Piotr Jaroszyński: Usermode debugging under Linux

Before I even started thinking about a proposal for my project I decided to do some pretty much random unrelated work to get a better feeling of gPXE in general. This resulted in a few trivial patches:

• [build] Add support for local configuration files
• [build] Look for isolinux.bin in more places
• [util] Hide an expected error from the 'which' command

And then I decided to tackle something involving proper coding - adding base64 support to iSCSI. This resulted in not yet merged changes:

• base64 branch

Even after doing some work on gPXE earlier I didn't know where to start the project I wanted to work on during summer. Josh helped me a lot with that.

Firstly I tried an efi build to see how the different ARCHs and PLATFORMs work. Unfortunately that resulted in some compilation as well as linking problems. I have managed to fix them:

• efi64 branch

With that behind me, I added a new linux PLATFORM, which involved the following:

• Remapping stdlib's symbols to avoid collisions with gPXE (see Kernel Interface)
• Adding linker scripts (for i386 and x86_64) just to handle tables (see include/gpxe/tables.h)
• Adding an empty linuxprefix

Having that base I implemented the very first linux subsytem: CONSOLE_LINUX, which got usermode gPXE as far as:

$ bin-x86_64-linux/rtl8139.linux

gPXE initialising devices...
 
Segmentation fault

That work can be seen on linux branch (up to the “[linux] Add console” commit).

After that I decided to add a network driver based on tun/tap and all the subsystems necessary for that:

• TIMER_LINUX
• UACCESS_LINUX
• UMALLOC_LINUX
• NAP_LINUX

It's in an ugly state, but seems to be working:

$ ./bin-x86_64-linux/virtual.linux
gPXE initialising devices...



gPXE 1.0.0+ -- Open Source Boot Firmware -- http://etherboot.org
Features: HTTP DNS TFTP
net0: 52:54:00:12:34:56 on  (open)
  [Link:up, TX:0 TXE:0 RX:0 RXE:0]
DHCP (net0 52:54:00:12:34:56).... ok
net0: 192.168.1.201/255.255.255.0 gw 192.168.1.1
Booting from filename "http://boot.kernel.org/bko/pxelinux.0"
http://boot.kernel.org/bko/pxelinux.0. Not an executable image (0x2e008001)
Could not boot from filename "http://boot.kernel.org/bko/pxelinux.0": Not an executable image (0x2e008001)
No more network devices

The changes can be seen currently on the linux-ugly branch with just one monolithic commit.

During that work usermode gPXE helped me track down and fix two bugs:

• [uri] Fix NULL dereference in parse_uri()
• [uri] Special case NULL in churi()