Out-of-tree module failed to build if the kernel source path is different

[Leo1003]

As most of the Linux distribution install the kernel with their package (.deb, .rpm, ...). I also tried to do the same thing to test Linux kernel with Rust support. However, when I build some out-of-tree Rust kernel module, it failed to compile with the following message:

error[E0461]: couldn't find crate `core` with expected target triple target-13637732820183936185
  |
  = note: the following crate versions were found:
          crate `core`, target triple target-4719486650351723570: /usr/src/linux-headers-5.18.0-rc2-2-rust/rust/libcore.rmeta

error[E0461]: couldn't find crate `compiler_builtins` with expected target triple target-13637732820183936185
  |
  = note: the following crate versions were found:
          crate `compiler_builtins`, target triple target-4719486650351723570: /usr/src/linux-headers-5.18.0-rc2-2-rust/rust/libcompiler_builtins
.rmeta

error[E0461]: couldn't find crate `kernel` with expected target triple target-13637732820183936185
  --> /home/leo/rust-kernel-modules/rust-random/rust_random.rs:11:5
   |
11 | use kernel::{
   |     ^^^^^^
   |
   = note: the following crate versions were found:
           crate `kernel`, target triple target-4719486650351723570: /usr/src/linux-headers-5.18.0-rc2-2-rust/rust/libkernel.rmeta

After digging some codes, I found that rustc record the target triple as the path of target.json file.
https://doc.rust-lang.org/stable/nightly-rustc/rustc_target/spec/enum.TargetTriple.html

And the target triple appears in the error messages can be calculated with the following codes:

use std::collections::hash_map::DefaultHasher;
use std::hash::{Hash, Hasher};
use std::path::PathBuf;

#[allow(unused_variables)]
fn main() {
    let mut hasher = DefaultHasher::new();
    // target-4719486650351723570
    let path = PathBuf::from("/home/leo/linux-rust/src/build/rust/target.json");
    // target-13637732820183936185
    let path = PathBuf::from("/usr/src/linux-headers-5.18.0-rc2-2-rust/rust/target.json");
    path.hash(&mut hasher);
    let hash = hasher.finish();
    println!("target-{}", hash)
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=aa52d204a7d4167cfd84076776ab957a

The above codes is reference from rust-lang/rust/compiler/rustc_target/src/spec/mod.rs:2445

P.s. I think that this is the problem of the compiler, however, this issue must be resolved if we want to support out-of-tree Rust kernel module in the future.

[bjorn3]

Yeah, I think rustc should hash the file contents rather than the file path.

[bjorn3]

rust-lang/rust#98225 has landed. This issue should be fixed in rustc 1.63.0.

[ojeda]

Thanks a lot @bjorn3!

[Leo1003]

Thank you for fixing this issue which troubled me for a long time!

[YakoYakoYokuYoku]

Now with Rust 1.63.0, which btw hasn't required further patching, I could confirm that I'm able to build out of tree modules with the kernel build system using v9 of the patchset.

$ make KDIR=/lib/modules/5.19.1-arch2-1/build LLVM=1
make -C /lib/modules/5.19.1-arch2-1/build M=$PWD
make[1]: Entering directory '/usr/lib/modules/5.19.1-arch2-1/build'
  RUSTC [M] ~/Codes/Rust/Tests/kernelmoduletest/printk/rust_printk.o
  MODPOST ~/Codes/Rust/Tests/kernelmoduletest/printk/Module.symvers
  CC [M]  ~/Codes/Rust/Tests/kernelmoduletest/printk/rust_printk.mod.o
  LD [M]  ~/Codes/Rust/Tests/kernelmoduletest/printk/rust_printk.ko
  BTF [M] ~/Codes/Rust/Tests/kernelmoduletest/printk/rust_printk.ko
die__process_class: tag not supported 0x33 (variant_part)!
die__process_class: tag not supported 0x2f (template_type_parameter)!
make[1]: Leaving directory '/usr/lib/modules/5.19.1-arch2-1/build'

Said module, alongside a C counterpart, was tested in a QEMU VM, both loading and removing work as expected. If Rust 1.63.0 becomes mandatory then this issue can be closed indeed.

[bjorn3]

Thanks for confirming the fix @YakoYakoYokuYoku! @ojeda will probably update to Rust 1.63.0 soon.

[ojeda]

Nowadays we require a way higher version.