用于模拟Linux内核Initcall机制的自定义ld脚本工作原理是什么？

Hey everyone, let me break down a common issue when trying to replicate the Linux kernel's Initcall mechanism in user space—something Trevor ran into recently while writing a test program to simulate init_module() calls.

The Problem #

When Trevor tried compiling his user-space program, the linker threw an error complaining that __initcall_start and __initcall_end were undefined. No executable could be generated at all.

Why This Happens #

The Linux kernel's Initcall system relies on two special symbols—__initcall_start and __initcall_end—that are defined in the kernel's custom link script. These symbols mark the start and end of the section where initialization functions are stored. But user-space programs use the default system linker script, which doesn't include these symbols. So the linker has no idea what they're supposed to point to.

The Fix #

We need to create a custom linker script that adds these missing symbols. Here's how to do it step by step:

1. Export the default linker script: Run ld --verbose in your terminal, and save the entire output to a file (let's call it my_initcall.ld). This gives you a base to modify.
2. Add the initcall symbols: Open my_initcall.ld and find the section where the .text segment is defined. Right before that, add code to define our initcall symbols and map the relevant sections. For example:

   __initcall_start = .;
   *(SORT(.initcall*.init))
   __initcall_end = .;
   

   This tells the linker to set __initcall_start at the current address, include all sections matching .initcall*.init (matching the kernel's initcall naming convention), then mark __initcall_end at the end of those sections.
3. Compile with the custom script: When compiling your program, add the -T my_initcall.ld flag to tell gcc (or ld) to use your modified linker script instead of the default one.

Proof It Works #

After making these changes, the linker no longer throws errors. Here's a snippet from objdump -d on the working executable, showing the symbols are correctly placed:

0000000000401000 <__initcall_start>:
401000: 55 push %rbp
401001: 48 89 e5 mov %rsp,%rbp
401004: b8 00 00 00 00 mov $0x0,%eax
401009: 5d pop %rbp
40100a: c3 retq

000000000040100b <__initcall_end>:
40100b: e9 f0 ff ff ff jmpq 400ff0 <_start>

内容的提问来源于stack exchange，提问作者Baroudi Safwen