Kernel Build Bug: X86-64-v2 Kernel, X86-64-v3 Host Tools

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Kernel Build Bug: x86-64-v2 Kernel, x86-64-v3 Host Tools

Hey guys, I ran into a bit of a head-scratcher while building a kernel, and I figured I'd share it with you all in case you've bumped into it too. The core issue revolves around a mismatch between the kernel's instruction set architecture (ISA) and the ISA required by the host tools used during the build process. Specifically, when building a kernel targeting x86-64-v2, the kernel itself runs fine, but certain host tools, like objtool, fixdep, and modpost, are built with a requirement for x86-64-v3. This creates a problem, especially when using Dynamic Kernel Module Support (DKMS) on CPUs that only support x86-64-v2. Let's dive into the details, the impact, and the potential workarounds.

The Core Problem: ISA Mismatch

So, the main issue arises from a discrepancy between what the kernel is built for and what the supporting tools require. When you configure your kernel build with a specific architecture, like x86-64-v2 (using something like _config=config_x86-64-v2 or the equivalent -march=x86-64-v2 compiler flag), you're essentially telling the compiler to optimize the code for CPUs that support that instruction set. This is super important for performance because it allows the code to take advantage of specific CPU features. The kernel itself, in this scenario, functions as expected. It boots, it runs, everything's peachy. However, the host tools, which are programs used during the kernel build process, are sometimes compiled with a higher ISA requirement. In this instance, even though the kernel is v2, tools like objtool are built demanding x86-64-v3. You can verify this using the readelf -n command on these tools; it will show an "ISA needed: x86-64-v3" message in the output. This is where things get tricky, especially if you're on older hardware.

This mismatch is a real pain in the neck because it directly impacts the ability to build external kernel modules using DKMS. DKMS allows you to install kernel modules without needing to rebuild the entire kernel. This is super useful for things like custom drivers or other add-ons. But, if the host tools required to build those modules can't run on your hardware (because of the ISA mismatch), the DKMS build will fail. This basically locks you out of customizing your kernel on systems that only support the lower ISA. For those of you who are unaware, the .note.gnu.property section is where the tools declare their ISA requirements. When this section specifies a higher ISA than the hardware supports, it's game over. The tools will simply refuse to run, causing the DKMS build process to halt.

So, what causes this? Well, it likely stems from how the build system and toolchain handle the ISA settings for the host tools versus the kernel itself. It seems like there might be a disconnect, where the host tools aren't consistently inheriting or respecting the ISA level specified for the kernel. This is particularly problematic with modern compilers and optimization techniques, such as Link Time Optimization (LTO), which can influence how the tools are built and what features they end up requiring. The bottom line is that a build system bug results in the tools, the kernel, and the underlying hardware to be out of sync. This leaves you, as the user, with a non-functional DKMS, as the host tools will not work.

Impact: DKMS Failure and Limited Customization

The most significant impact of this bug is the inability to build DKMS modules on x86-64-v2 only CPUs. This means you're stuck with the kernel as it is, without the ability to add custom drivers, modules, or make other kernel-level customizations. For anyone who relies on DKMS for their workflow, this is a major roadblock. Consider someone who needs a specific driver for a piece of hardware or wants to experiment with different kernel configurations. They're out of luck. They are completely unable to load these items because the system will not allow the host tools to execute. It's like having a car with a fancy engine but not being able to use it because the computer that controls the engine doesn't work with your car's features.

This limitation has wider implications. It restricts the flexibility and customization options available to users, especially those with older hardware. It prevents the addition of new features and could make it harder to troubleshoot hardware-specific problems. Essentially, the ISA mismatch undermines the modularity of the kernel, a core feature that makes Linux so adaptable. Furthermore, this limits the user's ability to maintain their system. Security patches and fixes often come in the form of kernel modules. This mismatch would prevent a user from applying these critical security updates, which would lead to an unsafe system. This is an enormous problem and affects many users. To give you some perspective, many server platforms and embedded systems may run older processors. These platforms, which might only support x86-64-v2, are suddenly unable to be customized. The security implications and system flexibility consequences are immense. This reduces the usefulness of CachyOS or other distributions that allow for such fine-grained control over their system. This bug essentially renders the build system non-functional for many use cases.

Reproducing the Bug: Steps and Environment

Reproducing this bug is fairly straightforward, making it easier to verify and troubleshoot. Here's a breakdown of the steps and the environment in which the issue was observed.

Steps to Reproduce:

  1. Kernel Configuration: The first step is to configure your kernel build for x86-64-v2. You can do this by using a specific configuration option, such as _config=config_x86-64-v2 during the build process. Alternatively, you can achieve the same result by setting the appropriate -march flag (e.g., -march=x86-64-v2) in your build commands. This tells the compiler to optimize the code for the v2 instruction set.
  2. Inspect Host Tools: After the kernel build completes, you need to examine the host tools that are generated. Focus on tools like objtool, fixdep, modpost, and resolve_btfids. Use the readelf -n command on these tools. This command will display the notes section of the ELF (Executable and Linkable Format) file. You're looking for the .note.gnu.property section within the output. The presence of this section and the declaration of "ISA needed: x86-64-v3" confirms the bug.

Build Environment:

  • CPU: The issue has been observed on an AMD Zen3 CPU. While the bug may be specific to certain architectures or compiler versions, it is likely to affect other x86-64 processors. Therefore, keep this in mind.
  • Compiler: The user reported the issue with Clang. Clang is a popular compiler that many distributions use, so this makes it even more relevant.
  • LTO Mode: Full LTO (Link Time Optimization) was enabled. LTO is an optimization technique that can potentially exacerbate the problem by influencing how the host tools are built. Be sure to check your LTO settings.

By following these steps, you can easily replicate the bug and confirm its presence in your build environment. This process also allows you to narrow down the potential causes and test potential fixes.

Expected vs. Actual Behavior: A Clear Disconnect

The difference between the expected and actual behavior highlights the core of the problem.

Expected Behavior:

  • When building a kernel for x86-64-v2, the host tools should also be compiled to match that ISA level (v2). The host tools need to be compatible with the ISA level specified for the kernel. This ensures that the tools can run correctly on v2-only hardware, and DKMS builds can proceed without issues. This is because the host tools directly support the architecture the kernel has been built for.

Actual Behavior:

  • The host tools incorrectly require x86-64-v3, even when the kernel is built for x86-64-v2. This means that the tools include instructions that are not supported by the v2 instruction set, making them unusable on hardware that only supports v2. This is what leads to the DKMS failure. The host tools are trying to use features that are not available to the architecture the kernel supports, so the DKMS build will fail.

This mismatch creates a fundamental incompatibility that prevents external modules from being built successfully on v2 hardware. The build environment's behavior does not align with the kernel's ISA, causing severe issues for those building external kernel modules.

Workaround: A Temporary Fix

There is a workaround that allows you to temporarily overcome this issue, but it's not a sustainable or ideal solution. This workaround involves manually removing the problematic .note.gnu.property section from the host tools after they've been built. This removes the declaration of the ISA requirement, allowing the tools to run on the v2 hardware.

Manual Stripping:

  1. Locate the Tools: After the kernel build completes, you need to locate the host tools, such as objtool. These tools are typically found in the tools/ directory of your kernel source tree.
  2. Use strip: Use the strip utility to remove the .note.gnu.property section from each tool. For example: strip --remove-section=.note.gnu.property ./tools/objtool/objtool. This command modifies the ELF file to remove the section that contains the ISA requirement.

While this workaround does enable DKMS to function, it is important to remember that it is a temporary solution. It's not a proper fix for the underlying build system bug. Therefore, you must repeat the stripping process every time you rebuild the kernel, which is a big hassle. It can also lead to issues if the build process changes or if other tools start relying on the information in the .note.gnu.property section. This is not an ideal solution, but it can allow users to temporarily restore DKMS functionality on v2 hardware. But, if a user forgets, they are again dead in the water.

Conclusion: A Call for a Fix

In conclusion, the x86-64-v2 to x86-64-v3 ISA mismatch in the kernel build process creates a significant problem. It effectively prevents users from building DKMS modules and customizing their kernels on older hardware. While the workaround of manually stripping the .note.gnu.property section provides a temporary fix, it is not a sustainable solution. The build system or the toolchain needs to be reviewed and adjusted to ensure that the host tools respect the ISA level selected for the kernel. This ensures compatibility and allows for seamless builds on all supported hardware. I hope the kernel developers will see this and implement a fix!