Previously we briefly mentioned shifted pointers but without details. What are they?
Shifted pointers is another custom extension to the C syntax. They are used by IDA and decompiler to represent a pointer to an object with some offset or adjustment (positive or negative). Let’s see how they work and several situations where they can be useful.
Shifted pointer description and syntax
A shifted pointer is a regular pointer with additional information about the name of the parent structure and the offset from its beginning. For example, consider this structure:
struct mystruct
{
char buf[16];
int dummy;
int value; // <- myptr points here
double fval;
};
And this pointer declaration:
int *__shifted(mystruct,20) myptr;
It means that myptr is a pointer to int and if we decrement it by 20 bytes, we end up with mystruct*.
In fact, the offset value is not limited to the containing structure and can even be negative. Also, the “parent” type does not have to be a structure but can be any type except void. This can be useful in some situations.
Whenever a shifted pointer is used with an adjustment, it will be displayed using the ADJ helper, a pseudo-operator which returns the pointer to the parent type (in our case mystruct). For example, if the pointer is dereferenced after adding 4 bytes, it can be represented like this:
ADJ(myptr)->fval
Optimized loop on array of structures
When compiling code which is processing an array of structures, a compiler may optimize the loop so that the “current item” pointer points into a middle of the structure instead of the beginning. This is especially common when only a small subset of fields are being accessed. Consider this example:
struct mydata
{
int a, b, c;
void *pad[2];
int d, e, f;
char path[260];
};
int sum_c_d(struct mydata *arr, int count)
{
int sum=0;
for (int i=0; i< count; i++)
{
sum+=arr[i].d+arr[i].c*43;
}
return sum;
}
When compiled with Microsoft Visual C++ x86, it can produce the following code:
?sum_c_d@@YAHPAUmydata@@H@Z proc near
arg_0 = dword ptr 4
arg_4 = dword ptr 8
mov edx, [esp+arg_4]
push esi
xor esi, esi
test edx, edx
jle short loc_25
mov eax, [esp+4+arg_0]
add eax, 14h
loc_12: ; CODE XREF: sum_c_d(mydata *,int)+23↓j
imul ecx, [eax-0Ch], 2Bh ; '+'
add ecx, [eax]
lea eax, [eax+124h]
add esi, ecx
sub edx, 1
jnz short loc_12
loc_25: ; CODE XREF: sum_c_d(mydata *,int)+9↑j
mov eax, esi
pop esi
retn
And initial decompilation looks quite strange even after adding and specifying the correct types:
int __cdecl sum_c_d(struct mydata *arr, int count)
{
int v2; // edx
int v3; // esi
int *p_d; // eax
int v5; // ecx
v2 = count;
v3 = 0;
if ( count <= 0 )
return v3;
p_d = &arr->d;
do
{
v5 = *p_d + 43 * *(p_d - 3);
p_d += 73;
v3 += v5;
--v2;
}
while ( v2 );
return v3;
}
Apparently the compiler decided to use the pointer to the d field and accesses c relative to it. How can we make this look nicer?
We can find out the offset at which d is situated in the structure via manual calculation, by inspecting disassembly, or by hovering the mouse over it in pseudocode.
Thus, we can change the type of p_d to int * __shifted(mydata, 0x14) to get improved pseudocode:
int __cdecl sum_c_d(struct mydata *arr, int count)
{
int v2; // edx
int v3; // esi
int *__shifted(mydata,0x14) p_d; // eax
int v5; // ecx
v2 = count;
v3 = 0;
if ( count <= 0 )
return v3;
p_d = &arr->d;
do
{
v5 = ADJ(p_d)->d + 43 * ADJ(p_d)->c;
p_d += 73;
v3 += v5;
--v2;
}
while ( v2 );
return v3;
}
Prepended metadata
This technique is used in situations where a raw block of memory needs to have some management info attached to it, i.e. heap allocators, managed strings and so on.
As a specific example, let’s consider the classic MFC 4.x CString class. It uses a structure placed before the actual character array:
struct CStringData
{
long nRefs; // reference count
int nDataLength; // length of data (including terminator)
int nAllocLength; // length of allocation
// TCHAR data[nAllocLength]
TCHAR* data() // TCHAR* to managed data
{
return (TCHAR*)(this+1);
}
};
The CStringclass itself has just one data member:
class CString
{
public:
// Constructors
[...skipped]
private:
LPTSTR m_pchData; // pointer to ref counted string data
// implementation helpers
CStringData* GetData() const;
[...skipped]
};
inline
CStringData*
CString::GetData(
) const
{
ASSERT(m_pchData != NULL);
return ((CStringData*)m_pchData)-1;
}
Here’s how it looks in memory:
┌───────────────┐
│ nRefs │
├───────────────┤
CStringData │ nDataLength │
├───────────────┤
│ nAllocLength │
├───────────────┴─────┐
┌──►│'H','e','l','l','o',0│
│ └─────────────────────┘
│
│
┌─┴────────┐
CString │m_pchData │
└──────────┘
Here’s how the CString’s destructor looks like in initial decompilation:
void __thiscall CString::~CString(CString *this)
{
if ( *(_DWORD *)this - (_DWORD)off_4635E0 != 12 && InterlockedDecrement((volatile LONG *)(*(_DWORD *)this - 12)) <= 0 )
operator delete((void *)(*(_DWORD *)this - 12));
}
Even after creating a CString structure with a single memberchar *m_pszDatait’s still somewhat confusing:
void __thiscall CString::~CString(CString *this)
{
if ( this->m_pszData - (char *)off_4635E0 != 12 && InterlockedDecrement((volatile LONG *)this->m_pszData - 3) <= 0 )
operator delete(this->m_pszData - 12);
}
Finally, if we create theCStringDatastruct as described above and change the type of the CString member to: char *__shifted(CStringData,0xC) m_pszData:
void __thiscall CString::~CString(CString *this)
{
if ( ADJ(this->m_pszData)->data - (char *)off_4635E0 != 12 && InterlockedDecrement(&ADJ(this->m_pszData)->nRefs) <= 0 )
operator delete(ADJ(this->m_pszData));
}
Now the code is more understandable: if the decremented reference count becomes zero, the CStringDatainstance is deleted.
More info: IDA Help: Shifted pointers
