amiga-bootcamp/05_reversing/dynamic/setfunction_patching.md

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SetFunction — Hooking Library Vectors at Runtime

Overview

SetFunction() is the official AmigaOS mechanism for patching a library's JMP table at runtime. It installs a custom function at a given LVO, replacing the original, and returns the old function pointer so a trampoline can be constructed.

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SetFunction() API

/* exec/execbase.h */
APTR SetFunction(struct Library *library, LONG funcOffset, APTR newFunction);
/* Returns: pointer to OLD function */

• library — target library base (e.g., DOSBase)
• funcOffset — negative LVO offset (e.g., -30 for dos.library Open)
• newFunction — your replacement function

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Installing a Hook

; Example: hook dos.library Write() at LVO -48

    MOVEA.L  _SysBase, A6
    JSR      (-120,A6)          ; Forbid() — prevent preemption during patch

    MOVEA.L  _DOSBase, A1
    MOVE.L   #-48, A0           ; LVO for Write
    LEA      _my_write(PC), A2
    JSR      (-420,A6)          ; SetFunction(DOSBase, -48, &my_write)
    MOVE.L   D0, _orig_write    ; save original function pointer

    JSR      (-126,A6)          ; Permit()

C equivalent:

static APTR orig_write;

void install_hook(void) {
    Forbid();
    orig_write = SetFunction((struct Library *)DOSBase, -48,
                             (APTR)my_write_hook);
    Permit();
}

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Writing a Trampoline

The hook function must:

1. Perform its instrumentation
2. Call the original via the saved pointer
3. Return with the original return value in D0

_my_write:
    ; D1 = file handle, D2 = buffer, D3 = length (Write args)
    MOVEM.L  D0-D7/A0-A6, -(SP)   ; save all (we may corrupt anything)

    ; ... instrumentation: log args, patch buffer, etc. ...

    MOVEM.L  (SP)+, D0-D7/A0-A6
    MOVEA.L  _orig_write, A0
    JMP      (A0)                  ; jump to original — not JSR; let original RTS

In C (with __asm constraints):

LONG __asm my_write_hook(register __d1 BPTR fh,
                          register __d2 APTR buf,
                          register __d3 LONG len) {
    /* instrumentation */
    return ((LONG (*)(BPTR,APTR,LONG))orig_write)(fh, buf, len);
}

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Restoring on Exit

Critical: Always restore the original function before the program exits. Failure leaves a dangling pointer in the library JMP table, causing crashes for any subsequent users of the library.

void remove_hook(void) {
    Forbid();
    SetFunction((struct Library *)DOSBase, -48, orig_write);
    Permit();
}

/* Register with atexit: */
atexit(remove_hook);

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Thread Safety Considerations

• Forbid() / Permit() disable task switching — keep the window minimal
• If the hook itself calls OS functions, use Disable() / Enable() instead only when interrupts must be excluded
• Hooks are system-global — all tasks using the library will go through your hook

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Common Use Cases in RE

Use	Hook	LVO
Trace file access	dos.library Open	−30
Intercept writes	dos.library Write	−48
Monitor memory allocation	exec.library AllocMem	−198
Log task creation	exec.library AddTask	−282
Spy on library opens	exec.library OpenLibrary	−552

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References

• NDK39: exec/execbase.h
• ADCD 2.1: SetFunction autodoc
• live_memory_probing.md — SysBase structure access
• Amiga ROM Kernel Reference Manual: Libraries — SetFunction chapter