arp                 Display or modify the ARP Table
ifconfig            Configure the Ethernet interface
show devices        Display information about the installed devices.
unsetenv            Delete an environment variable.
printenv            Display the environment variables
setenv              Set an environment variable.
help                Obtain help for CFE commands

For more information about a command, enter 'help command-name'
*** command status = 0
CFE> 

A command status of 0 is always a good thing, other command statuses are errors.

The next two commands are very important to booting a custon kernel image: ifconfig and boot.

ifconfig is just the Linux counterpart, it will set up the specified interface. For our router, we have the switch portion of the router connected to a xinu server (which is simply a TFTP and DHCP server). From there we type ifconfig -auto eth0 which will ask the xinu server for an IP address and set up the router.

CFE> ifconfig -auto eth0 � �
Device eth0:  hwaddr 00-16-B6-28-7D-4F, ipaddr 192.168.5.2, mask 255.255.254.0
        gateway 192.168.5.220, nameserver 192.168.5.220, domain xinu.mu.edu
*** command status = 0
CFE> 

We now have an IP and can transfer our boot image.

For our purposes, we name our boot images after the unit on which it will load (supervoc is our demo router).

CFE> boot -elf 192.168.5.220:supervoc.boot
Loader:elf Filesys:tftp Dev:eth0 File:192.168.5.220:supervoc.boot Options:(null)
Loading: 0x80001000/3145 0x80001c49/23 Entry at 0x80001000
Closing network.
Starting program at 0x80001000

Let's walk through these lines:

boot -elf 192.168.5.220:supervoc.boot

This will begin booting the supervoc.boot kernel that is located at 192.168.5.220 (our xinu server and, no, name resolution does not work).

Loader:elf Filesys:tftp Dev:eth0 File:192.168.5.220:supervoc.boot Options:(null)

A fairly explainitory line stating the file type it is loading (elf), the file system to be used (tftp), the device which it is using to transfer the image (eth0), and where that image is from (192.168.5.220:supervoc.boot).

Loading: 0x80001000/3145 0x80001c49/23 Entry at 0x80001000

This is also a line of explanation, the first portion (0x80001000/3145) tells us the 'physical' address of where we begin loading our image and the size of the image (in bytes). Next is the address of the end of the image (0x80001c49/23) and (I believe) the amount of padding to make the image size base 16. The last part is the address which CFE will branch to upon completion of upload, this is the start of your kernel.

Closing network.
Starting program at 0x80001000

The closes the network and begins execution the code at address 0x8000100. Any lines of text outputted after this are from your boot image (unless CFE throws an exception and shows a memory dump).

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