objtool warnings in KernelCI report

[nathanchance]

Apologies if this has been reported already! KernelCI recently started testing with Rust and it seems that there are quite a few objtool warnings for Rust code: https://lore.kernel.org/llvm/639aba22.170a0220.d9ab4.4ff4@mx.google.com/

[ojeda]

Thanks for the report as usual! We briefly discussed it on a meeting on Wednesday.

IBT/Retpoline/Rethunk require a few things to be in place:

• Passing -Zcf-protection=branch together with a new flag -Zno-jump-tables to mimic GCC/Clang's which rustc currently does not have. I have submitted a PR: Add -Zno-jump-tables rust-lang/rust#105812.

• New flags -Zfunction-return=thunk-extern to be implemented and so on.

In a patched rustc 1.66.0 with -Zno-jump-tables and a hacky -Zfunction-return=thunk-extern + an upgraded-to-Rust-1.66.0 kernel, the only warning remaining in defconfig I have is:

vmlinux.o: warning: objtool: rust_begin_unwind+0x70: unreachable instruction

The resulting kernel boots in QEMU, but will need to be tested properly in real hardware.

We will have to wait at least a couple months to get all the pieces in place, so I have submitted a PR to our KernelCI config to disable these: kernelci/kernelci-core#1580

[ojeda]

For reference, a couple updates:

• https://lore.kernel.org/rust-for-linux/20231009224347.2076221-1-mmaurer@google.com/
• https://lore.kernel.org/rust-for-linux/20231023174449.251550-1-ojeda@kernel.org/

[ojeda]

The series https://lore.kernel.org/rust-for-linux/20240725183325.122827-1-ojeda@kernel.org/ should fix all of those warnings.

[ojeda]

Applied to rust-next -- if anyone notices new warnings, please let us know. Thanks!