Mach-O File Format

What is a Mach-O file?

A Mach-O file (Mach Object) is the native executable file format used by macOS, iOS, watchOS, and tvOS. It serves as the container format for executables, object files, shared libraries, dynamically-loaded code, and core dumps on Apple's operating systems based on the Mach kernel.

File Extensions

• Usually no extension (executables)
• .dylib (dynamic libraries)
• .bundle (loadable bundles)
• .o (object files)

MIME Type

• application/x-mach-binary

History and Development

Mach-O was originally developed by Carnegie Mellon University as part of the Mach microkernel project in the 1980s. Apple adopted and adapted the format when they based Mac OS X on the Mach kernel and BSD userland. The format has evolved significantly since its adoption by Apple, with major updates accompanying new processor architectures and operating system features.

Key Milestones

• 1985: Original Mach-O format developed at CMU
• 2001: Adopted by Apple for Mac OS X
• 2005: Extended for Intel x86 transition (Universal Binaries)
• 2007: iOS adoption with ARM support
• 2012: 64-bit support for x86_64 and ARM64
• 2020: Apple Silicon (M1) support with ARM64 optimizations

Technical Specifications

File Structure

Mach-O files consist of three main regions:

1. Header: Contains metadata about the file
2. Load Commands: Describe file segments and dynamic linking information
3. Data: Contains actual code, data, and resources

Header Structure

struct mach_header_64 {
    uint32_t magic;        // Magic number (0xfeedfacf for 64-bit)
    cpu_type_t cputype;    // Target CPU type
    cpu_subtype_t cpusubtype; // Target CPU subtype
    uint32_t filetype;     // File type (executable, library, etc.)
    uint32_t ncmds;        // Number of load commands
    uint32_t sizeofcmds;   // Size of load commands
    uint32_t flags;        // File flags
    uint32_t reserved;     // Reserved field
};

CPU Types

• x86: Intel 32-bit processors
• x86_64: Intel 64-bit processors
• ARM: ARM 32-bit processors (iOS devices)
• ARM64: ARM 64-bit processors (Apple Silicon, modern iOS devices)
• PowerPC: Legacy PowerPC processors (pre-Intel Macs)

File Types

Executable Types

1. MH_EXECUTE: Main executable files
2. MH_DYLIB: Dynamic shared libraries
3. MH_BUNDLE: Loadable plugins and bundles
4. MH_OBJECT: Relocatable object files
5. MH_CORE: Core dump files
6. MH_DYLINKER: Dynamic linker

Load Commands

• LC_SEGMENT_64: Defines memory segments
• LC_LOAD_DYLIB: Specifies required dynamic libraries
• LC_MAIN: Entry point for executables
• LC_CODE_SIGNATURE: Code signing information
• LC_ENCRYPTION_INFO: Encryption metadata
• LC_VERSION_MIN: Minimum OS version requirements

Segments and Sections

Common Segments

• __TEXT: Executable code and read-only data
• __DATA: Initialized writable data
• __LINKEDIT: Linking and debugging information
• __PAGEZERO: Unmapped memory for null pointer protection

Important Sections

__TEXT,__text        // Executable code
__TEXT,__cstring     // C string literals
__TEXT,__const       // Constant data
__DATA,__data        // Initialized variables
__DATA,__bss         // Uninitialized variables
__DATA,__objc_*      // Objective-C runtime data

Universal Binaries

Fat Binaries

Universal Binaries (Fat Binaries) combine multiple architecture versions in a single file:

struct fat_header {
    uint32_t magic;     // FAT_MAGIC (0xcafebabe)
    uint32_t nfat_arch; // Number of architectures
};

struct fat_arch {
    cpu_type_t cputype;    // CPU type
    cpu_subtype_t cpusubtype; // CPU subtype
    uint32_t offset;       // File offset to architecture
    uint32_t size;         // Size of architecture slice
    uint32_t align;        // Alignment requirement
};

Advantages

• Single Distribution: One file works on multiple architectures
• Optimal Performance: Native code for each architecture
• Seamless Transition: Supports architecture migrations

Dynamic Linking

Library Loading

Mach-O supports sophisticated dynamic linking:

• Two-level namespaces: Libraries can have internal symbols
• Lazy loading: Functions loaded on first use
• Weak linking: Optional library dependencies
• Re-exports: Libraries can re-export other libraries

Symbol Resolution

# View dynamic library dependencies
otool -L /usr/bin/ls

# Examine symbol table
nm -g /usr/lib/libSystem.dylib

# Display load commands
otool -l /bin/ls

Security Features

Code Signing

• Digital Signatures: Verify code integrity and authenticity
• Entitlements: Define app capabilities and permissions
• Hardened Runtime: Additional security restrictions

Address Space Layout Randomization (ASLR)

• PIE (Position Independent Executable): Randomized load addresses
• Stack Canaries: Buffer overflow protection
• DEP (Data Execution Prevention): Non-executable stack and heap

System Integrity Protection (SIP)

• Protected Locations: System files and directories
• Code Signing Requirements: All system binaries must be signed
• Runtime Protections: Prevent modification of critical processes

Development Tools

Apple Development Tools

• Xcode: Integrated development environment
• clang/LLVM: Compiler toolchain
• ld: Linker for creating Mach-O files
• lipo: Tool for manipulating Universal Binaries

Analysis Tools

• otool: Object file display tool
• nm: Symbol table viewer
• strings: Extract string literals
• file: Identify file types and architectures
• codesign: Code signing utility

Third-party Tools

• Hopper Disassembler: Reverse engineering tool
• class-dump: Extract Objective-C class information
• MachOView: GUI hex editor for Mach-O files
• jtool: Advanced Mach-O analysis tool

Reverse Engineering

Static Analysis

# Basic file information
file /bin/ls
otool -h /bin/ls

# Disassemble specific sections
otool -t -V /bin/ls

# Extract strings
strings /bin/ls

# Analyze dependencies
otool -L /bin/ls

Dynamic Analysis

• LLDB: Apple's debugger
• dtrace: Dynamic tracing framework
• Instruments: Performance analysis tools
• Console.app: System log analysis

Common Use Cases

Application Development

• macOS Applications: Desktop software for Mac
• iOS Applications: Mobile apps for iPhone and iPad
• System Services: Background daemons and services
• Command-line Tools: Terminal utilities and scripts

System Programming

• Kernel Extensions: Low-level system drivers
• Framework Development: Shared libraries and frameworks
• Plugin Development: Loadable bundles and extensions
• Performance Optimization: Native code libraries

Best Practices

Development Guidelines

1. Code Signing: Always sign production binaries
2. Universal Binaries: Support multiple architectures when needed
3. Weak Linking: Use for optional features and backwards compatibility
4. Symbol Visibility: Hide internal symbols to reduce binary size

Security Recommendations

• Enable Hardened Runtime: For enhanced security
• Use Entitlements: Minimize required permissions
• Regular Updates: Keep development tools current
• Static Analysis: Use security analysis tools

Performance Optimization

• Link-time Optimization: Enable LTO for release builds
• Strip Debug Information: Reduce binary size for distribution
• Profile-guided Optimization: Use profiling data for optimization
• Minimize Dependencies: Reduce startup time and memory usage

Debugging and Troubleshooting

Common Issues

• Architecture Mismatches: Incompatible CPU types
• Missing Libraries: Unresolved dynamic dependencies
• Code Signing Errors: Invalid or expired signatures
• Entitlement Problems: Insufficient permissions

Diagnostic Commands

# Check architecture compatibility
arch -arch arm64 /path/to/binary

# Verify code signature
codesign -v -v /path/to/binary

# Examine crash reports
Console.app or ~/Library/Logs/DiagnosticReports/

Mach-O represents Apple's sophisticated approach to executable file formats, providing the foundation for all software running on Apple's platforms. Understanding its structure and capabilities is essential for macOS and iOS development, system administration, and security analysis.