rcrt1

Overview

rcrt1 is a Rust library developed by the Enarx project for relocating ELF dynamic symbols. It serves as a replacement for the traditional C runtime (crt1) and handles the crucial task of properly loading and relocating ELF binaries at runtime, specifically focusing on relocating dynamic symbols.

Core Functionality

• Dynamic Relocation: Provides functionality to relocate ELF dynamic symbols in a static Position Independent Executable (PIE)
• No Standard Library: Operates as a no_std crate, making it suitable for system-level programming without relying on the Rust standard library
• ELF Support: Specifically handles ELF64 binaries using the Goblin library for parsing ELF structures
• x86_64 Focus: Primarily targets x86_64 architecture, with specific handling for x86_64 relative relocations

Key Components

1. Dynamic Symbol Relocation: The dyn_reloc function handles the relocation of dynamic symbols by applying base address offsets
2. Runtime Initialization: The rcrt function serves as a replacement for the traditional crt1.o, handling initial program setup from the stack pointer
3. Startup Macro: Provides a convenient x86_64_linux_startup! macro for defining the program entry point

Technical Details

• Handles both REL and RELA relocation types for ELF binaries
• Works with Position Independent Executables (PIEs)
• Requires specific compiler optimizations in the binary's Cargo.toml for correct operation
• Manages program headers and the _DYNAMIC section to calculate load offsets
• Uses naked functions and assembly for low-level operations

Usage Requirements

When using this crate, specific compiler optimizations must be set in the binary's Cargo.toml:

[profile.dev.package.rcrt1]
opt-level = 3
debug-assertions = false
overflow-checks = false