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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, you can obtain one at http://mozilla.org/MPL/2.0/. */
// This file is an NSIS plugin which exports a function that starts a process
// from a provided path by using the shell automation API to have explorer.exe
// invoke ShellExecute. This roundabout method of starting a process is useful
// because it means the new process will use the integrity level and security
// token of the shell, so it allows starting an unelevated process from inside
// an elevated one. The method is based on
// https://blogs.msdn.microsoft.com/oldnewthing/20131118-00/?p=2643
// but the code has been rewritten to remove the need for ATL or the C runtime.
// Normally an NSIS installer would use the UAC plugin, which itself uses both
// an unelevated and an elevated process, and the elevated process can invoke
// functions in the unelevated one, so this plugin wouldn't be needed.
// But uninstallers are often directly run elevated because that's just how
// the Windows UI launches them, so there is no unelevated process. This
// plugin allows starting a needed unelevated process in that situation.
#include <windows.h>
#include <shlobj.h>
#pragma comment(lib, "shlwapi.lib")
static IShellView*
GetDesktopWindowShellView()
{
IShellView* view = nullptr;
IShellWindows* shell = nullptr;
CoCreateInstance(CLSID_ShellWindows, NULL, CLSCTX_LOCAL_SERVER,
IID_PPV_ARGS(&shell));
if (shell) {
VARIANT empty;
VariantInit(&empty);
VARIANT loc;
loc.vt = VT_VARIANT | VT_BYREF;
PIDLIST_ABSOLUTE locList;
SHGetFolderLocation(nullptr, CSIDL_DESKTOP, nullptr, 0, &locList);
loc.byref = locList;
HWND windowHandle = 0;
IDispatch* dispatch = nullptr;
shell->FindWindowSW(&loc, &empty, SWC_DESKTOP, (long*)&windowHandle,
SWFO_NEEDDISPATCH, &dispatch);
if (dispatch) {
IServiceProvider* provider = nullptr;
dispatch->QueryInterface(IID_PPV_ARGS(&provider));
if (provider) {
IShellBrowser* browser = nullptr;
provider->QueryService(SID_STopLevelBrowser, IID_PPV_ARGS(&browser));
if (browser) {
browser->QueryActiveShellView(&view);
browser->Release();
}
provider->Release();
}
dispatch->Release();
}
shell->Release();
}
return view;
}
static IShellDispatch2*
GetApplicationFromShellView(IShellView* view)
{
IShellDispatch2* shellDispatch = nullptr;
IDispatch* viewDisp = nullptr;
HRESULT hr = view->GetItemObject(SVGIO_BACKGROUND, IID_PPV_ARGS(&viewDisp));
if (SUCCEEDED(hr)) {
IShellFolderViewDual* shellViewFolder = nullptr;
viewDisp->QueryInterface(IID_PPV_ARGS(&shellViewFolder));
if (shellViewFolder) {
IDispatch* dispatch = nullptr;
shellViewFolder->get_Application(&dispatch);
if (dispatch) {
dispatch->QueryInterface(IID_PPV_ARGS(&shellDispatch));
dispatch->Release();
}
shellViewFolder->Release();
}
viewDisp->Release();
}
return shellDispatch;
}
static bool
ShellExecInExplorerProcess(wchar_t* path, wchar_t* args = nullptr)
{
bool rv = false;
if (SUCCEEDED(CoInitialize(nullptr))) {
IShellView *desktopView = GetDesktopWindowShellView();
if (desktopView) {
IShellDispatch2 *shellDispatch = GetApplicationFromShellView(desktopView);
if (shellDispatch) {
BSTR bstrPath = SysAllocString(path);
VARIANT vArgs;
VariantInit(&vArgs);
if (args) {
vArgs.vt = VT_BSTR;
vArgs.bstrVal = SysAllocString(args);
}
rv = SUCCEEDED(shellDispatch->ShellExecuteW(bstrPath, vArgs, VARIANT{},
VARIANT{}, VARIANT{}));
VariantClear(&vArgs);
SysFreeString(bstrPath);
shellDispatch->Release();
}
desktopView->Release();
}
CoUninitialize();
}
return rv;
}
struct stack_t {
stack_t* next;
TCHAR text[MAX_PATH];
};
/**
* Removes an element from the top of the NSIS stack
*
* @param stacktop A pointer to the top of the stack
* @param str The string to pop to
* @param len The max length
* @return 0 on success
*/
int
popstring(stack_t **stacktop, TCHAR *str, int len)
{
// Removes the element from the top of the stack and puts it in the buffer
stack_t *th;
if (!stacktop || !*stacktop) {
return 1;
}
th = (*stacktop);
lstrcpyn(str, th->text, len);
*stacktop = th->next;
HeapFree(GetProcessHeap(), 0, th);
return 0;
}
/**
* Adds an element to the top of the NSIS stack
*
* @param stacktop A pointer to the top of the stack
* @param str The string to push on the stack
* @param len The length of the string to push on the stack
* @return 0 on success
*/
void
pushstring(stack_t **stacktop, const TCHAR *str, int len)
{
stack_t *th;
if (!stacktop) {
return;
}
th = (stack_t*)HeapAlloc(GetProcessHeap(), 0, sizeof(stack_t) + len);
lstrcpyn(th->text, str, len);
th->next = *stacktop;
*stacktop = th;
}
/**
* Starts an executable or URL from the shell process.
*
* @param stacktop Pointer to the top of the stack, AKA the first parameter to
the plugin call. Should contain the file or URL to execute.
* @return 1 if the file/URL was executed successfully, 0 if it was not
*/
extern "C" void __declspec(dllexport)
Exec(HWND, int, TCHAR *, stack_t **stacktop, void *)
{
wchar_t path[MAX_PATH + 1];
wchar_t args[MAX_PATH + 1];
bool rv = false;
bool restoreArgString = false;
// We're skipping building the C runtime to keep the file size low, so we
// can't use a normal string initialization because that would call memset.
path[0] = L'\0';
args[0] = L'\0';
popstring(stacktop, path, MAX_PATH);
if (popstring(stacktop, args, MAX_PATH) == 0) {
// This stack item may not be for us, but we don't know yet.
restoreArgString = true;
}
if (lstrcmpW(args, L"/cmdargs") == 0) {
popstring(stacktop, args, MAX_PATH);
rv = ShellExecInExplorerProcess(path, args);
} else {
// If the stack wasn't empty, then we popped something that wasn't for us.
if (restoreArgString) {
pushstring(stacktop, args, lstrlenW(args));
}
rv = ShellExecInExplorerProcess(path);
}
pushstring(stacktop, rv ? L"1" : L"0", 2);
}
BOOL APIENTRY
DllMain(HMODULE, DWORD, LPVOID)
{
return TRUE;
}
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