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rev-struct - Structure Recovery

Recover data structure definitions by analyzing memory access patterns in functions and their call chains.

Adapted from P4nda0s/reverse-skills (MIT) for the Ghidra MCP toolchain (upstream targets IDA; the analysis methodology is unchanged).

Pre-check

This skill operates on decompiled native code. In this agent the toolchain is Ghidra MCP (ghidra_* tools):

1. Target .so not yet imported → adb pull it from the device, then ghidra_import_file (ELF auto-detects; explicit IDs: ARM64 AARCH64:LE:64:v8A, ARM32 ARM:LE:32:v7).
2. Query the loaded instance through Ghidra MCP: decompile functions by address/name, get callers/callees, inspect data types.
3. Exact tool names depend on the loaded tool groups — list or search them with ghidra_list_tool_groups / ghidra_search_tools when unsure.

If no Ghidra instance is connected, check ghidra_list_instances and connect with ghidra_connect_instance, or ask the user to start one.

Structure Recovery Steps

Step 1: Read Target Function

1. Decompile the function at the user-provided address/name via Ghidra MCP
2. Note its callers and callees (cross-references)
3. Identify pointer parameters in the function (potential structure pointers)

Step 2: Collect Memory Access Patterns

Search for the following patterns in the target function:

Direct offset access:

*(a1 + 0x10)           // offset 0x10
*(_DWORD *)(a1 + 8)    // offset 0x8, DWORD type
*(_QWORD *)(a1 + 0x20) // offset 0x20, QWORD type
*(_BYTE *)(a1 + 4)     // offset 0x4, BYTE type

Array access:

*(a1 + 8 * i)          // array, element size 8 bytes
a1[i]                  // array access

Nested structures:

*(*a1 + 0x10)          // first field of struct pointed by a1 is a pointer

Record format:

offset=0x00, size=8, access=read/write, type=QWORD
offset=0x08, size=4, access=read, type=DWORD
...

Step 3: Traverse Callers for Analysis

Decompile each caller (start from direct callers; expand one hop at a time and stop once the offset pattern stops changing) and analyze:

1. Parameter passing: What is passed when calling?
   `c
   sub_401000(v1); // v1 might be a struct pointer
   sub_401000(&v2); // v2 is a struct
   sub_401000(malloc(64)); // struct size is ~64 bytes
   `

2. Operations before/after the call:
   `c
   v1 = malloc(0x40); // allocate 0x40 bytes
   *v1 = 0; // offset 0x00 initialization
   *(v1 + 8) = callback; // offset 0x08 is a function pointer
   sub_401000(v1);
   `

3. Collect more offset accesses

Step 4: Traverse Callees for Analysis

Decompile each callee (direct callees first, same one-hop-at-a-time rule) and analyze:

1. How parameters are used:
   `c
   // In callee
   int callee(void *a1) {
   return *(a1 + 0x18); // accesses offset 0x18
   }
   `

2. Passed to other functions:
   `c
   another_func(a1 + 0x20); // offset 0x20 might be a nested struct
   `

Step 5: Aggregate and Infer

1. Merge all offset information, sort by offset
2. Calculate struct size: max(offset) + lastfieldsize

3. Infer field types:

   • Called as function pointer → function pointer
   • Passed to strlen/printf → string pointer
   • Compared with constants → enum/flags
   • Increment/decrement operations → counter/index

4. Identify common patterns:

   • Offset 0 is a function pointer table → vtable (C++ object)
   • next/prev pointers → linked list node
   • refcount field → reference counted object

Step 6: Apply the Struct in Ghidra

Define the recovered struct through Ghidra MCP datatype tools (create struct + fields at the recovered offsets), then re-decompile the functions — the decompiler now renders field names instead of raw offsets, and inconsistent accesses expose wrong guesses. Iterate until the view is consistent.

Output Format

/*
 * Structure Recovery Analysis
 * Source function: <func_address>
 * Analysis scope: <number of callers/callees analyzed>
 *
 * Functions using this struct:
 *   - 0x401000 (initialization)
 *   - 0x401100 (field access)
 *   - 0x401200 (destruction)
 */

// Estimated size: 0x48 bytes
// Confidence: High / Medium / Low

struct suggested_name {
    /* 0x00 */ void *vtable;           // vtable pointer, called: (*(*this))()
    /* 0x08 */ int refcount;           // reference count, has ++/-- operations
    /* 0x0C */ int flags;              // flags, AND with 0x1, 0x2
    /* 0x10 */ char *name;             // string, passed to strlen/printf
    /* 0x18 */ void *data;             // data pointer
    /* 0x20 */ size_t size;            // size field
    /* 0x28 */ struct node *next;      // linked list next pointer
    /* 0x30 */ struct node *prev;      // linked list prev pointer
    /* 0x38 */ callback_fn handler;    // callback function
    /* 0x40 */ void *user_data;        // user data
};

// Field access examples:
// 0x401000: *(this + 0x08) += 1;     // refcount++
// 0x401100: printf("%s", *(this + 0x10));  // print name