ReadProcessMemory failed with error code 299 (ERROR_PARTIAL_COPY)

Question

I'm trying to make a simple debugger as my homework. The debugger does 3 steps.

1. Read a exe file with rb flag, get its EntryPoint.
2. Call CreateProcess with flag CREATE_SUSPENDED | DEBUG_ONLY_THIS_PROCESS, then set a breakpoint at EntryPoint
3. When debugee was stopped at EntryPoint. Read codes from EntryPoint.

Now my problem is, CreateProcess returns true, but the following ReadProcessMemory returns false and GetLastError returns 299, AKA ERROR_PARTIAL_COPY, so I can't set breakpoint at EntryPoint. I've search on Stack Overflow with keywords ERROR_PARTIAL_COPY and ReadProcessMemory and read some questions and answers, but those answers cannot solve my problem.

My approach comes here: Syringe, it is a 32-bit program, which runs well on my 64-bit PC.

Environment:

• Windows 10 20H2 (19042), 64-bit.
• CMake(generated by Visual Studio 2019, I didn't specify anything except linking zydis) generator: Visual Studio 2019, compiler: MSVC v142.
• Compiled with x86-debug mode

Here is my code snippet

    auto* file = fopen("../debugee.exe", "rb");
    IMAGE_DOS_HEADER dosHdr;
    fread(&dosHdr, sizeof IMAGE_DOS_HEADER, 1, file);
    fseek(file, dosHdr.e_lfanew, SEEK_SET);

    IMAGE_NT_HEADERS ntHeaders;
    fread(&ntHeaders, sizeof IMAGE_NT_HEADERS, 1, file);
    auto IsWin32 = ntHeaders.OptionalHeader.Magic == IMAGE_NT_OPTIONAL_HDR32_MAGIC;
    std::cout << "parsing " << (IsWin32 ? "PE32" : "PE64") << " file\n";

    auto imageBase = ntHeaders.OptionalHeader.ImageBase;
    auto entry = ntHeaders.OptionalHeader.AddressOfEntryPoint + imageBase;
    auto codeSize = ntHeaders.OptionalHeader.SizeOfCode;

    STARTUPINFO startupInfo{sizeof STARTUPINFO};
    PROCESS_INFORMATION pInfo;
    auto r = CreateProcess(
        "../debugee.exe", nullptr, nullptr, nullptr, false,
        CREATE_SUSPENDED | DEBUG_ONLY_THIS_PROCESS,
        nullptr, nullptr, &startupInfo, &pInfo);
    printf("create process: %s, handle: %p\n", r ? "success" : "fail", pInfo.hProcess);

    char buffer = 0;
    char INT3 = 0xCC;
    printf("address of buffer: %p\n", &buffer);
    r = ReadProcessMemory(pInfo.hProcess, reinterpret_cast<void*>(entry), &buffer, 1, nullptr);

    auto err = GetLastError();
    printf("read: %s, error: %08X\n", r ? "success" : "fail", err);
    r = WriteProcessMemory(pInfo.hProcess, reinterpret_cast<void*>(entry), &INT3, 1, nullptr);
    err = GetLastError();
    printf("patch: %s, error: %08X\n\n", r ? "success" : "fail", err);
    ResumeThread(pInfo.hThread);

The output is

parsing PE32 file
create process: success, handle: 00000100
address of buffer: 00D3F5CF
read: fail, error: 0000012B
patch: fail, error: 000003E6 // WriteProcessMemory got another error ERROR_NOACCESS, I don't know if there are corelation, but I want to figure out why I got ERROR_PARTIAL_COPY first