标题: LLVM Pass转储类或结构的内存布局

创建: 2024-12-01 19:55
更新: 2024-12-09 15:05
链接: https://scz.617.cn/unix/202412011955.txt
      https://bbs.kanxue.com/thread-284643.htm

--------------------------------------------------------------------------

目录:

    ☆ 背景介绍
    ☆ dumpclass.cpp
    ☆ dumptarget.cpp
    ☆ 用dumpclass.so处理dumptarget.cpp
    ☆ pahole
    ☆ clang -Xclang -fdump-record-layouts
    ☆ VC有隐藏选项

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☆ 背景介绍

有次因故需要了解std::string类型内存布局，简单折腾一番，分享了一篇

《GDB查看结构或类的内存布局及分离终端》
https://scz.617.cn/unix/202411151604.txt

bluerust随即让我看下面这篇

STL容器逆向与实战 - [2023-02-07]
https://mp.weixin.qq.com/s/bfzeGbieYWaPS3_iB-gSeg

他的原话是，主要看"llvm pass dump data type"。看了这篇，于我而言，属于"每
个字都认识"系列，大概明白其基本原理是啥，但完全不了解所涉及的"LLVM Pass"技
术，看过之后，老虎吃天、无处下爪。我不会C++编程，基本未碰上过C++ STL容器逆
向需求，不在意上文中那些具体容器的实现细节。我感兴趣的是，如何转储类或结构
的内存布局，也就是上文第一部分的内容。原作者有句话，随便简单写个pass来dump，
深深刺激了我，别人随便简单弄的东西，代码都给了，我还是不知如何实践。或许有
些同道遭遇类似囧境，本文面向"LLVM Pass"小白提供完整可操作示例，聚焦"转储内
存布局"，是上文降阶后的狗尾续貂、画蛇添足。

☆ dumpclass.cpp

参看

--------------------------------------------------------------------------
Writing an LLVM Pass (legacy PM version)
https://llvm.org/docs/WritingAnLLVMPass.html

Writing an LLVM Pass
https://llvm.org/docs/WritingAnLLVMNewPMPass.html
--------------------------------------------------------------------------

看雪那篇是Legacy格式的"LLVM Pass"，此处dumpclass.cpp改写成New格式。支持两
个命令行参数，允许成员名中包含相对偏移或绝对偏移，允许过滤类或结构名。

--------------------------------------------------------------------------
#include "llvm/Passes/PassBuilder.h"
#include "llvm/Passes/PassPlugin.h"
#include "llvm/Support/raw_ostream.h"

#define DEFAULTSUBSTR   "<default>"

using namespace llvm;

namespace {

static cl::opt<int> passmode
(
"passmode",
cl::desc("absolute offset or not"),
cl::value_desc("int"),
cl::init(0)
);

static cl::opt<std::string> substr
(
"substr",
cl::desc("part of struct name"),
cl::value_desc("std::string"),
cl::init(DEFAULTSUBSTR)
);

struct DumpClass : PassInfoMixin<DumpClass>
{

    std::string getTypeName ( Type *type, const DataLayout &data )
    {
        if ( type->isIntegerTy() )
        {
            IntegerType    *i   = cast<IntegerType>( type );

            return "uint" + std::to_string( i->getBitWidth() ) + "_t";
        }
        else if ( type->isPointerTy() )
        {
            PointerType    *ptr = cast<PointerType>( type );

            return getTypeName( ptr->getPointerElementType(), data ) + "*";
        }
        else if ( type->isArrayTy() )
        {
            ArrayType      *arr = cast<ArrayType>( type );

            return getTypeName( arr->getArrayElementType(), data ) + "[" + std::to_string( arr->getArrayNumElements() ) + "]";
        }
        else if ( type->isFloatTy() )
        {
            return "float";
        }
        else if ( type->isStructTy() )
        {
            StructType     *stc = cast<StructType>( type );

            return std::string( stc->getStructName() );
        }
        else
        {
            return "unknown_" + std::to_string( data.getTypeAllocSizeInBits( type ) );
        }
    }

    void dumpType ( int depth, Type *type, const std::string &suffix, const DataLayout *data, unsigned base, int mode )
    {
        std::string blank( depth * 4, ' ' );

        if ( type->isStructTy() )
        {
            StructType         *stc = cast<StructType>( type );
            const StructLayout *sl  = data->getStructLayout( stc );

            errs() << blank + stc->getStructName() + "\n" + blank + "{\n";
            for ( size_t i = 0; i < stc->getStructNumElements(); i++ )
            {
                Type       *subType = stc->getStructElementType( i );
                unsigned    offset  = sl->getElementOffset( i );
                unsigned    size    = data->getTypeAllocSize( subType );

                if ( mode > 0 )
                {
                    offset += base;
                    dumpType( depth+1, subType, std::to_string(offset)+"_"+std::to_string(size), data, offset, mode );
                }
                else
                {
                    dumpType( depth+1, subType, std::to_string(offset)+"_"+std::to_string(size), data, 0, mode );
                }
            }
            errs() << blank + "} field_" + suffix + ";\n";
        }
        else
        {
            errs() << blank + getTypeName( type, *data ) + " field_" + suffix + ";\n";
        }
    }

    void visitor ( Function &F )
    {
        if ( F.getName() != "main" )
        {
            return;
        }

        std::set<StructType*>   types;
        const DataLayout       &data    = F.getParent()->getDataLayout();

        for ( auto &B : F )
        {
            for ( auto &I : B )
            {
                if ( auto *A = dyn_cast<AllocaInst>( &I ) )
                {
                    Type   *type    = A->getAllocatedType();
                    if ( type->isStructTy() )
                    {
                        StructType *stc = cast<StructType>( type );

                        if ( stc->isOpaque() )
                        {
                            continue;
                        }
                        std::string struct_name
                                        = std::string( stc->getStructName() );
                        if ( substr != DEFAULTSUBSTR && struct_name.find( substr ) == std::string::npos )
                        {
                            continue;
                        }
                        types.insert( stc );
                    }
                }
            }
        }

        int                     index = 0;

        for ( StructType *type : types )
        {
            dumpType( 0, type, std::to_string( index++ ), &data, 0, passmode );
        }
    }

    PreservedAnalyses run ( Function &F, FunctionAnalysisManager &FAM )
    {
        visitor( F );
        return PreservedAnalyses::all();
    }

};

}

PassPluginLibraryInfo getDumpClassPluginInfo ()
{
    const auto  callback = []( PassBuilder &PB )
    {
        PB.registerPipelineParsingCallback
        (
            [](
                StringRef               Name,
                FunctionPassManager    &FPM,
                ArrayRef<PassBuilder::PipelineElement>
            )
            {
                if ( Name == "DumpClass" )
                {
                    FPM.addPass( DumpClass() );
                    return true;
                }
                return false;
            }
        );
        PB.registerPipelineStartEPCallback
        (
            [&]( ModulePassManager &MPM, auto )
            {
                FunctionPassManager FPM;

                FPM.addPass( DumpClass() );
                MPM.addPass( createModuleToFunctionPassAdaptor( std::move( FPM ) ) );
                return true;
            }
        );
    };

    return { LLVM_PLUGIN_API_VERSION, "DumpClass", LLVM_VERSION_STRING, callback };
}

extern "C" LLVM_ATTRIBUTE_WEAK ::llvm::PassPluginLibraryInfo llvmGetPassPluginInfo ()
{
    return getDumpClassPluginInfo();
}
--------------------------------------------------------------------------

从dumpclass.cpp生成dumpclass.so

clang-14 \
$(llvm-config-14 --cxxflags) \
-Wall -pipe \
-fPIC -shared -Wl,-soname,dumpclass.so \
-O3 -s \
-o dumpclass.so dumpclass.cpp

后面会演示如何将dumpclass.so用作"LLVM Pass"来转储类或结构的内存布局。

☆ dumptarget.cpp

dumptarget.cpp是假想的目标程序，将来根据dumptarget.cpp转储其中的类或结构。

--------------------------------------------------------------------------
#include <deque>
#include <map>
#include <unordered_map>
#include <string>
#include <iostream>

class TargetClass
{
private:
    std::string                                                     unused;
public:
    std::deque<std::map<int, std::unordered_map<std::string, int>>> myDeque;
    std::map<int, std::unordered_map<std::string, int>>             myMap;
};

int main ( int argc, char * argv[] )
{
    TargetClass obj;

    obj.myMap[1]["one"] = 1;
    obj.myMap[2]["two"] = 2;

    obj.myDeque.push_back( obj.myMap );

    for ( const auto &d : obj.myDeque )
    {
        for ( const auto &pair : d )
        {
            std::cout << "Key : " << pair.first << " -> Value : ";
            for ( const auto &innerpair : pair.second )
            {
                std::cout << innerpair.first << " -> " << innerpair.second;
            }
            std::cout << std::endl;
        }
    }

    return 0;
}
--------------------------------------------------------------------------

☆ 用dumpclass.so处理dumptarget.cpp

有多种办法加载dumpclass.so，此处演示其中之一，依次执行这两条命令

clang-14 \
-Wall -pipe -S -emit-llvm \
-Xclang -disable-O0-optnone \
-o dumptarget.ll dumptarget.cpp

opt-14 \
-disable-output \
-load ./dumpclass.so -load-pass-plugin ./dumpclass.so \
-passes=DumpClass -passmode=1 -substr="::basic_string" \
dumptarget.ll 2>&1 | less

先从dumptarget.cpp生成dumptarget.ll，再用dumpclass.so处理dumptarget.ll。正
常情况下会得到

--------------------------------------------------------------------------
class.std::__cxx11::basic_string
{
    struct.std::__cxx11::basic_string<char>::_Alloc_hider
    {
        uint8_t* field_0_8;
    } field_0_8;
    uint64_t field_8_8;
    union.anon
    {
        uint64_t field_16_8;
        uint8_t[8] field_24_8;
    } field_16_16;
} field_0;
--------------------------------------------------------------------------

尝试不给opt指定passmode、substr参数，观察输出，加强理解。

☆ pahole

pahole也能转储类或结构的内存布局，不如dumpclass.cpp，出于完备性写在此处。

g++ -Wall -pipe -std=c++11 -O0 -g -o dumptarget_dbg dumptarget.cpp

pahole --hex -E -M -C TargetClass dumptarget_dbg | grep -A 25 "class basic_string"

正常情况下会得到

--------------------------------------------------------------------------
/* typedef string */ class basic_string<char, std::char_traits<char>, std::allocator<char> > {
        struct _Alloc_hider : allocator<char> {
                /* class allocator<char> : public new_allocator<char> {
                public:

                        /* class new_allocator<char> {
                        public:

                        }<ancestor>; */ /*     0     0 */

                        /* XXX last struct has 1 byte of padding */
                }<ancestor>; */ /*     0   0x1 */

                /* XXX last struct has 1 byte of padding */
                /* XXX 65535 bytes hole, try to pack */

                /* typedef pointer -> pointer -> pointer */ char *     _M_p;     /*     0   0x8 */
        }_M_dataplus; /*     0   0x8 */
        /* typedef size_type -> size_type -> size_type -> size_t */ long unsigned int  _M_string_length; /*   0x8   0x8 */
        union {
                char       _M_local_buf[16];                                     /*  0x10  0x10 */
                /* typedef size_type -> size_type -> size_type -> size_t */ long unsigned int _M_allocated_capacity; /*  0x10   0x8 */
        };                                                                       /*  0x10  0x10 */
public:

} unused; /*     0  0x20 */
--------------------------------------------------------------------------

☆ clang -Xclang -fdump-record-layouts

clang -Xclang -fdump-record-layouts dumptarget.cpp 2> /dev/null | grep -A 10 "0 | class std::basic_string" | less

正常情况下会得到

--------------------------------------------------------------------------
 0 | class std::basic_string<char>
 0 |   struct std::basic_string<char>::_Alloc_hider _M_dataplus
 0 |     class std::allocator<char> (base) (empty)
 0 |       class __gnu_cxx::new_allocator<char> (base) (empty)
 0 |     std::basic_string<char>::pointer _M_p
 8 |   std::basic_string<char>::size_type _M_string_length
16 |   union std::basic_string<char>::(anonymous at /usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/basic_string.h:179:7)
16 |     char[16] _M_local_buf
16 |     std::basic_string<char>::size_type _M_allocated_capacity
   | [sizeof=32, dsize=32, align=8,
   |  nvsize=32, nvalign=8]
...
--------------------------------------------------------------------------

☆ VC有隐藏选项

假设VirtualBaseClass.cpp如下

--------------------------------------------------------------------------
#include <stdio.h>
#include <windows.h>

class Base
{
public:
    int x;
};

class Derived1 : virtual public Base
{
public:
    int y;
};

class Derived2 : virtual public Base
{
public:
    int z;
};

class Multiple : public Derived1, public Derived2
{
public:
    int w;
};

int __cdecl main ( int argc, char * argv[] )
{
    Multiple    m;

    m.x = 10;
    return 0;
}
--------------------------------------------------------------------------

VC编译时有隐藏选项，查看C++类的内存布局

cl /d1reportSingleClassLayoutBase VirtualBaseClass.cpp
cl /d1reportSingleClassLayoutDerived1 VirtualBaseClass.cpp
cl /d1reportSingleClassLayoutDerived2 VirtualBaseClass.cpp
cl /d1reportSingleClassLayoutDerived VirtualBaseClass.cpp (子串匹配)
cl /d1reportSingleClassLayoutMultiple VirtualBaseClass.cpp
cl /d1reportAllClassLayout VirtualBaseClass.cpp (输出太多，慎用)

用ASCII图显示内存布局，向stdout输出，不影响其他编译选项。