Rust - Linux kernel module example

This example shows how to build a Rust Linux kernel module, and debug it using gdb.

This should enable relatively rapid iteration when developing (e.g. edit module file, run make debug_kernel, attach gdb with make gdb, repeat)

Prequisites

Build dependencies:

sudo apt-get install make gcc cpio

Fetch Rust-For-Linux:

git submodule update --init

Rust-For-Linux prequisites, see quick-start.rst. In brief (note! this overrides rustup config and installs specific version of bindgen)

cd linux
rustup override set $(scripts/min-tool-version.sh rustc)
rustup component add rust-src
cargo install --locked --version $(scripts/min-tool-version.sh bindgen) bindgen
make LLVM=1 rustavailable
# should print: "Rust is available!"

Building

Build the kernel module. Source located at sample_module path.

This uses kernel-config as a baseline config

make build_kernel

This will take a while

Running without gdb

Do-it-all-command, which

• builds the module (rust_minimal.rs)
• builds init-binary
• creates initramfs
• launches qemu

(see Makefile for details)

Run it:

make run_kernel

This should open up a busybox shell, with the module already loaded.

Running with gdb

1. Init, build module & Start qemu

Do-it-all-command, which

• builds the module (rust_minimal.rs)
• builds init-binary
• creates initramfs
• launches qemu

(see Makefile for details)

Run it:

make debug_kernel

It should print a few lines, and be in idle mode. Leave it running

2. Attach gdb to the qemu instance

Open another terminal window, and attach gdb to the emulator:

make gdb

Now, you should see the qemu-kernel booting up and emulator eventually breaking in the rust module init

Emulator (debug_kernel) should print:

...
[    1.210080] Run /init as init process
init - entered main()
init - loading kernel module (iteration=0)
init - module file fd=3

gdb should print:

Hardware assisted breakpoint 1 at 0xffffffffa0000474: file samples/rust/rust_minimal.rs, line 21.

Breakpoint 1, rust_minimal::{impl#0}::init (_name=..., _module=<optimized out>) at samples/rust/rust_minimal.rs:21
21	        pr_info!("Rust minimal sample (init)\n");
(gdb)

Troubleshooting

Finding module address

Hopefully the module address stays the same (for symbols). If not, this may help (but please make a pull request for documenting a proper way to find the address)

Look for the module kernel message:

[    5.532793] kobject: 'rust_minimal' (ffffffffa0002090): kobject_release, parent ffff888000928250 (delayed 2000)

Here the address is 0xffffffffa0002090.

TODO: why +0x2090 to base address 0xffffffffa0000000?

Update gdb.config add-symbol-file section with the base address.