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132 lines
3.3 KiB
Markdown
132 lines
3.3 KiB
Markdown
[← Home](../../README.md) · [Reverse Engineering](../README.md)
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# Live Memory Probing
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## Overview
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Live memory probing on a running Amiga means directly reading exec structures — `SysBase`, `LibList`, `TaskReady`, `MemList` — to observe system state without a traditional debugger.
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---
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## SysBase: The Root of Everything
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`SysBase` is always at absolute address `$4` (a pointer to the `ExecBase` structure):
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```c
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struct ExecBase *SysBase = *((struct ExecBase **)4);
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printf("exec version: %d.%d\n",
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SysBase->LibNode.lib_Version,
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SysBase->LibNode.lib_Revision);
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```
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In assembly:
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```asm
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MOVEA.L 4.W, A6 ; A6 = SysBase (exec.library base)
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MOVE.W ($16,A6), D0 ; lib_Version
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MOVE.W ($18,A6), D1 ; lib_Revision
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```
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---
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## Walking the Library List
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```c
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struct Node *n = SysBase->LibList.lh_Head;
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while (n->ln_Succ != NULL) {
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struct Library *lib = (struct Library *)n;
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printf("%-30s v%d.%d opens=%d\n",
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lib->lib_Node.ln_Name,
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lib->lib_Version, lib->lib_Revision,
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lib->lib_OpenCnt);
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n = n->ln_Succ;
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}
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```
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This enumerates all currently loaded libraries. Useful for:
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- Finding if a target library is loaded
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- Reading `lib_OpenCnt` to detect if your hook is installed
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- Checking `lib_Flags & LIBF_DELEXP` (expunge pending)
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---
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## Reading `lib_OpenCnt` Live
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```c
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/* Check if bsdsocket.library is loaded and its open count */
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struct Library *base = FindName(&SysBase->LibList, "bsdsocket.library");
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if (base) {
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printf("bsdsocket: OpenCnt=%d, Version=%d\n",
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base->lib_OpenCnt, base->lib_Version);
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}
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```
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`FindName` scans `ln_Name` in a linked list — it is an exec function at LVO −276.
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---
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## Memory Region Map
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`SysBase->MemList` lists all memory regions:
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```c
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struct MemHeader *mh = (struct MemHeader *)SysBase->MemList.lh_Head;
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while (mh->mh_Node.ln_Succ) {
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printf("Region: %s %08lx–%08lx free=%ld\n",
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mh->mh_Node.ln_Name,
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(ULONG)mh->mh_Lower,
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(ULONG)mh->mh_Upper,
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mh->mh_Free);
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mh = (struct MemHeader *)mh->mh_Node.ln_Succ;
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}
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```
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Output example:
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```
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Region: chip memory $000000–$1FFFFF free=524288
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Region: fast memory $200000–$9FFFFF free=6291456
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```
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---
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## Task List Inspection
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```c
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/* Running tasks: */
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Forbid();
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struct Task *t = (struct Task *)SysBase->TaskReady.lh_Head;
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while (t->tc_Node.ln_Succ) {
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printf("Task: %-20s pri=%d state=%d\n",
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t->tc_Node.ln_Name,
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t->tc_Node.ln_Pri,
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t->tc_State);
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t = (struct Task *)t->tc_Node.ln_Succ;
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}
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Permit();
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```
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`Forbid()` / `Permit()` are mandatory — the task list must not change while walking it.
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---
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## Patching Memory Live (Surgical Writes)
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For RE/patching: direct longword write to an OS structure:
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```c
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/* Example: force a library's version to 99 */
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Forbid();
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target_lib->lib_Version = 99;
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Permit();
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```
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> [!CAUTION]
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> Direct memory writes to OS structures bypass all synchronization. Always use `Forbid()` at minimum; use `Disable()` if modifying interrupt-visible data.
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---
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## References
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- NDK39: `exec/execbase.h`, `exec/memory.h`, `exec/tasks.h`
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- [exec_base.md](../../06_exec_os/exec_base.md) — full ExecBase offset table
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- [memory_management.md](../../06_exec_os/memory_management.md) — MemHeader structure
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- [setfunction_patching.md](setfunction_patching.md) — Forbid/Permit patterns
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