Marshalling a pointer to an array of pointer to structure Exception "cannot Read or Write protected memory"

Question

I am translating a native C++ library to C#. The original function allocates some memory of the struct, doing some memory manipulation of the current process and return the manipulated bytes to a byte buffer inside the struct.

The struct is defined as follow:

typedef struct tdMEM_SCATTER_HEADER {
    DWORD magic;            
    WORD version;           
    WORD Future1;
    ULONG64 qwA;            
    DWORD cbMax;            
    DWORD cb;               
    PBYTE pb;             
    PVOID pvReserved1;      
    PVOID pvReserved2;      
    PVOID Future2[8];
} MEM_SCATTER_HEADER, *PMEM_SCATTER_HEADER, **PPMEM_SCATTER_HEADER;

The original function is defined as follow:

DWORD Scatter(_Inout_ PPMEM_SCATTER_HEADER ppMEMs, _In_ DWORD count);

and the allocating sequence is

pbBuffer = LocalAlloc(0, count * (sizeof(PMEM_SCATTER_HEADER) + sizeof(MEM_SCATTER_HEADER) + 0x1000));
ZeroMemory(pbBuffer, count * (sizeof(PMEM_SCATTER_HEADER) + sizeof(MEM_SCATTER_HEADER)));
for(i = 0; i < cMEMs; i++) {
     pMEM->pb = pbData + ((QWORD)i << 12); // Allocating return bytes buffer.
}

Here is what I tried:

Struct marshalling:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
public struct MEM_SCATTER_HEADER
{
     public uint magic;            
     public ushort version;        
     public ushort Future1;
     public ulong qwA;            
     public uint cbMax;            
     public uint cb;                
     [MarshalAs(UnmanagedType.ByValArray)]
     public byte[] pb;             
     public IntPtr pvReserved1;     
     public IntPtr pvReserved2;                
     [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
     public IntPtr[] Future2;
}

Memory allocating:

MEM_SCATTER_HEADER[] header = new MEM_SCATTER_HEADER[count];

for (int i = 0; i < count; i++)
{
     header[i].pb = new byte[0x1000];
     Array.Clear(header[i].pb, 0, header[i].pb.Length); // Fill array with 0
}

DLLImport:

[DllImport("testC.dll", CharSet = CharSet.Auto, SetLastError = true)]
public static extern uint Scatter(ref MEM_SCATTER_HEADER ppMems, uint count);

And finally function call:

uint count = Scatter(ref header, count);

I have a had time debugging wether I marshal data wrongly or wether I have to do some manual marshalling to get rid of the exception: "Attempted to read or write protected memory. This is often an indication that other memory is corrupt.'" at function call.

https://stackoverflow.com/questions/15660722/why-are-cdecl-calls-often-mismatched-in-the-standard-p-invoke-convention — Hans Passant, Nov 03 '19 at 03:24

score 0 · Answer 1 · answered Nov 03 '19 at 00:49

The initial allocation is not zeroing all the memory it's allocating, and it doesn't look right. It's as if the code were doing:

int bigchunk = sizeof(PMEM_SCATTER_HEADER) + sizeof(MEM_SCATTER_HEADER) + 0x1000;
int smallchunk = sizeof(PMEM_SCATTER_HEADER) + sizeof(MEM_SCATTER_HEADER);

pBuffer = LocalAlloc(0, count * bigchunk);
ZeroMemory(pBuffer, count * smallchunk);

Might this matter?

Marshalling a pointer to an array of pointer to structure Exception "cannot Read or Write protected memory"

1 Answers1