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125 lines
3.6 KiB
Markdown
125 lines
3.6 KiB
Markdown
[← Home](../README.md) · [Exec Kernel](README.md)
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# Library Vectors — JMP Table, LVOs, MakeFunctions, SetFunction
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## Overview
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Every AmigaOS library exposes its functions via a **JMP table** at negative offsets from the library base. This document covers the structure of the table, how LVOs are assigned, and how to create or patch one programmatically.
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---
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## JMP Table Structure
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```
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Address Content Description
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lib_base - N×6: 4EF9 XXXXXXXX JMP <absolute address> ← function N
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...
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lib_base - 24: 4EF9 XXXXXXXX JMP Reserved
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lib_base - 18: 4EF9 XXXXXXXX JMP Expunge
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lib_base - 12: 4EF9 XXXXXXXX JMP Close
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lib_base - 6: 4EF9 XXXXXXXX JMP Open
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lib_base + 0: struct Library ← pointer returned by OpenLibrary
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```
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Each slot is exactly **6 bytes**: opcode `$4EF9` (JMP abs.l) + 4-byte target address.
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---
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## LVO Formula
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```
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LVO = −6 × slot_index
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where slot_index counts from 1 (Open) upward:
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Open = slot 1 → LVO = −6
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Close = slot 2 → LVO = −12
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Expunge = slot 3 → LVO = −18
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Reserved = slot 4 → LVO = −24
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First user fn = slot 5 → LVO = −30
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Second user fn = slot 6 → LVO = −36
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...
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```
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The `.fd` file `##bias` value is the positive LVO: `bias 30` → LVO `−30`.
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---
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## MakeFunctions — Building a JMP Table
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`exec.library MakeFunctions()` fills in the JMP table from a function pointer array:
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```c
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ULONG MakeFunctions(APTR targetLib, APTR funcArray, APTR funcDispBase);
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```
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Typical usage in library `InitLib`:
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```asm
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; funcArray: table of function pointers, terminated by -1
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_LibFuncTable:
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dc.l _LibOpen
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dc.l _LibClose
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dc.l _LibExpunge
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dc.l _LibNull ; Reserved — returns NULL
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dc.l _MyFunc1
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dc.l _MyFunc2
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dc.l -1 ; terminator
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LibInit:
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LEA _LibFuncTable(PC), A0
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MOVEA.L A6, A1 ; library base (passed in A6 by exec)
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MOVEQ #0, D0 ; funcDispBase = 0 (absolute addresses)
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MOVEA.L 4.W, A6
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JSR (-420,A6) ; MakeFunctions(A1, A0, D0)
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```
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`MakeFunctions` writes `JMP <ptr>` for each entry, filling the table downward from `lib_base − 6`.
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---
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## SetFunction — Patching a Single Slot
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```c
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APTR SetFunction(struct Library *library, LONG funcOffset, APTR newFunction);
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```
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- `funcOffset` is the negative LVO (e.g., `−30` for the first user function)
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- Returns the old function pointer
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```c
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/* Hook dos.library Write() */
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old_write = SetFunction((struct Library *)DOSBase, -48, my_write_hook);
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```
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See [setfunction_patching.md](../05_reversing/dynamic/setfunction_patching.md) for trampoline patterns.
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---
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## Checksum Maintenance
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After `MakeFunctions` or `SetFunction`, exec updates `lib_Sum` via `SumLibrary`:
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```c
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SumLibrary((struct Library *)myLib);
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```
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If `LIBF_SUMUSED` is set, exec verifies the checksum at `CloseLibrary` time. Patching the JMP table without calling `SumLibrary` will trigger a checksum failure (alert box or guru).
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---
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## Viewing Vectors in IDA Pro
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1. Navigate to `lib_base − 6` (first standard vector)
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2. Each 6-byte group: opcode `4EF9` + 4-byte address
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3. Press `C` to disassemble if not auto-detected
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4. The 4-byte value is the actual function address — press `G` (Go to) to navigate
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5. Name each function with the `.fd` file as reference
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---
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## References
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- NDK39: `exec/execbase.h`, `exec/libraries.h`
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- ADCD 2.1: `MakeFunctions`, `MakeLibrary`, `SetFunction`, `SumLibrary`
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- [library_jmp_table.md](../05_reversing/static/library_jmp_table.md) — reconstruction workflow
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- [lvo_table.md](../04_linking_and_libraries/lvo_table.md) — complete LVO reference tables
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