对象内的 Valgrind 数组溢出

Question

我有一个简单的程序如下。

struct Test
{
    int a[5];
    int b;
};

int main()
{
    Test* t = new Test;
    t->b = 1;
    t->a[5] = 5;          //This is an illegal write
    cout << t->b << endl; //Output is 5
    return 0;
}

运行它与Valgrind Memcheck 没有报告非法内存写入。

我注意到 Valgrind 声称 Memcheck 工具无法检测全局或堆栈数组溢出，但这个数组在堆中，对吗？只是数组在一个对象中。

是Valgrind真的检测不到这种错误还是我做错了什么？如果前者为真，那么是否有任何其他工具可以检测此类错误？

=========================================== =============================

更新：

我使用的编译命令是g++ -O0 -g main.cc。 valgrind 命令只是 valgrind ./a.out，它应该默认调用 memcheck 工具。

编译器版本为gcc version 4.4.7 20120313 (Red Hat 4.4.7-4) (GCC)，valgrind版本为valgrind-3.5.0。

当运行这个程序时 Valgrind 输出：

==7759== Memcheck, a memory error detector
==7759== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==7759== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
==7759== Command: ./a.out
==7759== 
5
==7759== 
==7759== HEAP SUMMARY:
==7759==     in use at exit: 24 bytes in 1 blocks
==7759==   total heap usage: 1 allocs, 0 frees, 24 bytes allocated
==7759== 
==7759== LEAK SUMMARY:
==7759==    definitely lost: 24 bytes in 1 blocks
==7759==    indirectly lost: 0 bytes in 0 blocks
==7759==      possibly lost: 0 bytes in 0 blocks
==7759==    still reachable: 0 bytes in 0 blocks
==7759==         suppressed: 0 bytes in 0 blocks
==7759== Rerun with --leak-check=full to see details of leaked memory
==7759== 
==7759== For counts of detected and suppressed errors, rerun with: -v
==7759== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 4 from 4)

Answer 1

我认为下面这句话你已经找到了：

Memcheck cannot detect every memory error your program has. For example, it can't detect out-of-range reads or writes to arrays that are allocated statically or on the stack. But it should detect many errors that could crash your program (eg. cause a segmentation fault).

如果您的 class 定义应该这样解释： 虽然class对象是动态分配的，但数组本身是静态分配的。

我验证过几个案例：

如果数组是动态分配的，Valgrind 将报告无效写入：

struct Test
{
    int *a;
    int b;
};

int main()
{
    Test* t = new Test;
    t->a = new int[5];
    t->b = 1;
    t->a[5] = 5;          //This is an illegal write
    cout << t->b << endl; //Output is 5
    delete [] t->a;
    delete t;
    return 0;
}

成员顺序改为：

也会报错

struct Test
{
  int b;  
  int a[5];
};

这是因为在尝试写入 [5] 时，我们已经落后于动态分配的对象。

使用原始 class 定义，如果您尝试写入 [6] - 因为那样我们落后于 b，所以落后于动态分配的对象。

更新：gcc sanitizer（我也怀疑 clang）通过编译在运行时检测到此越界访问：

g++ -fno-omit-frame-pointer -fsanitize=bounds m.cpp

输出：

m.cpp:15:7: runtime error: index 5 out of bounds for type 'int [5]'

对象内的 Valgrind 数组溢出

Valgrind array overrun within objects

c++

valgrind

overrun