Understanding CMAKE_RANLIB for Library Randomization in CMake

What is CMAKE_RANLIB?

• It's important to note that CMAKE_RANLIB is not used on Windows as Windows employs a different approach for static libraries.
• Library randomization is a technique that helps mitigate certain security vulnerabilities by altering the order of symbols within a static library. This makes it more difficult for attackers to exploit these libraries.
• In CMake, CMAKE_RANLIB is a predefined variable that stores the name of the tool used to perform library randomization on Unix-based systems (Linux, macOS, etc.).

How is CMAKE_RANLIB Used?

• You can set it in your CMakeLists.txt file using the set command like this:
• CMake typically detects the appropriate ranlib tool based on the underlying system. However, you can override the default behavior by setting the CMAKE_RANLIB variable to a specific path or name of the desired ranlib executable.

set(CMAKE_RANLIB /path/to/your/ranlib)

When is CMAKE_RANLIB Relevant?

• You might encounter situations where you need to use a non-standard ranlib tool, or you might want to disable library randomization altogether. In those cases, you can adjust the CMAKE_RANLIB variable accordingly.
• The CMAKE_RANLIB variable becomes relevant when CMake generates build instructions for creating static libraries on Unix-like systems. It instructs the build process to use the specified ranlib tool to randomize the library after it's been created.

• Since CMake 3.9, CMake has introduced variables like CMAKE_<LANG>_COMPILER_RANLIB (where <LANG> is C or CXX). These variables allow you to specify a wrapper around the ranlib tool that adds appropriate compiler-specific options for library randomization. This can be useful if your compiler has special flags for this purpose.

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Using the Default CMAKE_RANLIB

This code snippet demonstrates how CMake uses the system-detected ranlib tool:

# No explicit setting of CMAKE_RANLIB
add_library(mylib MY_SOURCE.cpp)

# CMake will automatically use the system's ranlib during the build process

Overriding CMAKE_RANLIB

This code snippet shows how to override the default ranlib with a custom one:

# Set CMAKE_RANLIB to a specific path
set(CMAKE_RANLIB /custom/path/to/ranlib)

add_library(mylib MY_SOURCE.cpp)

# CMake will use the specified ranlib tool for library randomization

Disabling Library Randomization

If you want to disable library randomization altogether (not recommended for security reasons), you can set CMAKE_RANLIB to an empty string:

# Disable library randomization
set(CMAKE_RANLIB "")

add_library(mylib MY_SOURCE.cpp)

# CMake won't call any ranlib tool during the build process

Using CMAKE_<LANG>_COMPILER_RANLIB (CMake 3.9+)

This code snippet (assuming CMake 3.9 or later) demonstrates using the compiler-specific ranlib wrapper:

# Check if CMAKE_CXX_COMPILER_RANLIB is set (assuming C++ project)
if(CMAKE_CXX_COMPILER_RANLIB)
  set(CMAKE_RANLIB ${CMAKE_CXX_COMPILER_RANLIB})
endif()

add_library(mylib MY_SOURCE.cpp)

# CMake will use the compiler-specific ranlib wrapper if available

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Compiler Flags (if applicable)

• Some compilers might offer specific flags for library randomization during the linking stage. This can potentially eliminate the need for a separate ranlib tool. You might need to consult your compiler's documentation for such flags.

Custom Script (advanced)

• For more complex scenarios, you could potentially create a custom script that wraps around the actual library creation process. This script could handle library randomization using alternative tools or techniques beyond the standard ranlib. However, this approach requires a deeper understanding of the build process and might not be suitable for most situations.

Disable Randomization (not recommended)

• As a last resort, you could choose to disable library randomization altogether by setting CMAKE_RANLIB to an empty string (""). However, this is strongly discouraged as library randomization helps mitigate security vulnerabilities. Only consider this option if you absolutely have to and fully understand the risks involved.

Consider Build System Alternatives

• If your project doesn't have strong ties to CMake and library randomization is a major concern, you might explore alternative build systems that handle randomization differently. However, this would be a significant change and should be carefully evaluated.

• Disabling randomization is generally not recommended due to security implications.
• For more advanced customization, a custom script might be an option, but it should be a well-considered approach.
• If you have specific compiler flags for randomization, consider using those within your CMake configuration.
• Stick with the default CMAKE_RANLIB behavior whenever possible. It leverages the standard approach on Unix systems and ensures security best practices are followed.