Feature request: tunable for extern function definition ELF relocation type

[jbaublitz]

I have a prototype of a toy Linux kernel module written in Rust. I previously had it working until recently when I rebuilt it with a newer version of Rust and LLVM and insmod module.ko started throwing an error in dmesg logs about unknown rela relocation 9. I did a readelf and realized that all of the extern "C" functions I'd declared were generating ELF entries with the R_X86_64_GOTPCREL relocation entry type which is defined as having a value of 9. This is apparently illegal for kernel modules after looking at the allowable ELF relocation types in the kernel module infrastructure.

I have a few questions:

1. Is this change recent?
2. If so, was this a change on the Rust side or LLVM side?
3. Would you be willing to work with me on either finding a linking-based solution (if possible) or a PR for tuning the resulting relocation type of extern-declared functions?

I am not sure if there's a simple linking flag I'm missing but the ones I've tested, as well as using any of the relocation strategies available through codegen options, yield the same results.

[jason-ni]

Another rust linux kernel module project is also blocking on this issue. fishinabarrel/linux-kernel-module-rust#67 .

We can use "needs-plt": true to avoid R_X86_64_GOTPCREL.

[jbaublitz]

@jason-ni Let me try that and see if this resolves it. Thanks for pointing me in that direction.

[jbaublitz]

Hey @jason-ni, thank you so much for your help! I was able to resolve this with the following options on newer Rust compilers: --crate-type=staticlib -C relocation-model=static -C code-model=kernel -Z plt=y. I'm gonna close this out.