When objtool Strikes: Diagnosing NVIDIA Driver Build Failures Against a Custom Kernel

Introduction

In the high-stakes world of provisioning a new machine learning server with 8× NVIDIA RTX PRO 6000 Blackwell GPUs, few things are more frustrating than a kernel module build failure. Message [msg 8453] captures a pivotal moment in the provisioning of kpro6, a Proxmox host destined to run DFlash drafter training workloads. At this point in the conversation, the assistant is deep in the trenches of building NVIDIA's open-source kernel driver (version 595.71.05) against a community-built 6.19 Proxmox VE kernel — a kernel that was compiled with a different GCC version and different toolchain than what the host system provides. The message is a diagnostic pivot point: the assistant has just discovered that objtool, the kernel's object file validation tool, is treating certain warnings as fatal errors, blocking the NVIDIA module build.

The Message

The subject message reads in full:

[assistant] The objtool MITIGATION_RETHUNK warnings are now errors (strict mode on 6.19). This is the known issue from the GitHub thread. Need to disable objtool warnings-as-errors for the nvidia build:

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[bash] ssh -o ConnectTimeout=10 root@10.1.2.6 ' cd /root/NVIDIA-Linux-x86_64-595.71.05-no-compat32/kernel-open

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# Build with KBUILD_EXTRA_WARN=0 to suppress objtool errors # Or set the objtool flag to ignore rethunk warnings IGNORE_CC_MISMATCH=1 make -j32 \ NV_EXCLUDE_KERNEL_MODULES="" \ SYSSRC="/lib/modules/6.19.5-2-pve/build" \ SYSOUT="/lib/modules/6.19.5-2-pve/build" \ KBUILD_MODPOST_WARN=1 \ modules 2>&1 | tail -30 echo "---EXIT: $?---" echo "---MODULES---" find . -name "*.ko" -exec ls -lh {} \; ' 2>&1 nvidia-modeset.o: error: objtool: _ZN11DisplayPort18DownRequestManagerD0Ev+0x1e: call without frame pointer save/setup nvidia-modeset.o: error: objtool: _ZTv0_n24_N11DisplayPort18DownRequestManagerD0Ev+0x26: call without frame pointer save/setup nvidia-modeset.o: error: objtool: _ZN11DisplayPort5Timer15PendingCallbackD0Ev+0x16: call without frame pointer save/setup nvidia-modeset.o: error: objtool: _ZTv0_n24_N11DisplayPort5Timer15PendingCallbackD0Ev+0x21: call without frame pointer save/setup nv...

The message contains both the assistant's reasoning (the italicized thinking block) and the tool call with its output. The assistant hypothesizes that the problem is objtool's MITIGATION_RETHUNK validation running in strict mode on kernel 6.19, and attempts to work around it by passing KBUILD_MODPOST_WARN=1 to the kernel build system. The output, however, reveals that the errors persist — objtool is still flagging calls in nvidia-modeset.o that lack frame pointer save/setup, and these are being treated as fatal errors rather than warnings.

Context and Motivation

To understand why this message was written, we must appreciate the broader context. The assistant is provisioning kpro6, a new Proxmox host with 8× Blackwell RTX PRO 6000 GPUs and a 14TB NVMe drive, intended for DFlash drafter training. Earlier in the session, the assistant had attempted to install a community-built 6.19 kernel from jaminmc's Proxmox repository — a kernel that promised newer features but was compiled with GCC 14 from Debian Trixie, while the host runs Debian Bookworm with GCC 12.2.0. This toolchain mismatch had already spawned an elaborate debugging spiral involving patched kernel headers, rebuilt gendwarfksyms and objtool binaries, hex-edited version strings, and a GLIBC_2.38 shim library that would eventually poison the system's dynamic linker and brick SSH access.

Message [msg 8453] occurs at a specific inflection point. In the immediately preceding message ([msg 8452]), the assistant had successfully extracted the NVIDIA .run installer and attempted to build all kernel modules at once (rather than sequentially, which had caused cross-module symbol issues). The output of that message was truncated, showing only the extraction. Now, in [msg 8453], the assistant sees the build result: it failed. The specific error is objtool complaining about "call without frame pointer save/setup" in the nvidia-modeset code.

The assistant's reasoning in the thinking block reveals two key pieces of knowledge being applied. First, it recognizes that these are MITIGATION_RETHUNK warnings — a specific class of objtool validation related to the kernel's retpoline mitigation for Spectre v2. When CONFIG_MITIGATION_RETHUNK is enabled, objtool validates that every call instruction is preceded by proper frame pointer setup, ensuring that the return stack can be safely unwound. Second, it recognizes that on kernel 6.19, objtool is running in "strict mode" where these warnings are treated as errors, unlike older kernels where they might have been non-fatal.

The assistant references "the known issue from the GitHub thread" — this is a reference to the NVIDIA/open-gpu-kernel-modules issue tracker, where similar build failures against modern kernels with strict objtool have been documented. This shows the assistant is drawing on external knowledge about known incompatibilities between the NVIDIA open driver and recent kernel hardening.

The Diagnostic Attempt and Its Flaw

The assistant's attempted fix is to pass KBUILD_MODPOST_WARN=1 to the kernel build system. This is a reasonable first guess: KBUILD_MODPOST_WARN controls whether module post-processing warnings are treated as errors. However, the assumption is subtly wrong. The errors shown in the output are not from modpost (the module post-processing stage) but from objtool (the object file validation tool). These are two different stages in the kernel build pipeline: objtool runs during compilation to validate individual object files, while modpost runs during linking to check module-wide consistency. The KBUILD_MODPOST_WARN flag has no effect on objtool's behavior.

The output confirms this: the errors are all prefixed with objtool: and reference specific symbols in nvidia-modeset.o. The fact that the build continues past these errors (we see multiple error lines, suggesting make is running with -k or similar) but ultimately fails shows that objtool's strict mode is indeed fatal.

Assumptions Made

Several assumptions underpin this message. The first is that the community 6.19 kernel is the right target for the NVIDIA driver build. At this point in the conversation, the assistant is still pursuing the community kernel approach, not yet aware that it will eventually abandon this entire strategy in favor of building a native Proxmox 6.14 kernel from source. The second assumption is that KBUILD_MODPOST_WARN=1 is the correct mechanism to suppress objtool errors — an assumption that proves incorrect, as the errors persist. The third assumption is that objtool's strict mode is a kernel configuration issue that can be worked around from the build command line, rather than a fundamental incompatibility between the NVIDIA code and the kernel's validation expectations.

There is also an implicit assumption that the NVIDIA open driver should compile cleanly against any reasonably modern kernel. In practice, the NVIDIA open driver contains code patterns (particularly around C++ destructors in the DisplayPort subsystem, as seen in the mangled symbol names like _ZN11DisplayPort18DownRequestManagerD0Ev) that trigger objtool's frame pointer validation. These are not bugs in the NVIDIA code per se, but rather a mismatch between the kernel's increasingly strict validation and the driver's code generation choices.

Input Knowledge Required

To fully understand this message, the reader needs knowledge of several domains. First, familiarity with the Linux kernel build system, particularly how external modules are compiled against kernel headers using SYSSRC and SYSOUT. Second, understanding of objtool's role in the kernel build: it is a host-side tool that performs static analysis on compiled object files, checking for violations of kernel-specific conventions like frame pointer discipline, retpoline safety, and ORC unwinder table correctness. Third, knowledge of the MITIGATION_RETHUNK feature and its relationship to Spectre v2 mitigations. Fourth, awareness of the NVIDIA open GPU kernel module project and its kernel-open directory structure. Fifth, familiarity with C++ name mangling (the _ZN... and _ZTv0_n24... prefixes in the error messages) to recognize that these are C++ destructor calls in the DisplayPort subsystem.

The reader also needs context about the broader provisioning effort: that this is a fresh Proxmox host with Blackwell GPUs, that the assistant is building the NVIDIA driver against a community kernel with a GCC version mismatch, and that the system has already survived one bricking incident from a GLIBC shim library.

Output Knowledge Created

This message produces several valuable outputs. First, it confirms that objtool's strict mode is the blocking issue for the NVIDIA driver build on kernel 6.19 — a concrete diagnosis that narrows the problem space. Second, it demonstrates that KBUILD_MODPOST_WARN=1 is insufficient to suppress objtool errors, ruling out one class of workarounds. Third, it reveals the specific nature of the violations: "call without frame pointer save/setup" in C++ destructor code paths within nvidia-modeset.o. This is actionable information that points toward the real solution: disabling CONFIG_OBJTOOL_WERROR in the kernel configuration, which the assistant will discover and apply in subsequent messages ([msg 8455] and [msg 8456]).

The message also implicitly documents that the NVIDIA open driver's nvidia-modeset module contains code that triggers objtool validation failures — a useful data point for anyone attempting similar builds. The specific mangled symbol names provide forensic evidence of which code paths are problematic.

The Thinking Process

The assistant's reasoning in the thinking block reveals a methodical diagnostic approach. It begins by categorizing the error: "objtool MITIGATION_RETHUNK warnings." This classification is important because it connects the observed error messages to a specific kernel feature, ruling out other possible causes like compiler bugs or missing symbols. The assistant then notes that these warnings "are now errors (strict mode on 6.19)," indicating awareness that this behavior is kernel-version-dependent — older kernels might have tolerated these same warnings.

The reference to "the known issue from the GitHub thread" shows that the assistant is drawing on institutional knowledge about NVIDIA driver build issues. This is not a novel problem; it has been encountered and documented by others attempting to build NVIDIA's open driver against modern kernels.

The proposed fix — "disable objtool warnings-as-errors for the nvidia build" — is the correct high-level strategy. The specific mechanism chosen (KBUILD_MODPOST_WARN=1) is a reasonable first attempt but ultimately targets the wrong tool. The assistant will iterate: in [msg 8454] it tries OBJTOOL="" (which also fails because the kernel Makefile hardcodes the objtool path), and in [msg 8455] it discovers CONFIG_OBJTOOL_WERROR=y in the kernel configuration, leading to the successful fix in [msg 8456].

Broader Significance

Message [msg 8453] is significant beyond its immediate diagnostic value. It represents a class of problems that increasingly arise when building out-of-tree kernel modules against modern, heavily hardened kernels. The Linux kernel's objtool validation has become progressively stricter with each release, catching code patterns that were previously tolerated. This creates friction for proprietary and out-of-tree drivers like NVIDIA's, which must either adapt their code generation or find ways to disable the validation.

The message also illustrates the tension between using community-built kernels and building from source. The community 6.19 kernel was convenient — a pre-built .deb package — but its toolchain mismatch cascaded into a series of increasingly desperate workarounds. The assistant's eventual pivot to building the Proxmox 6.14 kernel from source with the native GCC 12.2.0 toolchain would eliminate all of these problems at once. In retrospect, message [msg 8453] is a signpost on the wrong path: the assistant is still trying to make the community kernel work, not yet realizing that the fundamental approach is flawed.

For the reader, this message serves as a case study in kernel module build debugging. It demonstrates how to read objtool error output, how to connect error categories to kernel configuration options, and how to systematically test workarounds. It also shows the importance of understanding the kernel build pipeline's stages — confusing modpost warnings with objtool validation leads to ineffective fix attempts.