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142 lines
3.7 KiB
Markdown
142 lines
3.7 KiB
Markdown
[← Home](../../README.md) · [Reverse Engineering](../README.md)
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# Reconstructing Library JMP Tables
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## Overview
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Every AmigaOS library has a **JMP table** at negative offsets from its base pointer. Reconstructing this table maps LVOs to function names and is essential for identifying all OS calls made by a binary under analysis.
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---
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## JMP Table Layout
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```
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lib_base - N*6: JFF xxxx xxxx ; JMP to function N (6 bytes)
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...
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lib_base - 24: JMP Reserved()
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lib_base - 18: JMP Expunge()
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lib_base - 12: JMP Close()
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lib_base - 6: JMP Open()
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lib_base + 0: struct Library ; lib_Node, lib_Version, ...
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```
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Each entry is a 68k `JMP (abs.l)` — opcode `4EF9` followed by a 4-byte absolute address, totalling 6 bytes. Hence LVO = `−6 × slot_index`.
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---
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## Finding the Library Base
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### From SysBase LibList
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The `exec.library` maintains a doubly-linked list at `SysBase→LibList`:
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```c
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struct ExecBase {
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...
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struct List LibList; /* offset +378 — list of open libraries */
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...
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};
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/* Walk the list: */
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struct Node *n = SysBase->LibList.lh_Head;
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while (n->ln_Succ) {
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struct Library *lib = (struct Library *)n;
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printf("%s v%d\n", lib->lib_Node.ln_Name, lib->lib_Version);
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n = n->ln_Succ;
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}
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```
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### In IDA Pro
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After loading, `SysBase` is at `$4`. Use `Edit → Segments → Create Segment` pointed at `$4` with type `WORD` to follow the pointer to `ExecBase`. Then navigate to `LibList` at offset `+0x17A` and walk the linked list.
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---
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## Reading the JMP Table in IDA
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1. Know the library base address (e.g., `DOSBase` from the `OpenLibrary` result)
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2. Navigate to `lib_base - 6` — first user function slot
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3. IDA shows `JMP sub_XXXXXX` — the target is the actual function implementation
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4. Rename each `sub_` with the function name from the LVO table
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### Automated Script: `apply_lvo_names.py`
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```python
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import idaapi, idc
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LVO_DOS = {
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-30: "Open", # LVO -30 = Open(name, mode) d1/d2
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-36: "Close",
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-42: "Read",
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-48: "Write",
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-54: "Input",
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-60: "Output",
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-126: "WaitForChar",
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-138: "Delay",
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# ... extend from dos_lib.fd
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}
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DOS_BASE = idc.get_name_ea_simple("_DOSBase")
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dos_ptr = idc.get_wide_dword(DOS_BASE)
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for lvo, name in LVO_DOS.items():
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jmp_entry = dos_ptr + lvo
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# read the JMP target: opcode at jmp_entry is 4EF9, target at +2
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target = idc.get_wide_dword(jmp_entry + 2)
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idc.set_name(target, f"dos_{name}", idaapi.SN_NOWARN)
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print(f"LVO {lvo:+d}: {name} → {target:#010x}")
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```
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---
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## Mapping LVO → Function via `.fd` Files
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NDK39 `.fd` files define the exact register assignments and bias (LVO offset):
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```
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## NDK39/fd/dos_lib.fd (excerpt)
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##base _DOSBase
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##bias 30
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##public
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Open(name,accessMode)(d1,d2)
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##bias 36
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Close(file)(d1)
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##bias 42
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Read(file,buffer,length)(d1,d2,d3)
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##bias 48
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Write(file,buffer,length)(d1,d2,d3)
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```
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The `##bias` value **is** the positive LVO — the actual call offset is `−bias`.
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---
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## JSR −LVO(A6) Pattern in Disassembly
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```asm
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; Typical OS call site in disassembly:
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MOVEA.L (_DOSBase).L, A6
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JSR (-30,A6) ; Open(d1=name, d2=mode)
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; D0 = file handle (BPTR) or 0 on error
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```
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In IDA, this appears as `jsr ($fffffffe2,a6)` with displacement `-30` (`$FFFFFFE2` in two's complement 16-bit). Applying LVO names makes this `jsr (Open,a6)`.
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---
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## Common Library Bases and LVO Tables
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See [`../../../[lvo_table.md](../../04_linking_and_libraries/lvo_table.md)](../../../04_linking_and_libraries/lvo_table.md) for complete LVO offset tables for:
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- `exec.library`
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- `dos.library`
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- `graphics.library`
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- `intuition.library`
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---
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## References
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- NDK39: `fd/` directory — all library `.fd` files
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- [lvo_table.md](../../04_linking_and_libraries/lvo_table.md)
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- ADCD 2.1: `Libraries_Manual_guide/`
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- IDA Pro scripting: `idc.py` reference
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