如何着手理解x86_64架构下GNU链接器的源码？

Hey there! Awesome that you’re digging into the GNU linker’s internals—with your year of GNU toolchain experience and progress through John R. Levine’s Linkers&Loaders, you’re already off to a strong start. Let’s break down how to jump into the ld source code for x86_64, step by step.

1. First, Map Out the Binutils Source Tree #

Unpack your binutils source archive, and head straight to the ld/ directory—that’s the heart of the linker code. Here are the key subdirectories and files to prioritize:

• ld/emulparams/: Holds architecture-specific configuration. For x86_64, look at elf_x86_64.sh—it defines default linker script templates, target-specific flags, and other architecture quirks you’ll care about.
• ld/scripttempl/: Contains generic linker script templates that the x86_64 config pulls from. Great for understanding how default link scripts are constructed.
• ld/elfcpp/: Handles low-level ELF format parsing and manipulation—critical since the linker’s entire job revolves around ELF files.
• ld/link.c & ld/merge.c: These hold the core linking logic: symbol resolution, relocation processing, and section merging. Start here once you’re oriented.

2. Start with the Entry Point & a Debug Build #

The linker’s main entry point is ld/ldmain.c in the main() function. Tracing from here will let you follow the high-level flow: parsing command-line args, initializing the x86_64 target, processing input files, and generating the output binary.

First, build ld with debug symbols to make debugging easier:

cd binutils-x.y.z
./configure --target=x86_64-linux-gnu --enable-debug
make -j$(nproc)

The --enable-debug flag ensures you can step through the code with gdb without losing context.

3. Tie Linkers&Loaders Concepts to the Source #

Leverage the book you’re reading to guide your exploration—map chapters directly to source files:

• When you hit symbol resolution and relocation (Chapter 4 in most editions), dive into ld/symtab.c (symbol table management) and ld/reloc.c (relocation handling for all architectures, including x86_64-specific cases).
• For linker scripts (Chapter 7), look at ld/script.c and ld/parse.c—these handle parsing both user-provided scripts and the default templates.
• When studying output file generation (Chapter 5), check ld/output.c and ld/sections.c—they manage section merging, address assignment, and writing the final ELF executable.

4. Debug Small, Simple Test Cases #

Don’t try to trace a complex program’s link process first—start tiny:

1. Write a minimal C program:

   // test.c
   int main() { return 0; }
   

2. Compile it to an object file, then link it with your custom-built ld:

   gcc -c test.c -o test.o
   ./binutils-x.y.z/ld/ld -o test test.o /lib/x86_64-linux-gnu/crt1.o /lib/x86_64-linux-gnu/crti.o /lib/x86_64-linux-gnu/crtn.o -lc
   

3. Fire up gdb to trace the linker’s execution:

   gdb --args ./binutils-x.y.z/ld/ld -o test test.o /lib/x86_64-linux-gnu/crt1.o /lib/x86_64-linux-gnu/crti.o /lib/x86_64-linux-gnu/crtn.o -lc
   

   Set a breakpoint at main() and step through. Watch how ld loads each input file, parses their symbol tables, resolves references, applies relocations, and stitches everything into the final executable.

5. Zero in on x86_64-Specific Logic #

Most linker code is architecture-agnostic, but x86_64 has unique behaviors:

• Check ld/emulparams/elf_x86_64.sh for how the linker sets up x86_64-specific memory layouts and default scripts.
• Search for x86_64 relocation types (like R_X86_64_32 or R_X86_64_RELATIVE) in ld/reloc.c—you’ll find branches that handle these architecture-specific relocation operations.

6. Lean on Source Comments & Built-In Docs #

Don’t overlook the comments in the source code—core functions almost always have a header comment explaining their purpose and flow. Also, check the doc/ld.texi file in the binutils source tree; it’s the official linker documentation, and while it’s not deep into source details, it’ll help you understand the linker’s high-level architecture.

内容的提问来源于stack exchange，提问作者uma sraz