With the release of Linux 6.1, minimal Rust
support landed and it should be possible to easily build a hello
world kernel module in Rust.
Installing Linux 6.1
At the time of writing Linux 6.1 was still in testing on ArchLinux, so I enabled the testing repositories:
--- pacman.conf 2022-12-18 15:25:10.742819293 +0100
+++ /etc/pacman.conf 2022-12-18 00:32:56.404801925 +0100
@@ -69,8 +69,8 @@
# repo name header and Include lines. You can add preferred servers immediately
# after the header, and they will be used before the default mirrors.
-#[testing]
-#Include = /etc/pacman.d/mirrorlist
+[testing]
+Include = /etc/pacman.d/mirrorlist
[core]
Include = /etc/pacman.d/mirrorlist
@@ -78,8 +78,8 @@
[extra]
Include = /etc/pacman.d/mirrorlist
-#[community-testing]
-#Include = /etc/pacman.d/mirrorlist
+[community-testing]
+Include = /etc/pacman.d/mirrorlist
[community]
Include = /etc/pacman.d/mirrorlist
@@ -87,8 +87,8 @@
# If you want to run 32 bit applications on your x86_64 system,
# enable the multilib repositories as required here.
-#[multilib-testing]
-#Include = /etc/pacman.d/mirrorlist
+[multilib-testing]
+Include = /etc/pacman.d/mirrorlist
[multilib]
Include = /etc/pacman.d/mirrorlist
A first start
Since I just want to play around, I decided to build an out-of-tree Rust module. Luckily, there is an example module available for that already: https://github.com/Rust-for-Linux/rust-out-of-tree-module.
So I went ahead, cloned the repo and tried to just execute make:
rust-out-of-tree-module (git)-[main] % make
make -C /lib/modules/`uname -r`/build M=$PWD
make[1]: Entering directory '/usr/lib/modules/6.1.0-arch1-1/build'
RUSTC [M] /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.o
error: target file "./rust/target.json" does not exist
make[2]: *** [scripts/Makefile.build:307: /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.o] Error 1
make[1]: *** [Makefile:1992: /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module] Error 2
make[1]: Leaving directory '/usr/lib/modules/6.1.0-arch1-1/build'
make: *** [Makefile:6: default] Error 2
Well...
Reading through https://www.kernel.org/doc/html/latest/rust/quick-start.html, it seemed that I have everything I need to get started.
The README mentions the following though:
The kernel tree (KDIR) requires the Rust metadata to be available. These are generated during the kernel build, but may not be available for installed/distributed kernels (the scripts that install/distribute kernel headers etc. for the different package systems and Linux distributions are not updated to take into account Rust support yet).
Trying an in-kernel build
To check if it is just a tooling issue, I cloned the https://github.com/Rust-for-Linux/linux repo and executed the check according to the documentation:
Rust-for-Linux/linux (git)-[rust] % make LLVM=1 rustavailable
***
*** Rust compiler 'rustc' is too new. This may or may not work.
*** Your version: 1.66.0
*** Expected version: 1.62.0
***
***
*** Rust bindings generator 'bindgen' is too new. This may or may not work.
*** Your version: 0.63.0
*** Expected version: 0.56.0
***
Rust is available!
Since my toolchain seems to be too new, I installed the older versions with
rustup override set $(scripts/min-tool-version.sh rustc)
rustup component add rust-src
cargo install --locked --version $(scripts/min-tool-version.sh bindgen) bindgen
Then I enabled Rust support with make menuconfig in the General setup menu.
The option is only shown if the toolchain is installed, and in my case I also
had to disable the GCC_PLUGINS option.
I then just compiled the whole kernel to test it:
make LLVM=1
This went smoothly, so I continued to build the actual ArchLinux kernel which matches my running kernel:
git remote add archlinux git@github.com:archlinux/linux.git
git fetch archlinux v6.1-arch1
git checkout v6.1-arch1
To be sure, I ran make menuconfig again and applied the same changes again.
A few minutes later, I had my kernel ready:
Rust-for-Linux/linux (git)-[rust] % make -j8 LLVM=1
...
Kernel: arch/x86/boot/bzImage is ready (#2)
make -j8 LLVM=1 1557.69s user 133.21s system 745% cpu 3:46.77 total
Back to the out-of-tree module
Having successfully built the whole kernel with Rust support, I went back to the
rust-out-of-tree-module repository and tried to build it:
Rust-for-Linux/rust-out-of-tree-module (git)-[main] % make KDIR=../linux LLVM=1
make -C ../linux M=$PWD
make[1]: Entering directory '/home/roughl/projects/github/Rust-for-Linux/linux'
RUSTC [M] /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.o
error: proc macro panicked
--> /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.rs:7:1
|
7 | / module! {
8 | | type: RustOutOfTree,
9 | | name: "rust_out_of_tree",
10 | | author: "Rust for Linux Contributors",
11 | | description: "Rust out-of-tree sample",
12 | | license: "GPL",
13 | | }
| |_^
|
= help: message: Expected byte string
error[E0412]: cannot find type `CStr` in this scope
--> /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.rs:20:29
|
20 | fn init(_name: &'static CStr, _module: &'static ThisModule) -> Result<Self> {
| ^^^^ not found in this scope
|
help: consider importing this struct
|
5 | use core::ffi::CStr;
|
error[E0425]: cannot find value `__LOG_PREFIX` in the crate root
--> /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.rs:21:9
|
21 | pr_info!("Rust out-of-tree sample (init)\n");
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in the crate root
|
= note: this error originates in the macro `$crate::print_macro` (in Nightly builds, run with -Z macro-backtrace for more info)
error[E0425]: cannot find value `__LOG_PREFIX` in the crate root
--> /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.rs:34:9
|
34 | pr_info!("My numbers are {:?}\n", self.numbers);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in the crate root
|
= note: this error originates in the macro `$crate::print_macro` (in Nightly builds, run with -Z macro-backtrace for more info)
error[E0425]: cannot find value `__LOG_PREFIX` in the crate root
--> /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.rs:35:9
|
35 | pr_info!("Rust out-of-tree sample (exit)\n");
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in the crate root
|
= note: this error originates in the macro `$crate::print_macro` (in Nightly builds, run with -Z macro-backtrace for more info)
error[E0050]: method `init` has 2 parameters but the declaration in trait `init` has 1
--> /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.rs:20:20
|
20 | fn init(_name: &'static CStr, _module: &'static ThisModule) -> Result<Self> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected 1 parameter, found 2
|
= note: `init` from trait: `fn(&'static kernel::ThisModule) -> core::result::Result<Self, kernel::error::Error>`
error: aborting due to 6 previous errors
Some errors have detailed explanations: E0050, E0412, E0425.
For more information about an error, try `rustc --explain E0050`.
make[2]: *** [scripts/Makefile.build:307: /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.o] Error 1
make[1]: *** [Makefile:1992: /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module] Error 2
make[1]: Leaving directory '/home/roughl/projects/github/Rust-for-Linux/linux'
make: *** [Makefile:6: default] Error 2
Looking into the compile errors, it seemed that the fn init signature changed
and that the module! marco expects byte strings.
Changing that:
diff --git a/rust_out_of_tree.rs b/rust_out_of_tree.rs
index e280409..58cc9ba 100644
--- a/rust_out_of_tree.rs
+++ b/rust_out_of_tree.rs
@@ -6,10 +6,10 @@ use kernel::prelude::*;
module! {
type: RustOutOfTree,
- name: "rust_out_of_tree",
- author: "Rust for Linux Contributors",
- description: "Rust out-of-tree sample",
- license: "GPL",
+ name: b"rust_out_of_tree",
+ author: b"Rust for Linux Contributors",
+ description: b"Rust out-of-tree sample",
+ license: b"GPL",
}
struct RustOutOfTree {
@@ -17,7 +17,7 @@ struct RustOutOfTree {
}
impl kernel::Module for RustOutOfTree {
- fn init(_name: &'static CStr, _module: &'static ThisModule) -> Result<Self> {
+ fn init(_module: &'static ThisModule) -> Result<Self> {
pr_info!("Rust out-of-tree sample (init)\n");
let mut numbers = Vec::new();
We get a successful build!
Rust-for-Linux/rust-out-of-tree-module (git)-[main] % make KDIR=../linux LLVM=1
make -C ../linux M=$PWD
make[1]: Entering directory '/home/roughl/projects/github/Rust-for-Linux/linux'
RUSTC [M] /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.o
MODPOST /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/Module.symvers
LD [M] /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.ko
make[1]: Leaving directory '/home/roughl/projects/github/Rust-for-Linux/linux'
But trying to load the resulting module fails:
sudo insmod rust_out_of_tree.ko
insmod: ERROR: could not insert module rust_out_of_tree.ko: Invalid module format
We see this in the dmesg output as well:
[24593.691184] rust_out_of_tree: version magic '6.1.0-arch1 SMP preempt mod_unload ' should be '6.1.0-arch1-1 SMP preempt mod_unload '
Of course we aren't setting the version the same as the ArchLinux kernel. I copied a few steps from https://github.com/archlinux/svntogit-packages/blob/packages/linux/trunk/PKGBUILD to set the same version (we could also just copy the config from there, but lets see what happens first)
Rust-for-Linux/linux (git)-[tags/v6.1-arch1] % echo "-1" > localversion.10-pkgrel
Rust-for-Linux/linux (git)-[tags/v6.1-arch1] % make -s kernelrelease > version
Rust-for-Linux/linux (git)-[tags/v6.1-arch1] % cat version
6.1.0-arch1-1
It didn't work again:
Rust-for-Linux/rust-out-of-tree-module (git)-[fix-build-for-linux-6.1] % sudo insmod rust_out_of_tree.ko
insmod: ERROR: could not insert module rust_out_of_tree.ko: Unknown symbol in module
The dmesg output shows:
[25300.463334] rust_out_of_tree: Unknown symbol _RNvNtNtCsfATHBUcknU9_6kernel5print14format_strings4INFO (err -2)
[25300.463353] rust_out_of_tree: Unknown symbol _RNvNtCsfATHBUcknU9_6kernel5print11call_printk (err -2)
[25300.463371] rust_out_of_tree: Unknown symbol __rust_dealloc (err -2)
[25300.463386] rust_out_of_tree: Unknown symbol __rust_realloc (err -2)
[25300.463400] rust_out_of_tree: Unknown symbol __rust_alloc (err -2)
[25300.463414] rust_out_of_tree: Unknown symbol _RNvNtCs3yuwAp0waWO_4core9panicking5panic (err -2)
[25300.463428] rust_out_of_tree: Unknown symbol _RNvMs7_NtCs3yuwAp0waWO_4core3fmtNtB5_9Formatter15debug_lower_hex (err -2)
[25300.463442] rust_out_of_tree: Unknown symbol _RNvXsS_NtNtCs3yuwAp0waWO_4core3fmt3numlNtB7_8LowerHex3fmt (err -2)
[25300.463456] rust_out_of_tree: Unknown symbol _RNvMs7_NtCs3yuwAp0waWO_4core3fmtNtB5_9Formatter15debug_upper_hex (err -2)
[25300.463469] rust_out_of_tree: Unknown symbol _RNvXsT_NtNtCs3yuwAp0waWO_4core3fmt3numlNtB7_8UpperHex3fmt (err -2)
[25300.463483] rust_out_of_tree: Unknown symbol _RNvXs2_NtNtNtCs3yuwAp0waWO_4core3fmt3num3implNtB9_7Display3fmt (err -2)
[25300.463497] rust_out_of_tree: Unknown symbol _RNvMs7_NtCs3yuwAp0waWO_4core3fmtNtB5_9Formatter10debug_list (err -2)
[25300.463510] rust_out_of_tree: Unknown symbol _RNvMs5_NtNtCs3yuwAp0waWO_4core3fmt8buildersNtB5_9DebugList5entry (err -2)
[25300.463524] rust_out_of_tree: Unknown symbol _RNvMs5_NtNtCs3yuwAp0waWO_4core3fmt8buildersNtB5_9DebugList6finish (err -2)
[25300.463538] rust_out_of_tree: Unknown symbol _RNvXs_NtCsfATHBUcknU9_6kernel5errorNtB4_5ErrorINtNtCs3yuwAp0waWO_4core7convert4FromNtNtCsdvv6pRyacSq_5alloc11collections15TryReserveErrorE4from (err -2)
[25300.463552] rust_out_of_tree: Unknown symbol _RNvMNtCsfATHBUcknU9_6kernel5errorNtB2_5Error15to_kernel_errno (err -2)
So lets try with the ArchLinux config:
wget https://raw.githubusercontent.com/archlinux/svntogit-packages/packages/linux/trunk/config
make olddefconfig
diff -u config .config
The configuration difference isn't big
--- config 2022-12-18 20:51:38.269359290 +0100
+++ .config 2022-12-18 20:53:48.896025137 +0100
@@ -11,6 +11,7 @@
CONFIG_LD_IS_BFD=y
CONFIG_LD_VERSION=23900
CONFIG_LLD_VERSION=0
+CONFIG_RUST_IS_AVAILABLE=y
CONFIG_CC_CAN_LINK=y
CONFIG_CC_CAN_LINK_STATIC=y
CONFIG_CC_HAS_ASM_GOTO_OUTPUT=y
A bit strange that CONFIG_RUST_IS_AVAILABLE=y isn't set, but
CONFIG_HAVE_RUST=y is enabled, so all should be good.
Quite some time later, we built the ArchLinux kernel as well:
make -j8 LLVM=1 15202.68s user 1464.06s system 800% cpu 34:42.57 total
But this lead to build failures in the kernel module!
Rust-for-Linux/rust-out-of-tree-module (git)-[fix-build-for-linux-6.1] % make KDIR=../linux LLVM=1
make -C ../linux M=$PWD
make[1]: Entering directory '/home/roughl/projects/github/Rust-for-Linux/linux'
MODPOST /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/Module.symvers
ERROR: modpost: "_RNvNtNtCsfATHBUcknU9_6kernel5print14format_strings4INFO" [/home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.ko] undefined!
ERROR: modpost: "_RNvNtCsfATHBUcknU9_6kernel5print11call_printk" [/home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.ko] undefined!
ERROR: modpost: "__rust_dealloc" [/home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.ko] undefined!
ERROR: modpost: "__rust_realloc" [/home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.ko] undefined!
ERROR: modpost: "__rust_alloc" [/home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.ko] undefined!
ERROR: modpost: "_RNvMs7_NtCs3yuwAp0waWO_4core3fmtNtB5_9Formatter15debug_lower_hex" [/home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.ko] undefined!
ERROR: modpost: "_RNvXsS_NtNtCs3yuwAp0waWO_4core3fmt3numlNtB7_8LowerHex3fmt" [/home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.ko] undefined!
ERROR: modpost: "_RNvMs7_NtCs3yuwAp0waWO_4core3fmtNtB5_9Formatter15debug_upper_hex" [/home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.ko] undefined!
ERROR: modpost: "_RNvXsT_NtNtCs3yuwAp0waWO_4core3fmt3numlNtB7_8UpperHex3fmt" [/home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.ko] undefined!
ERROR: modpost: "_RNvXs2_NtNtNtCs3yuwAp0waWO_4core3fmt3num3implNtB9_7Display3fmt" [/home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/rust_out_of_tree.ko] undefined!
WARNING: modpost: suppressed 5 unresolved symbol warnings because there were too many)
make[2]: *** [scripts/Makefile.modpost:126: /home/roughl/projects/github/Rust-for-Linux/rust-out-of-tree-module/Module.symvers] Error 1
make[1]: *** [Makefile:1944: modpost] Error 2
make[1]: Leaving directory '/home/roughl/projects/github/Rust-for-Linux/linux'
make: *** [Makefile:6: default] Error 2
These are now the compile time equivalent of the runtime errors we got before!
Apparently, the ArchLinux kernel doesn't have Rust support, despite it being
set in the config. Executing make menuconfig reveals, that options which
conflict with CONFIG_HAVE_RUST=y are enabled:
│ Symbol: RUST [=n] │
│ Type : bool │
│ Defined at init/Kconfig:1925 │
│ Prompt: Rust support │
│ Depends on: HAVE_RUST [=y] && RUST_IS_AVAILABLE [=y] && !MODVERSIONS [=n] && !GCC_PLUGINS [=y] && !RANDSTRUCT [=n] && !DEBUG_INFO_BTF [=y] │
Giving up for now
At this point I'd probably need to compile and run a custom kernel which actually has Rust support enabled.
I'll probably try this at a later point, but for now I'll give it a bit more time to settle.
I'm also considering reporting this to the ArchLinux developers, but I'm not sure how much priority Rust kernel modules should have, given the early stage. I'll probably wait until Linux 6.2 is released before investigating further.