Passing variable length struct from User Mode to Kernel Mode

Question

I'm writing virtual disk driver, and there I have structure defined like that:

typedef struct _MOUNT_NEW_QUERY {
    PWCHAR imagePath;
    WCHAR letter;
    PCHAR key;
} MOUNT_NEW_QUERY, *PMOUNT_NEW_QUERY;

So I have sort of dynamically sized structure.

How must I pass it from User Mode to my driver?

Answer 1

Allocate a continuous chunk of memory, sufficient to hold both your struct and the data for "key" and "path" - something like this:

/* we add + 1 for terminating NULLs to make life easy */
size_t keyLen = (strlen(key) +  1); 
size_t imgLen = (wcslen(imagePath) + 1) * sizeof(WCHAR);

PMOUNT_NEW_QUERY pMNQ = malloc(sizeof(MOUNT_NEW_QUERY) + keyLen + imgLen);

if(pMNQ != NULL) 
{       
   /* make imagePath point to the allocated buffer immediately after 
    * the MOUNT_NEW_QUERY portion 
    */
   pMNQ->imagePath = (PWCHAR)((PBYTE)pMNQ + sizeof(MOUNT_NEW_QUERY));

   /* make the key point to the allocated buffer immediately after 
    * the imagePath portion (including a NULL WCHAR terminator)
    */
   pMNQ->key = (PCHAR)((PBYTE)pMNQ + sizeof(MOUNT_NEW_QUERY) + imgLen);

   /* populate the data here appropriately, typically with strcpy
    * and wcscpy, and then send the IOCTL 
    */
   fun(pMNQ);
}

When you call the IOCTL for your driver, pass the total size of the buffer, not just the sizeof the MOUNT_NEW_QUERY structure.