Question

I need to use GetVolumeInformationW. For reasons uknown, I decided to load Kernel32.dll dynamically, resolve function address at runtime… Result is memory corruption problem and some strange side-effects of modifying stack. Though static version works like a charm and I can just stick with it and move on, I want to investigate the matter.

Source code is self-explaining (comment if you need some more information, full version can be found here):

#include <QtDebug>
#include <QByteArray>
#include <QLibrary>
#include <QDir>

static inline QString LAT1(const char *str, const int len = -1) {
    return QString::fromLatin1(str, len);
}

template <typename T>
static inline QByteArray createByteArray(const T *from, const int numElements) {
    return QByteArray(reinterpret_cast<const char*>(from), sizeof(T) * numElements);
}

// This one resolves functions from Kernel32.dll dynamically and uses standard types.
// (Dynamic linking.)
QByteArray fingerprintDynamic() {
    const uint32_t kMaxPath = 260 + 1;  // MAX_PATH + 1
    wchar_t path[kMaxPath]       = {0};
    wchar_t name[kMaxPath]       = {0};
    wchar_t fileSystem[kMaxPath] = {0};
    uint32_t serial = 0;
    uint32_t maximumComponentLength = 0;
    uint32_t fileSystemFlags = 0;

    QLibrary kernel32("kernel32");
    typedef uint32_t (*fnGetLastError)(void);
    typedef bool (*fnGetVolumeInformationW)(const wchar_t*, wchar_t*, uint32_t, uint32_t*, uint32_t*,
                                            uint32_t*, wchar_t*, uint32_t);
    fnGetVolumeInformationW GetVolumeInformationW = reinterpret_cast<fnGetVolumeInformationW>(kernel32.resolve("GetVolumeInformationW"));
    fnGetLastError GetLastError = reinterpret_cast<fnGetLastError>(kernel32.resolve("GetLastError"));

    if (!GetVolumeInformationW) {
        qWarning(LAT1("GetVolumeInformationW() not resolved: %1").arg(kernel32.errorString()).toLatin1().constData());
        return QByteArray();
    }
    else if (!GetLastError) {
        qWarning(LAT1("GetLastError() not resolved: %1").arg(kernel32.errorString()).toLatin1().constData());
        return QByteArray();
    }

    QDir::toNativeSeparators(QDir::rootPath()).toWCharArray(path);
    bool apiCall = GetVolumeInformationW(path, name, kMaxPath, &serial, &maximumComponentLength,
                                         &fileSystemFlags, fileSystem, kMaxPath);
    if (!apiCall)
        qWarning(LAT1("GetVolumeInformationW() failed: %1").arg(GetLastError()).toLatin1().constData());

    // At this point, fileSystem is correct and contains
    // L"NTFS"

    // ONLY HAPPENS IN DEBUG MODE
    //
    // After this call memory becomes corrupted. wcslen() is not a problem.
    // And createByteArray<>() is ok too, I believe.
    //size_t len;  // But if I change stack a bit (like uncomment this line),
                   // result will be correct, so I guess it's related to memory offset.
    return createByteArray<wchar_t>(fileSystem, wcslen(fileSystem));
}

void print(const QByteArray &bytes) {
    qDebug() << QString::fromWCharArray(reinterpret_cast<const wchar_t*>(bytes.constData()));
    qDebug() << bytes.size() << "bytes" << bytes.toHex();
    qDebug() << "";
}

int main(int, char**)
{    
    qDebug() << "dynamic";
    print(fingerprintDynamic());

    return 0;
}

Sample output:

// this is DEBUG build
dynamic 
"(?(" 
8 bytes "280052f828000400"

// this is RELEASE build
// (same with `size_t len` uncommenented before `return` in DEBUG)
dynamic 
"NTFS" 
8 bytes "4e00540046005300"

The question is: what is the reason of such behaviour? where’s my mistake is hiding?

I’m using g++.exe (GCC) 4.4.0, Qt 4.8.1 and Creator 2.5.2.

Answer 1

The problem is probably caused by a mismatch in the calling convention. GetVolumeInformationW() has calling convention WINAPI so change the function pointer type to:

typedef BOOL (WINAPI *fnGetVolumeInformationW)(const wchar_t*,
                                               wchar_t*,
                                               uint32_t,
                                               uint32_t*,
                                               uint32_t*,
                                               uint32_t*,
                                               wchar_t*,
                                               uint32_t);

WINAPI is calling convention __stdcall, whereas the default is __cdecl.
Note the return type is BOOL, and not bool.