Troubleshooting Xdp-tools Build Failure Num Of Global Symbols Mismatch

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Introduction

This article addresses a critical issue encountered while building xdp-tools within the OpenWrt environment. The build process fails due to a mismatch in the number of global symbols between the compiled objects and the shared library. Specifically, the error message indicates that the number of global symbols in ./sharedobjs/libxdp.o ./sharedobjs/xsk.o does not match the number of versioned symbols in libxdp.so. This discrepancy prevents the successful compilation of xdp-tools, hindering the utilization of XDP (eXpress Data Path) functionalities in the OpenWrt ecosystem. Understanding the root cause of this issue and implementing appropriate solutions are crucial for developers and network engineers relying on XDP for high-performance packet processing.

Understanding the Issue

The core problem lies in the symbol versioning within the libxdp library. Symbol versioning is a mechanism used to manage compatibility between different versions of a shared library. It ensures that applications compiled against a specific version of the library continue to function correctly even when a newer version of the library is installed on the system. The error message “Num of global symbols in ./sharedobjs/libxdp.o ./sharedobjs/xsk.o (1) does NOT match with num of versioned symbols in libxdp.so (69)” signifies that the number of symbols marked for global access in the object files (libxdp.o and xsk.o) does not align with the number of symbols that have explicit versioning information in the shared library (libxdp.so). This mismatch typically arises when symbols are not properly versioned in the library’s map file (libxdp.map). The map file is essential for specifying which symbols should be versioned and how they should be versioned. When the build system detects this discrepancy, it halts the compilation process to prevent potential runtime issues that could arise from linking against an improperly versioned library. This issue can be particularly challenging in embedded systems like OpenWrt, where resource constraints and the need for stable, predictable behavior are paramount.

Detailed Analysis of the Error

To further dissect the error, it is important to understand the components involved and their roles in the build process. The files libxdp.o and xsk.o are object files resulting from the compilation of the XDP library and related components. These files contain the compiled code and data, including symbols that are intended to be exposed for linking with other parts of the system. The libxdp.so file is the shared library, a collection of compiled code that can be dynamically linked into programs at runtime. Shared libraries are crucial for reducing code duplication and conserving system resources. The libxdp.map file is a critical piece of the puzzle; it is a text file that specifies the symbol versioning information for the libxdp library. This file dictates which symbols should be versioned and how they should be versioned. The versioning process ensures that different versions of the library can coexist on the system without causing conflicts. When the build system encounters a discrepancy between the symbols in the object files and the versioning information in the map file, it triggers the error. This check is a safeguard to prevent the creation of a shared library with inconsistent symbol versioning, which could lead to unpredictable behavior and crashes. Identifying the specific symbols that are missing versioning information in the libxdp.map file is a key step in resolving this issue. This often involves examining the output of the build process and comparing the symbol tables of the object files and the shared library.

Troubleshooting Steps

Resolving this build error requires a systematic approach. The following steps outline a comprehensive troubleshooting process:

  1. Verify the libxdp.map File: The first step is to examine the libxdp.map file. This file should contain entries for all global symbols in libxdp.so. Ensure that all necessary symbols are listed and that the versioning information is correctly specified. Common mistakes include typos, missing symbols, or incorrect version numbers. It is also important to check the syntax of the map file, as even a small error can prevent the build system from correctly interpreting the versioning information. Comparing the libxdp.map file with a known good version (if available) can help identify discrepancies. Additionally, tools like nm and objdump can be used to inspect the symbol tables of the object files and the shared library, providing valuable insights into which symbols are missing versioning information.

  2. Check Build Environment: Ensure that the build environment is correctly set up. This includes having the necessary tools and libraries installed and configured properly. In the context of OpenWrt, this means verifying that the toolchain is correctly installed and that all dependencies for xdp-tools are satisfied. The build environment should also be clean, meaning that there are no remnants of previous builds that could interfere with the current build process. Running make clean can help ensure a clean build environment. Additionally, it is important to check for any environment variables that might be affecting the build process, such as incorrect paths or compiler flags.

  3. Update Dependencies: Outdated dependencies can sometimes cause build issues. Make sure that all required libraries and tools are up to date. This includes the XDP library itself, as well as any other libraries that xdp-tools depends on. In OpenWrt, this typically involves updating the package feeds and rebuilding the toolchain. It is also important to check for any known compatibility issues between different versions of the dependencies. Consulting the release notes and documentation for the dependencies can help identify potential problems.

  4. Examine Compiler and Linker Flags: Incorrect compiler or linker flags can lead to symbol versioning issues. Review the build scripts and Makefiles to ensure that the correct flags are being used. Pay close attention to flags related to symbol visibility and versioning, such as -fvisibility=hidden and -Wl,--version-script. These flags control how symbols are exported from the shared library and how versioning information is applied. Incorrect usage of these flags can result in symbols being inadvertently hidden or not properly versioned. It is also important to ensure that the linker script is correctly configured to handle symbol versioning.

  5. Clean and Rebuild: Sometimes, a clean rebuild can resolve the issue. Use the make clean command followed by the build command to ensure that all previously compiled files are removed and the project is rebuilt from scratch. This can help eliminate any potential issues caused by stale object files or build artifacts. A clean rebuild also ensures that the build process is starting from a consistent state, making it easier to diagnose any remaining issues. It is often a good practice to perform a clean rebuild after making significant changes to the build environment or the project’s configuration.

OpenWrt Specific Considerations

When dealing with OpenWrt, there are specific considerations to keep in mind:

  • OpenWrt Build System: OpenWrt uses a specific build system that manages the compilation and packaging of software for embedded devices. Understanding the OpenWrt build system is crucial for troubleshooting build issues. This includes knowing how to use the make command, how to configure the build environment, and how to manage dependencies. The OpenWrt build system also provides specific tools and utilities for debugging build issues, such as verbose build output and error logging.
  • Package Feeds: OpenWrt uses package feeds to manage software packages. Ensure that the package feeds are correctly configured and up to date. Outdated package feeds can lead to dependency issues and build errors. The feeds command in OpenWrt is used to manage package feeds. It allows you to update, install, and remove packages from the feeds.
  • Target Architecture: OpenWrt supports a wide range of target architectures. Ensure that you are building for the correct target architecture. Building for the wrong architecture can lead to compatibility issues and build errors. The target architecture is typically specified in the OpenWrt configuration file.
  • Customizations: If you have made any customizations to the OpenWrt build system or the xdp-tools package, review these customizations carefully. Customizations can sometimes introduce build issues. It is important to document any customizations that you make, so that you can easily revert them if necessary.

Proposed Solution

Based on the error message and the troubleshooting steps, the most likely solution is to ensure that all global symbols in libxdp.so are properly versioned in the libxdp.map file. This involves:

  1. Identifying Missing Symbols: Use tools like nm and objdump to list the global symbols in libxdp.so and compare them with the entries in libxdp.map. Identify any symbols that are present in libxdp.so but missing in libxdp.map.
  2. Adding Versioning Information: Add the missing symbols to libxdp.map, specifying the appropriate versioning information. This typically involves adding an entry for each missing symbol, indicating the symbol name and the version tag.
  3. Rebuilding xdp-tools: After updating libxdp.map, clean and rebuild xdp-tools to incorporate the changes. This ensures that the new versioning information is used during the build process.

Example Scenario and Resolution

Consider a scenario where the symbol xdp_socket__create is present in libxdp.so but missing from libxdp.map. The following steps outline the resolution:

  1. Identify the Missing Symbol:
    nm -gD ./sharedobjs/libxdp.so | grep xdp_socket__create
    This command will output the symbol xdp_socket__create if it is present in libxdp.so.
  2. Inspect libxdp.map: Open the libxdp.map file and verify that there is no entry for xdp_socket__create.
  3. Add the Symbol to libxdp.map: Add the following line to libxdp.map:
    xdp_socket__create XDP_1.0
    This entry specifies that the symbol xdp_socket__create should be versioned as XDP_1.0.
  4. Rebuild xdp-tools:
    make clean
make
    This will clean the build directory and rebuild xdp-tools, incorporating the updated libxdp.map file.

By following these steps, the build error should be resolved, and xdp-tools should compile successfully.

Conclusion

The error “Num of global symbols in ./sharedobjs/libxdp.o ./sharedobjs/xsk.o (1) does NOT match with num of versioned symbols in libxdp.so (69)” during the build of xdp-tools in OpenWrt indicates a mismatch in symbol versioning within the libxdp library. This issue can be resolved by carefully examining the libxdp.map file, identifying missing symbols, adding the appropriate versioning information, and rebuilding xdp-tools. A systematic approach to troubleshooting, including verifying the build environment, updating dependencies, and examining compiler and linker flags, is crucial for resolving this and similar build issues. By addressing this issue, developers and network engineers can ensure the successful compilation and utilization of xdp-tools in OpenWrt, enabling the use of XDP for high-performance packet processing in embedded systems.

This article provides a comprehensive guide to understanding and resolving this specific build error. However, symbol versioning and build system intricacies can be complex. It is always recommended to consult the official documentation and community resources for further assistance and best practices.