rev-struct

SKILL.md

rev-struct - Structure Recovery

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

Pre-check

Determine which IDA access method is available:

Option A — IDA Pro MCP (preferred if connected):
Check if the IDA Pro MCP server is connected (look for an active ida-pro or equivalent MCP connection). If connected, you can query IDA directly via MCP tools — no exported files needed. Proceed with the analysis using MCP.

Option B — IDA-NO-MCP exported data:
If MCP is not connected, check if IDA-NO-MCP exported data exists in the current directory:

Check if decompile/ directory exists

Check if there are .c files inside

If neither MCP nor exported data is available, prompt the user:

No IDA access method detected. Choose one of the following:

Option A — IDA Pro MCP (recommended):
  Connect the IDA Pro MCP server so Claude can query IDA directly.

Option B — IDA-NO-MCP export:
  1. Download plugin: https://github.com/P4nda0s/IDA-NO-MCP
  2. Copy INP.py to IDA plugins directory
  3. Press Ctrl-Shift-E in IDA to export
  4. Open the exported directory with Claude Code

Export Directory Structure

./
├── decompile/              # Decompiled C code directory
│   ├── 0x401000.c          # One file per function, named by hex address
│   ├── 0x401234.c
│   └── ...
├── decompile_failed.txt    # Failed decompilation list
├── decompile_skipped.txt   # Skipped functions list
├── strings.txt             # String table (address, length, type, content)
├── imports.txt             # Import table (address:function_name)
├── exports.txt             # Export table (address:function_name)
└── memory/                 # Memory hexdump (1MB chunks)

Function File Format (decompile/*.c)

Each .c file contains function metadata comments and decompiled code:

/*
 * func-name: sub_401000
 * func-address: 0x401000
 * callers: 0x402000, 0x403000    // List of functions that call this function
 * callees: 0x404000, 0x405000    // List of functions called by this function
 */

int __fastcall sub_401000(int a1, int a2)
{
    // Decompiled code...
}

Structure Recovery Steps

Step 1: Read Target Function

Based on the user-provided address, read decompile/<address>.c

Parse function metadata, extract callers and callees lists

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

Read each caller function and analyze:

Parameter passing: What is passed when calling?

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

Operations before/after the call:

v1 = malloc(0x40);     // allocate 0x40 bytes
*v1 = 0;               // offset 0x00 initialization
*(v1 + 8) = callback;  // offset 0x08 is a function pointer
sub_401000(v1);

Collect more offset accesses

Step 4: Traverse Callees for Analysis

Read each callee function and analyze:

How parameters are used:

// In callee
int callee(void *a1) {
    return *(a1 + 0x18);  // accesses offset 0x18
}

Passed to other functions:

another_func(a1 + 0x20);  // offset 0x20 might be a nested struct

Step 5: Aggregate and Infer

Merge all offset information, sort by offset

Calculate struct size: max(offset) + last_field_size

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

Identify common patterns:

Offset 0 is a function pointer table → vtable (C++ object)

next/prev pointers → linked list node

refcount field → reference counted object

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