Unable to Extract vmlinux from vmlinuz on Fedora 35: A Comprehensive Troubleshooting Guide

Encountering the “extract-vmlinux: Cannot find vmlinux” error while attempting to extract the uncompressed kernel image from a compressed vmlinuz file on Fedora 35 can be a frustrating experience. As revWhiteShadow, this guide delves into the reasons behind this issue and provides a step-by-step approach to resolving it, ensuring you can successfully obtain the vmlinux file for debugging, analysis, or modification purposes. Understanding the intricacies of kernel image packaging and the tools involved is paramount.

Understanding vmlinuz and vmlinux

Before diving into the troubleshooting steps, let’s clarify the difference between vmlinuz and vmlinux.

  • vmlinuz: This is the compressed, bootable Linux kernel image. It contains the kernel itself, often compressed using gzip or bzip2, along with a minimal filesystem (initrd) that allows the kernel to boot. In Fedora, vmlinuz files are typically located in the /boot directory.

  • vmlinux: This is the uncompressed, statically linked Linux kernel image. It contains all the kernel code and data in a single, executable file. This file is primarily used for debugging and analysis.

The extract-vmlinux script, located in the kernel source tree, is designed to extract the vmlinux file from a vmlinuz image. However, its effectiveness depends on the format of the vmlinuz file and the presence of the embedded vmlinux.

Common Reasons for extract-vmlinux Failure

Several factors can contribute to the “Cannot find vmlinux” error. Let’s examine the most common culprits:

Kernel Configuration and Build Options

The most frequent cause is related to how the kernel was built and configured. If the kernel was not built with the option to embed the vmlinux image within the vmlinuz file, the extract-vmlinux script will fail. This is particularly common in distributions like Fedora, where security and size optimizations often lead to the exclusion of the full vmlinux image from the bootable kernel image.

Incorrect Kernel Source Tree

The extract-vmlinux script relies on the correct kernel source tree to function properly. If you are using a script from a different kernel version than the vmlinuz file you are trying to extract, the script may not be able to correctly parse the file format. Ensure that you are using the extract-vmlinux script that corresponds exactly to the kernel version of your vmlinuz file.

Missing Debug Symbols

Even if the vmlinux file is technically present, it might lack the necessary debug symbols. Without debug symbols, tools like debuggers (gdb) and performance analyzers (perf) are significantly less effective.

Incompatible Compression

While vmlinuz files are commonly compressed with gzip, other compression algorithms might be used. The extract-vmlinux script may not support all possible compression methods, leading to extraction failures.

Kernel Hardening Features

Kernel hardening techniques, such as kernel address space layout randomization (KASLR) and stack protection, can sometimes interfere with the extraction process, especially if the extract-vmlinux script isn’t designed to handle them.

Troubleshooting Steps to Extract vmlinux

Here’s a systematic approach to troubleshoot and resolve the extract-vmlinux error on Fedora 35.

1. Verify Kernel Version and Source Tree

The first step is to ensure that you have the correct kernel source tree installed for the vmlinuz file you are trying to extract.

  • Identify the kernel version:

    Use the uname -r command to determine the running kernel version. Alternatively, inspect the filename of the vmlinuz file in the /boot directory (e.g., vmlinuz-5.14.15-200.fc34.x86_64).

  • Install the kernel development package:

    Use dnf (the Fedora package manager) to install the corresponding kernel development package:

    sudo dnf install kernel-devel-$(uname -r)

    This package will install the kernel headers and the extract-vmlinux script in the /usr/src/kernels/<kernel_version>/scripts/ directory.

2. Use the Correct extract-vmlinux Script

Navigate to the kernel source directory corresponding to your kernel version and use the extract-vmlinux script located there.

cd /usr/src/kernels/$(uname -r)
sudo scripts/extract-vmlinux /boot/vmlinuz-$(uname -r) > vmlinux

3. Check Kernel Configuration

Verify if the kernel was configured to include the vmlinux file within the vmlinuz image. This requires access to the kernel configuration file (.config) used during the kernel build.

  • Locate the kernel configuration file:

    The configuration file is often located in the /boot directory as config-<kernel_version>.

  • Check for relevant configuration options:

    Use grep to search for relevant options in the configuration file:

    grep -i "CONFIG_IKCONFIG" /boot/config-$(uname -r)
grep -i "CONFIG_IKCONFIG_PROC" /boot/config-$(uname -r)

    If CONFIG_IKCONFIG and CONFIG_IKCONFIG_PROC are enabled, you can extract the kernel configuration directly from the running kernel:

    zcat /proc/config.gz > .config

    After obtaining the .config file, search for CONFIG_DEBUG_INFO=y. If this option is not enabled, the kernel was built without debug information, and the vmlinux file will be minimal and may not be easily extracted.

4. Alternative Extraction Methods

If the standard extract-vmlinux script fails, consider alternative extraction methods.

  • Using objcopy:

    The objcopy utility can sometimes be used to extract sections from the vmlinuz file. However, this method requires a deep understanding of the kernel image format and memory layout.

    objcopy -O binary -j .text /boot/vmlinuz-$(uname -r) kernel_text

    This command attempts to extract the .text section (containing the executable code) from the vmlinuz file. Further processing might be required to obtain a usable vmlinux image.

  • Manual Extraction with dd:

    In some cases, you can manually extract the kernel image using the dd command. This involves identifying the start and end offsets of the compressed kernel data within the vmlinuz file and extracting those bytes. This method is highly dependent on the specific kernel version and compression format and requires significant expertise.

5. Obtain vmlinux from Debug Packages

A more reliable approach is to obtain the vmlinux file from the corresponding debug packages. Fedora provides debug packages that contain the full, unstripped vmlinux image with all debug symbols.

  • Install the kernel debuginfo package:

    Use dnf to install the kernel debuginfo package:

    sudo dnf debuginfo-install kernel-$(uname -r)

  • Locate the vmlinux file:

    The vmlinux file will be located in the /usr/lib/debug/lib/modules/$(uname -r)/ directory.

6. Dealing with Compressed Kernel Images

If the vmlinuz file is compressed with a non-standard compression algorithm, you might need to decompress it manually before attempting to extract the vmlinux file.

  • Identify the compression algorithm:

    Use the file command to identify the compression algorithm used:

    file /boot/vmlinuz-$(uname -r)

  • Decompress the image:

    Use the appropriate decompression tool (e.g., gzip, bzip2, xz) to decompress the vmlinuz file.

    gzip -d /boot/vmlinuz-$(uname -r)

    After decompression, you can attempt to use the extract-vmlinux script on the decompressed file.

7. Kernel Module Considerations

Keep in mind that extracting the vmlinux file alone might not be sufficient for all debugging or analysis tasks. Kernel modules often rely on specific kernel symbols and data structures. If you are working with kernel modules, you might also need to obtain the corresponding module debugging information.

8. Using SystemTap

SystemTap is a powerful tracing and probing tool that can be used to dynamically analyze the running kernel. It doesn’t require the vmlinux file in all cases, but having it can greatly enhance its capabilities. SystemTap can often infer the necessary information directly from the running kernel.

9. Rebuilding the Kernel with Debug Options

If all other methods fail, consider rebuilding the kernel from source with the necessary debug options enabled. This provides the most control over the kernel build process and ensures that the vmlinux file is generated with the desired debug information.

  • Download the kernel source:

    Obtain the kernel source code from the Fedora repositories or the official kernel website (kernel.org).

  • Configure the kernel:

    Use make menuconfig or make xconfig to configure the kernel. Ensure that the following options are enabled:

    • CONFIG_DEBUG_INFO=y
    • CONFIG_IKCONFIG=y
    • CONFIG_IKCONFIG_PROC=y

  • Build and install the kernel:

    Build and install the kernel using the standard kernel build process.

    make
sudo make modules_install
sudo make install

    After rebuilding the kernel, the vmlinux file will be located in the kernel source directory.

10. Verifying the Extracted vmlinux File

After attempting to extract the vmlinux file, verify its integrity and usability.

  • Check the file size:

    The vmlinux file should be significantly larger than the vmlinuz file.

  • Use file command to verify the file type:

    file vmlinux

    The output should indicate that it is an ELF executable.

  • Attempt to load it into a debugger:

    Try loading the vmlinux file into a debugger like GDB:

    gdb vmlinux

    If GDB can successfully load the file and access its symbols, the extraction was likely successful.

Conclusion

Extracting the vmlinux file from a vmlinuz image on Fedora 35 can be challenging, but by following these detailed troubleshooting steps, you can increase your chances of success. Remember to verify the kernel version, use the correct extract-vmlinux script, check the kernel configuration, and consider alternative extraction methods or debug packages. When facing persistent issues, rebuilding the kernel with debug options enabled provides the most reliable solution. As revWhiteShadow, we are dedicated to providing you with the resources to conquer your kernel-related challenges. This detailed guide should serve as your primary weapon!