Class: Omnizip::Filters::BcjArm

Inherits:
FilterBase
  • Object
show all
Defined in:
lib/omnizip/filters/bcj_arm.rb

Overview

BCJ filter for 32-bit ARM executables.

This filter preprocesses ARM machine code by converting relative addresses in BL (Branch and Link - 0xEB) instructions to absolute addresses. ARM uses 4-byte aligned instructions with little-endian encoding.

The filter improves compression by making branch targets position-independent. The offset in ARM BL instructions is stored as word offset (divided by 4), and is relative to PC+8.

Constant Summary collapse

OPCODE_BL =

ARM BL (Branch and Link) opcode

0xEB
INSTRUCTION_SIZE =

Size of ARM instruction (4 bytes, little-endian)

4
OFFSET_MASK =

Offset mask (24-bit offset in BL instruction)

0x00FFFFFF

Class Method Summary collapse

Instance Method Summary collapse

Class Method Details

.metadata ⇒ Hash

Get metadata about this filter.

Returns:

  • (Hash) —

    Filter metadata



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# File 'lib/omnizip/filters/bcj_arm.rb', line 133

def metadata
  {
    name: "BCJ-ARM",
    description: "Branch converter for 32-bit ARM executables",
    architecture: "ARM (32-bit)",
    alignment: 4,
    endian: "little",
  }
end

Instance Method Details

#decode(data, position = 0) ⇒ String

Decode (postprocess) ARM executable data after decompression.

Reverses the encoding by converting absolute word offsets back to relative word offsets.

Parameters:

  • data (String) —

    Binary executable data

  • position (Integer) (defaults to: 0) —

    Current stream position

Returns:

  • (String) —

    Decoded binary data



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# File 'lib/omnizip/filters/bcj_arm.rb', line 93

def decode(data, position = 0)
  return data.dup if data.bytesize < INSTRUCTION_SIZE

  result = data.b
  size = data.bytesize & ~(INSTRUCTION_SIZE - 1)
  i = 0
  # PC starts at position + 4 (as per C implementation)
  pc = position + 4

  while i < size
    # Advance PC to current instruction position
    current_pc = pc + i

    # Check if last byte is 0xEB (BL instruction)
    if result.getbyte(i + 3) == OPCODE_BL
      # Extract full 32-bit instruction value
      instruction = extract_uint32_le(result, i)

      # Calculate word offset from PC
      word_offset = current_pc >> 2

      # Subtract word offset from instruction value
      instruction -= word_offset

      # Mask to 24-bit and combine with opcode
      instruction = (instruction & OFFSET_MASK) | 0xEB000000

      write_uint32_le(result, i, instruction)
    end

    i += INSTRUCTION_SIZE
  end

  result
end

#encode(data, position = 0) ⇒ String

Encode (preprocess) ARM executable data for compression.

Scans for BL (0xEB) opcodes and converts relative word offsets to absolute word offsets. ARM branch offset is relative to PC+8.

Parameters:

  • data (String) —

    Binary executable data

  • position (Integer) (defaults to: 0) —

    Current stream position

Returns:

  • (String) —

    Encoded binary data



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# File 'lib/omnizip/filters/bcj_arm.rb', line 49

def encode(data, position = 0)
  return data.dup if data.bytesize < INSTRUCTION_SIZE

  result = data.b
  size = data.bytesize & ~(INSTRUCTION_SIZE - 1)
  i = 0
  # PC starts at position + 4 (as per C implementation)
  pc = position + 4

  while i < size
    # Advance PC to current instruction position
    current_pc = pc + i

    # Check if last byte is 0xEB (BL instruction)
    if result.getbyte(i + 3) == OPCODE_BL
      # Extract full 32-bit instruction value
      instruction = extract_uint32_le(result, i)

      # Calculate word offset from PC
      word_offset = current_pc >> 2

      # Add word offset to instruction value
      instruction += word_offset

      # Mask to 24-bit and combine with opcode
      instruction = (instruction & OFFSET_MASK) | 0xEB000000

      write_uint32_le(result, i, instruction)
    end

    i += INSTRUCTION_SIZE
  end

  result
end