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CO-RE.md

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CO-RE for eBPF

CO-RE, which stands for "Compile Once - Run Everywhere," is a concept within the eBPF (extended Berkeley Packet Filter) ecosystem. eBPF is a powerful technology that allows for the dynamic insertion of bytecode into the Linux kernel without changing kernel source code or loading kernel modules.

Understanding CO-RE

  1. Context: eBPF programs are used for various purposes like networking, security, and performance monitoring. They are often loaded into the kernel to customize its behavior.

  2. Challenge: Traditionally, eBPF programs are tightly coupled with the kernel version and configuration they were compiled against. This means that an eBPF program compiled for one kernel might not work on a different kernel version or configuration, due to differences in kernel internals, such as data structures.

  3. CO-RE Solution: CO-RE addresses this challenge by allowing an eBPF program to be compiled once and then run on different kernel versions and configurations without modification. This is achieved through a combination of technologies and techniques:

    • BTF (BPF Type Format): BTF provides rich type information about the kernel data structures, enabling eBPF programs to adapt to different kernel versions.
    • Relocation: During the loading of an eBPF program, certain adjustments (relocations) are made based on the BTF information to ensure compatibility with the target kernel's internal structures.
    • Libbpf: A userspace library that aids in developing eBPF applications. It handles the complexities of loading and managing eBPF programs and maps, including the relocations needed for CO-RE.

Why We Need CO-RE

  1. Portability: Ensures that eBPF programs can run on various kernel versions, enhancing their portability across different systems.

  2. Ease of Deployment: Simplifies the deployment process of eBPF programs in environments with diverse kernel configurations.

  3. Development Efficiency: Reduces the need for multiple versions of eBPF programs for different kernel versions, streamlining development and maintenance.

  4. Adaptability: CO-RE allows eBPF programs to automatically adapt to changes in kernel data structures, making them more resilient to kernel updates.

  5. Performance: Despite the abstraction, eBPF with CO-RE maintains high performance, crucial for kernel-level operations.

Example of CO-RE in Userspace

Suppose the host sofeware has a data structure like this:

struct data {
	int a;
	int b;
    int c;
} __attribute__((__packed__));

And we write a eBPF program like this, which use c field of data structure, as a plugin of the host software:

int bpf_main(struct data *d)
{
	uint64_t n = print_func(str);
	int x = (int)n + d->c;
	return add_func(x, 12);
}

However, in production, the host software has a lot of different versions. the data structure may change in different versions. For example, in another, the data structure is like this:

struct data {
	int a;
	int b;
    int add_field;
    int c;
} __attribute__((__packed__));

The CO-RE technology can still ensure the eBPF program can access the c field of data structure correctly, even if the data structure is changed.