The Shim That Wouldn't Shim: A Debugging Spiral into GLIBC Version Mismatches

Introduction

In any systems engineering effort, there comes a moment when the accumulation of small workarounds, patches, and hacks collapses under its own weight. Message [msg 8443] captures precisely such a moment in the provisioning of kpro6, a new Proxmox host destined to become an 8× Blackwell RTX PRO 6000 GPU training node. The assistant has been locked in a multi-hour battle with a fundamental toolchain incompatibility: the community-built 6.19.5-2-pve kernel was compiled on Debian Trixie with GCC 14 and linked against GLIBC_2.38, but the host runs Debian Bookworm with GLIBC_2.36. This one-version gap has broken four critical kernel build tools — objtool, modpost, insert-sys-cert, and resolve_btfids — each of which demands __isoc23_strtoul, __isoc23_strtoull, or __isoc23_strtol symbols that simply don't exist in Bookworm's libc.

This message is the climax of a failed workaround strategy. The assistant has already tried rebuilding tools from source (the headers package lacks the full source tree), using LD_PRELOAD with a shim library (the dynamic linker checks version requirements before loading preloaded libraries), and hex-editing the version string from GLIBC_2.38 to GLIBC_2.17 (the symbols don't exist under that version tag). Now, in this message, the assistant attempts one final hack: building a proper shared library that exports the missing symbols under the GLIBC_2.38 version tag and installing it into the system library path. It fails again. This message is the turning point that forces a complete strategic pivot — from patching binary incompatibilities to building everything from source with a consistent toolchain.

The Message in Full

The assistant begins with a moment of reflection and diagnosis:

Wait — GLIBC_2.17 shows up in the version list but the dynamic linker says it's not found. This is probably because the VERNEED in the patched binary now references GLIBC_2.17 for a symbol (__isoc23_strtoul) that doesn't exist under that version tag in libc.

>

The issue is that VERNEED entries bind a specific symbol to a specific version. When we patched GLIBC_2.38 → GLIBC_2.17, the linker looks for __isoc23_strtoul@GLIBC_2.17 in libc and can't find it.

>

Let me use a completely different approach — provide a separate .so that has the GLIBC_2.38 version tag and the symbols:

The assistant then executes a series of commands over SSH to the remote host at 10.1.2.6:

  1. Restores the original binaries from .orig backups
  2. Writes a C source file implementing the three missing functions as wrappers around the standard strtoul, strtoull, and strtol calls
  3. Creates a linker version script that binds these symbols under the GLIBC_2.38 version tag
  4. Compiles the shared library with -fPIC and the version script
  5. Registers the library path with ldconfig
  6. Tests each binary The output shows:
Restored: /usr/src/linux-headers-6.19.5-2-pve/tools/objtool/objtool
Restored: /usr/src/linux-headers-6.19.5-2-pve/scripts/mod/modpost
Restored: /usr/src/linux-headers-6.19.5-2-pve/scripts/insert-sys-cert
Restored: /usr/src/linux-headers-6.19.5-2-pve/tools/bpf/resolve_btfids/resolve_btfids
---BUILD: 0---
/usr/src/linux-headers-6.19.5-2-pve/tools/objtool/objtool: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.38' not found (required by /usr/src/linux-headers-6.19.5-2-pve/tools/objtool/objtool)

The build succeeds — the shim library compiles cleanly — but the test still fails with the same error. The dynamic linker still refuses to load the binaries because it checks the version requirement against libc.so.6 itself, and libc.so.6 does not advertise GLIBC_2.38 as a version it provides. The shim library, even though it exports the right symbols under the right version tag, cannot satisfy the VERNEED check because the version tag must be found in the directly linked library (libc), not in an auxiliary library discovered through the library path.

Why This Message Was Written

This message exists because the assistant has been chasing a fundamentally flawed strategy: trying to make a community-built kernel binary work on a system with an older glibc by patching around the incompatibility. The motivation is understandable — the community kernel (6.19.5-2-pve) offers features that the stock Proxmox kernel doesn't, and the NVIDIA 595.71.05 open driver requires a modern kernel. Rather than building from source (which would take time and require understanding the Proxmox kernel build system), the assistant has been attempting to retrofit the pre-built binaries.

The reasoning visible in the opening paragraph shows the assistant working through why the previous hex-edit approach failed. It correctly identifies the mechanism: VERNEED (Version Need) entries in ELF binaries bind specific symbols to specific version tags. When you hex-edit GLIBC_2.38 to GLIBC_2.17, you're not just changing a version string — you're changing the version tag under which the dynamic linker expects to find __isoc23_strtoul. Since libc doesn't export that symbol under GLIBC_2.17, the linker fails. This is a correct diagnosis.

The proposed solution — providing a separate .so that exports the symbols under GLIBC_2.38 — is a reasonable next attempt. If the shim library can be found by the dynamic linker and provides the right symbols under the right version tag, the VERNEED check might be satisfied. The assistant builds the library correctly, using a linker version script to bind the symbols to GLIBC_2.38, and registers it with ldconfig.

Assumptions Made

The assistant makes several assumptions in this message, some of which prove incorrect:

  1. That a shim library can satisfy VERNEED checks: The core assumption is that if a library in the search path provides __isoc23_strtoul@GLIBC_2.38, the dynamic linker will accept it. In reality, the VERNEED check is performed against the directly linked libraries — specifically libc.so.6. The dynamic linker reads the version requirement from the binary's .gnu.version_r section, finds that libc.so.6 is the provider of the GLIBC_2.38 version tag (because the binary was linked against a libc that advertised that version), and checks whether libc.so.6 actually provides that version. Since Bookworm's libc only goes up to GLIBC_2.36, the check fails before any auxiliary libraries are consulted.
  2. That ldconfig and library path registration would make the shim visible: The assistant adds /usr/local/lib to the library search path via /etc/ld.so.conf.d/glibc238-shim.conf and runs ldconfig. This would make the shim library available for future linking, but it doesn't change the VERNEED resolution for already-linked binaries. The dynamic linker resolves version requirements against the libraries already specified in the binary's DT_NEEDED entries.
  3. That the build succeeded means the approach is viable: The shim library compiles with exit code 0, which the assistant treats as progress. But the fundamental issue — that the dynamic linker checks version requirements against the specific library that originally provided that version — remains unaddressed.
  4. That restoring original binaries was sufficient cleanup: The assistant restores the four patched binaries from their .orig backups, but the earlier hex-editing may have left other traces. This is a minor concern, but in a debugging spiral, incomplete cleanup can lead to confusing state.

Mistakes and Incorrect Assumptions

The central mistake in this message — and in the broader workaround strategy — is underestimating the rigor of the GNU dynamic linker's version checking. The ELF versioning mechanism (.gnu.version_r / VERNEED) is designed precisely to prevent the kind of shimming the assistant is attempting. When a binary is linked against a library that advertises GLIBC_2.38, the linker records not just the version string but the association between that version and the library that provides it. At runtime, the dynamic linker verifies that the specific library (by soname) still provides that version. A separate shim library, even one that exports the right symbols under the right version tag, cannot satisfy this check because it's not libc.so.6.

A secondary mistake is not recognizing earlier that the community kernel approach was fundamentally incompatible with Bookworm. The assistant had already spent many messages on this path — rebuilding gendwarfksyms, patching binaries, creating shims — before this message finally demonstrates the approach's impossibility. The sunk cost fallacy is visible: each failed workaround leads to a slightly more elaborate workaround, rather than a clean break.

There's also a subtle error in the shim implementation itself. The __isoc23_strtoul function in glibc 2.38 has different behavior from strtoul — it implements C23 semantics where the function does not skip leading whitespace and has different error handling for certain inputs. The assistant's shim simply forwards to strtoul, which means even if the dynamic linker accepted it, the kernel build tools might behave incorrectly in edge cases. For kernel module building, this could lead to subtle bugs. The assistant acknowledges this ("for our purposes shims that call the old versions will work") but the assumption is risky.

Input Knowledge Required

To fully understand this message, the reader needs knowledge in several areas:

ELF Binary Format: Understanding of how shared libraries are linked, what DT_NEEDED entries are, how the .gnu.version_r section records version requirements, and how the dynamic linker resolves them at load time. The concept of VERNEED — version need entries that bind specific symbols to specific version tags from specific libraries — is central to understanding why the shim approach fails.

GNU libc Versioning: Knowledge that glibc uses a symbol versioning scheme where functions are associated with version tags (like GLIBC_2.38) and that these tags are advertised by libc.so.6 itself. The dynamic linker checks that the library provides the required version, not just the required symbol.

Linux Kernel Build System: Understanding that kernel build tools like objtool, modpost, insert-sys-cert, and resolve_btfids are compiled as part of the kernel build and are used during module building. These tools must be compatible with the running system's libraries because they execute on the host during DKMS builds.

Debian/Proxmox Packaging: Knowledge of the difference between kernel headers packages (which contain header files and pre-built tools) and full kernel source packages (which contain the complete build system). The assistant discovered earlier that the headers package doesn't include the full tools/build/ infrastructure needed to rebuild these tools from source.

Dynamic Linker Behavior: Understanding of LD_PRELOAD, ldconfig, library search paths, and the order of operations during dynamic linking. Specifically, that version requirement checks happen before LD_PRELOAD libraries are loaded, which is why earlier attempts with LD_PRELOAD also failed.

Output Knowledge Created

This message creates several pieces of knowledge:

  1. Definitive proof that the shim approach cannot work: The test output shows that even a properly-built shared library with the correct version tag cannot satisfy the VERNEED check. This is a concrete demonstration that the dynamic linker's version checking is tied to the specific library (by soname) that originally provided the version, not just to any library that exports the right symbols.
  2. A clear diagnosis of the VERNEED mechanism: The assistant's opening paragraph provides a correct and concise explanation of why the hex-edit approach failed. This diagnosis is reusable knowledge for anyone encountering similar glibc version mismatches.
  3. Restored binaries: The assistant restores the original binaries from their .orig backups, cleaning up the state from the hex-editing experiment. This is important because the next approach (building from source) will need the original, unmodified binaries as reference.
  4. A working shim library: Although the approach fails, the shim library itself is correctly built. The C code implements the three missing functions, the version script correctly binds them to GLIBC_2.38, and the compilation succeeds. This library could potentially be useful in other contexts (e.g., if the binaries were re-linked against it).
  5. The boundary of what's possible with workarounds: This message establishes that the community kernel approach, with its GCC 14 / GLIBC_2.38 toolchain, cannot be retrofitted onto Bookworm. This negative result is valuable because it forces the strategic pivot to building from source — which ultimately succeeds.

The Thinking Process

The assistant's reasoning is visible in several places in this message. The opening paragraph shows the diagnostic process:

Wait — GLIBC_2.17 shows up in the version list but the dynamic linker says it's not found. This is probably because the VERNEED in the patched binary now references GLIBC_2.17 for a symbol (__isoc23_strtoul) that doesn't exist under that version tag in libc.

The "Wait" indicates a moment of realization — the assistant had assumed that changing the version string would be sufficient, but the test failure prompted a deeper analysis. The reasoning connects the observed behavior (dynamic linker says "not found") to the mechanism (VERNEED binds symbols to specific versions in specific libraries).

The assistant then formulates a new hypothesis:

Let me use a completely different approach — provide a separate .so that has the GLIBC_2.38 version tag and the symbols

This is a reasonable next step: if the problem is that the symbols don't exist under GLIBC_2.17, provide them under GLIBC_2.38. The assistant correctly identifies that the version tag in the shim must match what the binaries expect.

The implementation shows careful attention to detail:

The Broader Context

This message sits within a larger narrative about the provisioning of kpro6. The segment summary describes the overall arc:

Provisioned kpro6, a new Proxmox host with 8× Blackwell RTX PRO 6000 GPUs, by building a custom 6.14 kernel and NVIDIA 595.71.05 open driver from source with a consistent GCC 12.2.0 toolchain after recovering from a bricked system caused by toolchain incompatibility.

The "bricked system" refers to a later event (not in this message) where the GLIBC_2.38 shim library, when installed system-wide via ldconfig, poisons the dynamic linker and breaks SSH access. This message is the precursor to that disaster — the moment where the assistant is still trying to make the shim approach work, unaware that it's about to make things much worse.

The chunk summary for this segment notes:

Workarounds involving patched kernel headers, rebuilt gendwarfksyms and objtool binaries, and a GLIBC_2.38 shim library culminated in the shim poisoning the system's dynamic linker, bricking SSH access and requiring physical rescue from a live ISO.

This message is the last attempt before that poisoning occurs. The assistant builds the shim library and installs it via ldconfig — and while the immediate test fails, the library is now in the system path, ready to cause havoc when the dynamic linker encounters it in other contexts.

Conclusion

Message [msg 8443] is a study in the limits of workarounds. The assistant demonstrates solid diagnostic skills — correctly identifying the VERNEED mechanism, formulating a reasonable alternative approach, and implementing it carefully. But the approach fails because the dynamic linker's version checking is more fundamental than the assistant assumed: it checks not just that a symbol exists under a version tag, but that the specific library the binary was linked against provides that version. A shim library, no matter how correctly built, cannot satisfy this check.

This message is the turning point in the provisioning of kpro6. It's the moment where the assistant exhausts the workaround strategy and must confront the fundamental incompatibility. The path forward — building the kernel and driver from source with a consistent toolchain — is more work, but it's the only reliable approach. The lesson is one that every systems engineer learns eventually: when you find yourself fighting the toolchain, you're fighting the wrong battle. Sometimes the only way forward is to go back to the beginning and build it right.

The message also serves as a cautionary tale about the ELF dynamic linking model. The GNU dynamic linker's version checking is robust and well-designed — it prevents exactly the kind of shimming the assistant attempted. Understanding this mechanism is essential for anyone working with binary compatibility across different Linux distributions or glibc versions. The VERNEED mechanism is not a bug or an annoyance to be worked around; it's a safety feature that prevents subtle runtime errors from incompatible library versions. This message demonstrates, through failure, why that safety feature exists.