Why ELF Files Include PHDR Segment Headers

The PHDR segment in ELF program headers serves a crucial purpose that goes beyond what the ELF header provides: it specifies where the program header table itself will be located in the program’s memory image, not just where it resides in the file. While the ELF header (e_phoff) tells you where to find the program header table in the file, the PHDR segment tells the operating system where to load that table in memory, making it accessible during program execution.

Contents

• Understanding the ELF Header vs PHDR Segment
• Purpose of the PHDR Segment
• Usage During Program Loading
• Practical Purposes in ELF Format
• Implementation Considerations
• Examples and Scenarios

Understanding the ELF Header vs PHDR Segment

The ELF header contains three key fields related to program headers:

• e_phoff: Offset from the beginning of the file where the program header table starts
• e_phentsize: Size of each program header entry in bytes
• e_phnum: Number of program header entries

These fields tell you where to find the program header table in the file and how to parse it. However, they don’t tell you anything about where the program header table should be loaded in memory.

The PHDR segment (with type PT_PHDR) provides this missing information by specifying:

• The memory location where the program header table will reside when loaded
• The size of the program header table in memory
• File offset and other loading parameters

According to the Linux manual page, “PT_PHDR The array element, if present, specifies the location and size of the program header table itself, both in the file and in the memory image of the program.”

Purpose of the PHDR Segment

The PHDR segment serves several important purposes:

1. Memory Layout Specification

The PHDR segment tells the operating system exactly where in memory the program header table should be placed. This is crucial for creating a complete memory image of the program.

2. Self-Referential Information

As noted in k3170makan’s blog, “0x00000006 PHDR - Indicates the beginning of the program header table itself.” The PHDR segment essentially describes its own location in memory.

3. Runtime Accessibility

Once loaded, the program header table may need to be accessible to the running program for introspection or debugging purposes. The PHDR segment ensures this table is properly mapped into memory.

4. Consistency with Other Segments

Just like other segments (LOAD, DYNAMIC, etc.), the program header table itself becomes part of the program’s memory space and needs to be described by the program headers.

Usage During Program Loading

Yes, the PHDR segment is actually used during program loading, but not necessarily by all loaders. Here’s how it works:

Loading Process:

1. The loader reads the ELF header to find the program header table location in the file
2. It processes all program headers, including the PHDR segment
3. For the PHDR segment specifically, it loads the program header table into memory at the specified virtual address
4. The program header table is now accessible at both its file location and its memory location

From the OSDev Wiki: “Copy the segment data from the file offset specified by the p_offset member to the virtual memory address specified by the p_vaddr member.”

When It’s Used:

• Dynamic linkers often use the PHDR segment to quickly locate the program header table
• Debuggers and runtime introspection tools may use it to access program metadata
• Some security tools use it to verify program integrity
• Not strictly required for basic loading, but provides important metadata

Practical Purposes in ELF Format

1. Runtime Program Analysis

The PHDR segment allows programs and tools to access program header information at runtime without needing to parse the file again.

2. Dynamic Linking

Dynamic linkers use the program header table extensively for symbol resolution and relocation. Having it readily available in memory speeds up these operations.

3. Memory Management

The PHDR segment helps the operating system maintain a complete view of the program’s memory layout, including all metadata structures.

4. Debug Support

Debuggers can use the PHDR segment to quickly locate program metadata, making debugging more efficient.

5. Security Applications

Security tools may use the PHDR segment to verify that the program header table is properly loaded and hasn’t been tampered with.

Implementation Considerations

When implementing ELF loading in your operating system, consider these points:

PHDR Segment Processing:

for (int i = 0; i < elf_header->e_phnum; i++) {
    Elf64_Phdr *phdr = &program_headers[i];
    
    switch (phdr->p_type) {
        case PT_LOAD:
            // Load segment into memory
            break;
        case PT_PHDR:
            // This segment describes the program header table itself
            // Load the program header table into memory at phdr->p_vaddr
            break;
        // ... other segment types
    }
}

Memory Mapping:

The PHDR segment typically has the same memory location as where the program header table is actually loaded. This creates a self-referential structure.

Optional Nature:

The PHDR segment is optional. Some ELF files may not include it, but most modern executables and shared libraries do.

Examples and Scenarios

Example PHDR Segment:

From a real ELF file (as seen in reverse engineering examples):

PHDR 0x000034 0x00008034 0x00008034 0x00100 0x00100 R 0x4

This means:

• File offset: 0x34 bytes from file start
• Virtual address: 0x00008034 in memory
• Physical address: 0x00008034 (same as virtual)
• File size: 0x100 bytes
• Memory size: 0x100 bytes
• Flags: Readable only
• Alignment: 4 bytes

Loading Scenario:

1. Your OS reads the ELF header at offset 0
2. Finds e_phoff = 0x34 (program header table location in file)
3. Reads program headers starting at offset 0x34
4. Processes the PHDR segment (type PT_PHDR at index 0)
5. Loads the program header table from file offset 0x34 to memory address 0x00008034
6. The program header table is now accessible in memory at 0x00008034

Why Not Just Use ELF Header?

The ELF header is at a fixed location (start of file), but the program header table can be loaded anywhere in memory. The PHDR segment provides the runtime location, which may be different from the file offset and is essential for programs that need to access their own metadata.

Conclusion

The PHDR segment serves several critical purposes in the ELF format:

1. Memory location specification: It tells the operating system where to load the program header table in memory, which the ELF header alone cannot provide
2. Runtime accessibility: It makes the program header table accessible to the running program for introspection and debugging
3. Complete memory mapping: It ensures all program metadata, including the headers themselves, are properly mapped into the program’s memory space
4. Self-description: The program headers describe their own location, creating a consistent and complete metadata structure

For your OS development, you should process the PHDR segment when loading ELF files, as it provides important information about where the program header table will reside in memory. While basic loading could technically skip it, proper implementation includes loading the PHDR segment to maintain compatibility with standard ELF tools and provide a complete memory image.

Sources

1. Linux manual page - elf(5)
2. Introduction to the ELF Format Part II - Understanding Program Headers
3. Program Header - Linux Base Specifications
4. ELF - OSDev Wiki
5. Oracle - Program Header (Linker and Libraries Guide)
6. Why ELF headers are included in the process memory - Reverse Engineering Stack Exchange