Lab Setup¶

You’ll use two sets of tools in this class: an x86 emulator, QEMU, for running your kernel; and a compiler toolchain, including assembler, linker, C compiler, and debugger, for compiling and testing your kernel. This page has the information you’ll need to download and install your own copies. This class assumes familiarity with Unix commands throughout.

For your convenience, the CS 519 staff has prepared a server environment that pre-installed all required toolchain. You can get access to the server by following the steps listed below.

(In case if you would like to setup your own virtual machine to run the lab, you may follow the directions in UW CSE451.

Connect to OSU VPN¶

By following the FERPA law, all the servers are residing at Oregon State University, and following the network administration rules at OSU, you can access to the servers only if you are in the OSU network.

If you are on campus network (via LAN, Wi-Fi, etc.), you do not have to connect to VPN, however, if you are at the out-of-campus, then you must connect to OSU VPN before connecting to our lab server.

Please follow the guideline provided by EECS IT to get access to OSU VPN.

Creating an SSH key¶

If you already have your SSH public/private key pair, then you do not need to follow this step. What you need is just preparing id_rsa.pub or id_ecdsa.pub, etc., to register your public key to the server.

If you do not have one:

In linux:

[host] $ ssh-keygen -t ecdsa

After running this, you may find id_ecdsa.pub from ~/.ssh or your specified directory.

In windows, use puttygen: https://devops.profitbricks.com/tutorials/use-ssh-keys-with-putty-on-windows/

After generating the key, do not forget to copy the text shown on the screen:

Copy the text starts with ssh-rsa or ssh-ecdsa (the entire text)

Server Registration¶

The next step is to create an account on the vm-jos.unexploitable.systems server.

Please copy the content of SSH public key (a string starts with ssh-rsa, ssh-ecdsa, or ssh-ed25519, etc.) before connecting to the registration website <https://register.unexploitable.systems/>.

In the page, you can choose your preferred username (only use lowercase character and numbers), then paste your public key, and then click ‘Submit’ to create an account.

For example,

You can register your username and ssh-public key

You can follow the figure above. If you see messages something like ‘Adding user ????’, then you are registered to the server.

Please make sure that you must be on the OSU network (or VPN) to register yourself to the server.

Server Connection¶

You can connect to the server via following ways:

In Linux:

[host] $ ssh your-username@vm-jos.unexploitable.systems

if your ssh private key is not at ~/.ssh, then,

[host] $ ssh your-username@vm-jos.unexploitable.systems -i location-of-private-key

For example, if your username is yellow9057, and your key is at ~/.ssh/id_rsa,

[host] $ ssh yellow9057@vm-jos.unexploitable.systems -i ~/.ssh/id_rsa

will let you connect to the server.

In Windows:

Remember where you have stored your private key file (.ppk). After that, set the key at the ‘Connection - SSH - Auth’ menu of PuTTY (see this).

Accessing GitLab Repository¶

We will use CS 519 GitLab to track and submit your lab assignment. Please visit the website and register your account.

On the registration, you must use your @oregonstate.edu e-mail address.

After the registration, go to your profile page to register your SSH public key for accessing repositories.

Put your ssh public key

Forking the Lab¶

After registering yourself to the GitLab, you can ‘fork’ the lab as your repository. To do this, please visit the lab repository and click ‘fork’.

Cloning the lab¶

To clone the lab, you must put your private key to the server’s ~/.ssh or use ssh-agent to forward the authentication.

To put your private key, you may use sftp client such as Filezilla

If you wish to use a separate private key for the git repository, then you can create a new key on the server (via ssh-keygen) and register that public key to the GitLab.

# in server, clone the repository
[Server] $ git clone git@gitlab.unexploitable.systems:your-id/jos.git
Cloning into jos...
$ cd jos

To commit and push to the repository, you may want to setup your git information by running the following commands:

[Server] git config --global user.email "your@email.address.com"
[Server] git config --global user.name your-name

Tool Guide¶

Familiarity with your environment is crucial for productive development and debugging. This page gives a brief overview of the JOS environment and useful GDB and QEMU commands. Don’t take our word for it, though. Read the GDB and QEMU manuals. These are powerful tools that are worth knowing how to use.

Note

Debugging tips:

Kernel
User environments

Reference:

JOS makefile
JOS obj/
GDB
QEMU

Debugging tips¶

Kernel¶

GDB is your friend. Use the qemu-gdb target (or its qemu-gdb-nox variant) to make QEMU wait for GDB to attach. See the GDB reference below for some commands that are useful when debugging kernels.

If you’re getting unexpected interrupts, exceptions, or triple faults, you can ask QEMU to generate a detailed log of interrupts using the -d argument.

To debug virtual memory issues, try the QEMU monitor commands info mem (for a high-level overview) or info pg (for lots of detail). Note that these commands only display the current page table.

(Lab 4+) To debug multiple CPUs, use GDB’s thread-related commands like thread and info threads.

User environments (lab 3+)¶

GDB also lets you debug user environments, but there are a few things you need to watch out for, since GDB doesn’t know that there’s a distinction between multiple user environments, or between user and kernel.

You can start JOS with a specific user environment using make run-*name* (or you can edit kern/init.c directly). To make QEMU wait for GDB to attach, use the run-*name*-gdb variant.

You can symbolically debug user code, just like you can kernel code, but you have to tell GDB which symbol table to use with the symbol-file command, since it can only use one symbol table at a time. The provided .gdbinit loads the kernel symbol table, obj/kern/kernel. The symbol table for a user environment is in its ELF binary, so you can load it using symbol-file obj/user/name. Don’t load symbols from any .o files, as those haven’t been relocated by the linker (libraries are statically linked into JOS user binaries, so those symbols are already included in each user binary). Make sure you get the right user binary; library functions will be linked at different EIPs in different binaries and GDB won’t know any better!

(Lab 4+) Since GDB is attached to the virtual machine as a whole, it sees clock interrupts as just another control transfer. This makes it basically impossible to step through user code because a clock interrupt is virtually guaranteed the moment you let the VM run again. The stepi command works because it suppresses interrupts, but it only steps one assembly instruction. Breakpoints generally work, but watch out because you can hit the same EIP in a different environment (indeed, a different binary altogether!).

Reference¶

JOS makefile¶

The JOS Makefile includes a number of phony targets for running JOS in various ways. All of these targets configure QEMU to listen for GDB connections (the *-gdb targets also wait for this connection). To start once QEMU is running, simply run gdb from your lab directory. We provide a .gdbinit file that automatically points GDB at QEMU, loads the kernel symbol file, and switches between 16-bit and 32-bit mode. Exiting GDB will shut down QEMU.

make qemu: Build everything and start QEMU with the VGA console in a new window and the serial console in your terminal. To exit, either close the VGA window or press Ctrl-c or Ctrl-a x in your terminal.
make qemu-nox: Like make qemu, but run with only the serial console. To exit, press Ctrl-a x. This is particularly useful over SSH connections to Athena dialups because the VGA window consumes a lot of bandwidth.
make qemu-gdb: Like make qemu, but rather than passively accepting GDB connections at any time, this pauses at the first machine instruction and waits for a GDB connection.
make qemu-nox-gdb: A combination of the qemu-nox and qemu-gdb targets.
make run-name: (Lab 3+) Run user program name. For example, make run-hello runs user/hello.c.
make run-name-nox, run-name-gdb, run-name-gdb-nox,: (Lab 3+) Variants of run-name that correspond to the variants of the qemu target.

The makefile also accepts a few useful variables:

make V=1 …: Verbose mode. Print out every command being executed, including arguments.
make V=1 grade: Stop after any failed grade test and leave the QEMU output in jos.out for inspection.
make QEMUEXTRA=’args’ …: Specify additional arguments to pass to QEMU.

JOS obj/¶

When building JOS, the makefile also produces some additional output files that may prove useful while debugging:

obj/boot/boot.asm, obj/kern/kernel.asm, obj/user/hello.asm, etc.: Assembly code listings for the bootloader, kernel, and user programs.
obj/kern/kernel.sym, obj/user/hello.sym, etc.: Symbol tables for the kernel and user programs.
obj/boot/boot.out, obj/kern/kernel, obj/user/hello, etc: Linked ELF images of the kernel and user programs. These contain symbol information that can be used by GDB.

GDB¶

See the GDB manual for a full guide to GDB commands. Here are some particularly useful commands for CS 444/544, some of which don’t typically come up outside of OS development.

Ctrl-c: Halt the machine and break in to GDB at the current instruction. If QEMU has multiple virtual CPUs, this halts all of them.
c (or continue): Continue execution until the next breakpoint or Ctrl-c.
si (or stepi): Execute one machine instruction.
b function or b file:line (or breakpoint): Set a breakpoint at the given function or line.
b *addr (or breakpoint): Set a breakpoint at the EIP addr.
set print pretty: Enable pretty-printing of arrays and structs.
info registers: Print the general purpose registers, eip, eflags, and the segment selectors. For a much more thorough dump of the machine register state, see QEMU’s own info registers command.
x/Nx addr: Display a hex dump of N words starting at virtual address addr. If N is omitted, it defaults to 1. addr can be any expression.
x/Ni addr: Display the N assembly instructions starting at addr. Using $eip as addr will display the instructions at the current instruction pointer.
symbol-file file: (Lab 3+) Switch to symbol file file. When GDB attaches to QEMU, it has no notion of the process boundaries within the virtual machine, so we have to tell it which symbols to use. By default, we configure GDB to use the kernel symbol file, obj/kern/kernel. If the machine is running user code, say hello.c, you can switch to the hello symbol file using symbol-file obj/user/hello.

Note

QEMU represents each virtual CPU as a thread in GDB, so you can use all of GDB’s thread-related commands to view or manipulate QEMU’s virtual CPUs.

thread n: GDB focuses on one thread (i.e., CPU) at a time. This command switches that focus to thread n, numbered from zero.
info threads: List all threads (i.e., CPUs), including their state (active or halted) and what function they’re in.

QEMU¶

QEMU includes a built-in monitor that can inspect and modify the machine state in useful ways. To enter the monitor, press Ctrl-a c in the terminal running QEMU. Press Ctrl-a c again to switch back to the serial console.

For a complete reference to the monitor commands, see the QEMU manual. Here are some particularly useful commands:

xp/Nx paddr

Display a hex dump of N words starting at physical address paddr. If N is omitted, it defaults to 1. This is the physical memory analogue of GDB’s x command.

info registers

Display a full dump of the machine’s internal register state. In particular, this includes the machine’s hidden segment state for the segment selectors and the local, global, and interrupt descriptor tables, plus the task register. This hidden state is the information the virtual CPU read from the GDT/LDT when the segment selector was loaded. Here’s the CS when running in the JOS kernel in lab 1 and the meaning of each field:

CS =0008 10000000 ffffffff 10cf9a00 DPL=0 CS32 [-R-]

CS =0008: The visible part of the code selector. We’re using segment 0x8. This also tells us we’re referring to the global descriptor table (0x8&4=0), and our CPL (current privilege level) is 0x8&3=0.
10000000: The base of this segment. Linear address = logical address + 0x10000000.
ffffffff: The limit of this segment. Linear addresses above 0xffffffff will result in segment violation exceptions.
10cf9a00: The raw flags of this segment, which QEMU helpfully decodes for us in the next few fields.
DPL=0: The privilege level of this segment. Only code running with privilege level 0 can load this segment.
CS32: This is a 32-bit code segment. Other values include DS for data segments (not to be confused with the DS register), and LDT for local descriptor tables.
[-R-]: This segment is read-only.

info mem

(Lab 2+) Display mapped virtual memory and permissions. For example,

ef7c0000-ef800000 00040000 urw
efbf8000-efc00000 00008000 -rw

tells us that the 0x00040000 bytes of memory from 0xef7c0000 to 0xef800000 are mapped read/write and user-accessible, while the memory from 0xefbf8000 to 0xefc00000 is mapped read/write, but only kernel-accessible.

QEMU also takes some useful command line arguments, which can be passed into the JOS makefile using the QEMUEXTRA variable.

make QEMUEXTRA=’-d int’ …

Log all interrupts, along with a full register dump, to qemu.log. You can ignore the first two log entries, “SMM: enter” and “SMM: after RMS”, as these are generated before entering the boot loader. After this, log entries look like

4: v=30 e=0000 i=1 cpl=3 IP=001b:00800e2e pc=00800e2e SP=0023:eebfdf28 EAX=00000005
EAX=00000005 EBX=00001002 ECX=00200000 EDX=00000000
ESI=00000805 EDI=00200000 EBP=eebfdf60 ESP=eebfdf28
...

The first line describes the interrupt. The 4: is just a log record counter. v gives the vector number in hex. e gives the error code. i=1 indicates that this was produced by an int instruction (versus a hardware interrupt). The rest of the line should be self-explanatory. See info registers for a description of the register dump that follows.

Note: If you’re running a pre-0.15 version of QEMU, the log will be written to /tmp instead of the current directory.

Questions or comments regarding CS 444/544? Send e-mail to the TAs at *cs444-staff@gmail.com*.