一
概述
if else语句转换成switch语句。switch结构体包含多个分支，各个分支的执行顺序是随机的，但并不影响真正的程序逻辑。然后在switch结构的外层，再套一个或多个while循环。下面编写一段代码进行测试，代码的功能是在
main函数中执行一个名称为add的自定义函数。add函数里会判断参数num1是否等于100，如果等于，则返回0，否则继续执行，而后将参数num1和num2相加，结果赋值给num3，并返回num3。具体代码如下：int add(int num1, int num2){
if (num1 == 100) {
return 0;
}
int num3 = num1 + num2;
return num3;
}int main(){
int num1 = 10;
int num2 = 20;
int num3 = add(num1,num2);
return 0;
}
二
源码分析
namespace {
struct Flattening : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
bool flag;Flattening() : FunctionPass(ID) {}
Flattening(bool flag) : FunctionPass(ID) { this->flag = flag; }bool runOnFunction(Function &F);
bool flatten(Function *f);
};
}char Flattening::ID = 0;
static RegisterPass<Flattening> X("flattening", "Call graph flattening");
Pass *llvm::createFlattening(bool flag) { return new Flattening(flag); }bool Flattening::runOnFunction(Function &F) {
Function *tmp = &F;
// Do we obfuscate
if (toObfuscate(flag, tmp, "fla")) {
if (flatten(tmp)) {
++Flattened;
}
}return false;
}
po f->dump()，打印参数 F，输出 add函数的所有中间层代码，代码如下，为了方便理解，每一行都做了相应注释：; Function Attrs: noinline nounwind ssp uwtable
define i32 @add(i32 %num1, i32 %num2) #0 {entry: ;第 1 个 BasicBlock，名称是 entry
%retval = alloca i32, align 4 ;为返回值分配 4 字节空间
%num1.addr = alloca i32, align 4 ;为变量 num1.addr 分配 4 字节空间
%num2.addr = alloca i32, align 4 ;为变量 num2.addr 分配 4 字节空间
%num3 = alloca i32, align 4 ;为变量 num3 分配 4 字节的空间
store i32 %num1, i32* %num1.addr, align 4 ;将变量 num1 保存到 num1.addr
store i32 %num2, i32* %num2.addr, align 4 ;将变量 num2 保存到 num2.addr
%0 = load i32, i32* %num1.addr, align 4 ;将变量 num1.addr 保存到变量 0
%cmp = icmp ne i32 %0, 100 ;比较变量 0 是否等于 100
br i1 %cmp, label %if.then, label %if.end ;如果条件比较成立则会跳转到 if.thenif.then: ;第 2 个 BasicBlock，名称是 if.then ; preds = %entry
store i32 0, i32* %retval, align 4
br label %return ;跳转到 returnif.end: ;第 3 个 BasicBlock，名称是 if.end ; preds = %entry
%1 = load i32, i32* %num1.addr, align 4 ;将变量 num1.addr 保存到变量 1
%2 = load i32, i32* %num2.addr, align 4 ;将变量 num2.addr 保存到变量 2
%add = add nsw i32 %1, %2 ;将变量 1 和变量 2 相加，结果保存到变量 add
store i32 %add, i32* %num3, align 4 ;将变量 add 保存到变量 num3
%3 = load i32, i32* %num3, align 4 ;将变量 num3 保存到变量 3
store i32 %3, i32* %retval, align 4 ;将变量 3 保存到变量 retval
br label %return ;跳转到 returnreturn: ;第 4 个 BasicBlock，名称是 return ; preds = %if.end, %if.then
%4 = load i32, i32* %retval, align 4
ret i32 %4
}
entry、if.then、if.end和return。第 63 行是flatten函数要执行的第一条代码，第 63 行和第 64 行的作用是生成随机数（下方代码），这个在后面会用到：
62 //生成随机数
63 char scrambling_key[16];
64 llvm::cryptoutils->get_bytes(scrambling_key, 16);
switch语句转换成if语句：67 //转换 switch 语句
68 FunctionPass *lower = createLowerSwitchPass();
69 lower->runOnFunction(*f);
BasicBlock到origBB容器：71 //保存所有原始的 BasicBlock
72 for (Function::iterator i = f->begin(); i != f->end(); ++i) {
73 BasicBlock *tmp = &*i;
74 origBB.push_back(tmp);
75
76 BasicBlock *bb = &*i;
77 if (isa<InvokeInst>(bb->getTerminator())) {
78 return false;
79 }
80 }
origBB容器里的第一个BasicBlock，因为第一个BasicBlock需要单独处理：87 //清空第一个 BasicBlock
88 origBB.erase(origBB.begin());
89
90 //获取指向第一个 BasicBlock 的指针
91 Function::iteratortmp = f->begin(); //++tmp;
92 BasicBlock *insert = &*tmp;
93
BasicBlock是否含有条件语句，如果有，就获取条件语句的内容，并赋给br变量：94 //如果第一个 BasicBlock 含有条件语句
95 BranchInst *br = NULL;
96 if (isa<BranchInst>(insert->getTerminator())) {
97 br = cast<BranchInst>(insert->getTerminator());
98 }
br i1 %cmp, label %if.then, label %if.end
BasicBlock的倒数第二行：100 if ((br != NULL && br->isConditional()) ||
101 insert->getTerminator()->getNumSuccessors() > 1) {
102 BasicBlock::iterator i = insert->end();
103 --i;
104
105 if (insert->size() > 1) {
106 --i;
107 }
108
%cmp = icmp ne i32 %0, 100
BasicBlock进行分割，第 110 行是将分割后的内容放入origBB容器的第一条记录中，还记得第88 行清空了origBB容器的第一条记录吧？109 BasicBlock *tmpBB = insert->splitBasicBlock(i, "first");
110 origBB.insert(origBB.begin(), tmpBB);
111 }
112
tmp得到的内容如下：first: ; preds = %entry
%cmp = icmp ne i32 %0, 100
br i1 %cmp, label %if.then, label %if.end
insert的内容如下：entry:
%retval = alloca i32, align 4
%num1.addr = alloca i32, align 4
%num2.addr = alloca i32, align 4
%num3 = alloca i32, align 4
store i32 %num1, i32* %num1.addr, align 4
store i32 %num2, i32* %num2.addr, align 4
%0 = load i32, i32* %num1.addr, align 4
br label %first
br label%first移除：113 // 移除跳转
114 insert->getTerminator()->eraseFromParent();
switch语句需用的变量switchVar，变量的值就是最前面第 63 行和第 64 行随机生成的scrambling_key：116 //创建 switchVar，并按其进行设置
117 switchVar = new AllocaInst(Type::getInt32Ty(f->getContext()), 0, "switchVar", insert);
118
119 new StoreInst(
120 ConstantInt::get(Type::getInt32Ty(f->getContext()),
121 llvm::cryptoutils->scramble32(0, scrambling_key)),
122 switchVar, insert);
switchVar之后，再打印insert，可以看到其内容的最后多了两条语句：%switchVar = alloca i32
store i32 1207049111, i32* %switchVar
loopEntry和loopEnd，此时里面的代码还是空的。第 128 行是创建一条load指令，将switchvar放入loopEntry中。124 //创建主循环
125 loopEntry = BasicBlock::Create(f->getContext(), "loopEntry", f, insert);
126 loopEnd = BasicBlock::Create(f->getContext(), "loopEnd", f, insert);
127
128 load = new LoadInst(switchVar, "switchVar", loopEntry);
129
130 //在顶部移动第一个 BasicBlock
131 insert->moveBefore(loopEntry);
132 BranchInst::Create(loopEntry, insert);
133
134 //从 loopEnd 跳转到 loopEntry
135 BranchInst::Create(loopEntry, loopEnd);
136
137 BasicBlock *swDefault = BasicBlock::Create(f->getContext(), "switchDefault", f, loopEnd);
138
139 BranchInst::Create(loopEnd, swDefault);
140
141 //创建 switch 语句本身，并设置条件
142 switchI = SwitchInst::Create(&*f->begin(), swDefault, 0, loopEntry);
143 switchI->setCondition(load);
144
145 //移除第一个BasicBlock 中的跳转分支，并且创建一个到 while 循环的跳转
146 f->begin()->getTerminator()->eraseFromParent();
147
148 BranchInst::Create(loopEntry, &*f->begin()); //添加上 br label %loopEntry
origBB里每个的BasicBlock填充switch分支：150 //把所有 BasicBlock 都放入 switch 分支
151 for (vector<BasicBlock *>::iterator b = origBB.begin(); b != origBB.end();
152 ++b) {
153 BasicBlock *i = *b;
154 ConstantInt *numCase = NULL;
155
156 //把 BasicBlock 移动到 switch 内（只是视觉上的，不涉及代码逻辑）
157 i->moveBefore(loopEnd);
158
159 //给 switch 添加 case，switchVar 是随机数
160 numCase = cast<ConstantInt>(ConstantInt::get(
161 switchI->getCondition()->getType(),
162 llvm::cryptoutils->scramble32(switchI->getNumCases(), scrambling_key)));
163 switchI->addCase(numCase, i); //添加 switch case
164 }
(lldb) po f->dump()
; Function Attrs: noinline nounwind ssp uwtable
define i32 @add(i32 %num1, i32 %num2) #0 {
entry:
%retval = alloca i32, align 4
%num1.addr = alloca i32, align 4
%num2.addr = alloca i32, align 4
%num3 = alloca i32, align 4
store i32 %num1, i32* %num1.addr, align 4
store i32 %num2, i32* %num2.addr, align 4
%0 = load i32, i32* %num1.addr, align 4
%switchVar = alloca i32
store i32 1207049111, i32* %switchVar
br label %loopEntryloopEntry: ; preds = %entry,%loopEnd
%switchVar1 = load i32, i32* %switchVar
switch i32 %switchVar1, label %switchDefault [
i32 1207049111, label %first
i32 -677357051, label %if.then
i32 -1251090459, label %if.end
i32 1194405227, label %return
]switchDefault: ; preds = %loopEntry
br label %loopEndfirst: ; preds = %loopEntry
%cmp = icmp ne i32 %0, 100
br i1 %cmp, label %if.then, label %if.endif.then: ; preds = %loopEntry,%first
store i32 0, i32* %retval, align 4
br label %returnif.end: ; preds = %loopEntry, %first
%1 = load i32, i32* %num1.addr, align 4
%2 = load i32, i32* %num2.addr, align 4
%add = add nsw i32 %1, %2
store i32 %add,i32* %num3, align 4
%3 = load i32, i32* %num3, align 4
store i32 %3, i32* %retval, align 4
br label %returnreturn: ; preds = %loopEntry, %if.end, %if.then
%4 = load i32, i32* %retval, align 4
ret i32 %4loopEnd: ; preds = %switchDefault
br label %loopEntry
}
loopEntry和loopEnd是一个while循环，while循环中有一个switch结构，switch结构中有 4 个case分支，分别是first、if.then、if.end、return。如果switchVar的值是1207049111，就跳转到first分支；如果是-677357051，则跳转到if.then；如果是-125090459，跳转到if.end分支；如果是1194405227，跳转到return分支。可以看到，上面得到的中间层代码中已经有
switch结构了，但是在执行跳转时并没有更新switchVar，这样会使真实的程序逻辑没有正确执行。第 166 行到第 237 行的作用就是更新switchVar，因为只有一个后续块相当于无条件跳转，所以直接将switchVar更新成后续块对应的case：166 //重新计算 switchVar
167 for (vector<BasicBlock *>::iterator b = origBB.begin(); b != origBB.end();
168 ++b) {
169 BasicBlock *i = *b;
170 ConstantInt *numCase = NULL;
171
172 // Ret BasicBlock 是个返回块，不需要更新 caseVar
173 if (i->getTerminator()->getNumSuccessors() == 0) {
174 continue;
175 }
176
177 //如果是无条件跳转
178 if (i->getTerminator()->getNumSuccessors() == 1) {
179 // Get successor and delete terminator
180 BasicBlock *succ = i->getTerminator()->getSuccessor(0);
181 i->getTerminator()->eraseFromParent();
182
183 //获取下一个 case 分支
184 numCase = switchI->findCaseDest(succ);
185
186 // If next case == default case (switchDefault)
187 if (numCase == NULL) {
188 numCase = cast<ConstantInt>(
189 ConstantInt::get(switchI->getCondition()->getType(),
190 llvm::cryptoutils->scramble32(
191 switchI->getNumCases() - 1, scrambling_key)));
192 }
193
194 //更新 switchVar，并且跳转到循环尾部
195 new StoreInst(numCase, load->getPointerOperand(), i); //store 给%switchVar 赋值为 numCase
196 BranchInst::Create(loopEnd, i); //插入 br label %loopEnd
197 continue;
198 }
199
200 //If it's a conditional jump //有两个后续块，也就是条件跳转，successor 对象就是后续块的意思
201 if (i->getTerminator()->getNumSuccessors() == 2) {
202 //获取接下来的 case 分支
203 ConstantInt *numCaseTrue =
204 switchI->findCaseDest(i->getTerminator()->getSuccessor(0));
205 ConstantInt *numCaseFalse =
206 switchI->findCaseDest(i->getTerminator()->getSuccessor(1));
207
208 //检查 next case 和 default case (switchDefault) 是否相等
209 if (numCaseTrue == NULL) {
210 numCaseTrue = cast<ConstantInt>(numCaseTrue = cast<ConstantInt>(
211 ConstantInt::get(switchI->getCondition()->getType(),
212 llvm::cryptoutils->scramble32(
213 switchI->getNumCases() - 1, scrambling_key)));
214 }
215
216 if (numCaseFalse == NULL) {
217 numCaseFalse = cast<ConstantInt>(
218 ConstantInt::get(switchI->getCondition()->getType(),
219 llvm::cryptoutils->scramble32(
220 switchI->getNumCases() - 1, scrambling_key)));
221 }
222
223 // Create a SelectInst %1 = select i1 %cmp, i32 117441206, i32 -880348549
224 BranchInst *br = cast<BranchInst>(i->getTerminator());
225 SelectInst *sel =
226 SelectInst::Create(br->getCondition(), numCaseTrue, numCaseFalse, "",
227 i->getTerminator());
228
229 //清除终止符
230 i->getTerminator()->eraseFromParent();
231
232 //更新 switchVar，并且跳转到循环尾部
233 new StoreInst(sel, load->getPointerOperand(), i);
234 BranchInst::Create(loopEnd, i);
235 continue;
236 }
237 }
238
239 fixStack(f);
(lldb) po f->dump() ; Function Attrs: noinline nounwind ssp uwtable
define i32 @add(i32 %num1, i32 %num2) #0 {entry:
%.reg2mem = alloca i32
%retval = alloca i32, align 4
%num1.addr = alloca i32, align 4
%num2.addr = alloca i32, align 4
%num3 = alloca i32, align 4
store i32 %num1, i32* %num1.addr, align 4
store i32 %num2,i32* %num2.addr, align 4
%0 = load i32, i32* %num1.addr, align 4
store i32 %0, i32* %.reg2mem
%switchVar = alloca i32
store i32 1207049111, i32* %switchVar
br label %loopEntryloopEntry: ; preds = %entry, %loopEnd
%switchVar1 = load i32, i32* %switchVar
switch i32 %switchVar1, label %switchDefault [
i32 1207049111, label %first
i32 -677357051, label %if.then
i32 -1251090459, label %if.end
i32 1194405227, label %return
]switchDefault: ; preds = %loopEntry
br label %loopEndfirst: ; preds = %loopEntry
%.reload = load volatile i32, i32* %.reg2mem
%cmp = icmp ne i32 %.reload, 100
%1 = select i1 %cmp, i32 -677357051, i32 -1251090459
store i32 %1, i32* %switchVar
br label %loopEndif.then: ; preds = %loopEntry
store i32 0, i32* %retval, align 4
store i32 1194405227, i32* %switchVar
br label %loopEndif.end: ; preds = %loopEntry
%2 = load i32, i32* %num1.addr, align 4
%3 = load i32, i32* %num2.addr, align 4
%add = add nsw i32 %2, %3
store i32 %add, i32* %num3, align 4
%4 = load i32, i32* %num3, align 4
store i32 %4, i32* %retval, align 4
store i32 1194405227, i32* %switchVar
br label %loopEndreturn: ; preds = %loopEntry
%5 = load i32, i32* %retval, align 4
ret i32 %5loopEnd: ; preds = %if.end,%if.then, %first, %switchDefault
br label %loopEntry}
first、if.then、if.end这三个BasicBlock的代码都会更新switchVar，这样便达到了混淆代码逻辑的效果，并且不影响真实的程序逻辑。至此，整个代码扁平化的过程全部分析完。三
总结
看雪ID：ElainaDaemon
https://bbs.kanxue.com/user-home-945395.htm
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