关于SetEnvironmentStringsW在空环境进程中调用返回ERROR_INVALID_PARAMETER的技术问询

大家好，我最近调试Windows环境变量相关逻辑时，碰到了SetEnvironmentStringsW函数的一个奇怪问题，想请教下有没有人遇到过或者知道原因。

正常来说，SetEnvironmentStringsW应该能用来恢复之前通过GetEnvironmentStringsW备份的环境变量块，这本来是个很方便的方式来临时修改然后恢复环境。但我发现，如果一个进程是用空环境创建的——也就是调用CreateProcessW时，lpEnvironment参数传L"\0"，同时指定CREATE_UNICODE_ENVIRONMENT标志——之后再调用SetEnvironmentStringsW去恢复之前获取的环境块，就会直接失败：返回0，调用GetLastError()得到ERROR_INVALID_PARAMETER。这个问题我在Win7、Win10、Win11的x64版本上都复现了。

我写了个小工具passenv来复现这个问题，它支持三种调用方式：

• passenv 1：给子进程传递只有一个变量的环境
• passenv 0：给子进程传递空环境
• 直接运行passenv：当它作为子进程被递归调用时，会输出最关键的测试结果

测试程序源码 #

文件名：passenv.c

// Demonstrate an issue in `SetEnvironmentStringsW` kernel32 function
// Compile:
// $ gcc passenv.c -municode -o passenv
#include <stdio.h>
#include <windows.h>

void ShowGetEnvironmentStringsW() {
    printf("--- get environment ---\n");
    WCHAR* env_block = GetEnvironmentStringsW();
    if (env_block) {
        for (WCHAR* p = env_block; *p; ++p) {
            wprintf(L"%ls\n", p);
            while (*p) ++p;
        }
        FreeEnvironmentStringsW(env_block);
    }
    printf("-----------------------\n");
}

void ShowSetEnvironmentStringsW(WCHAR* env) {
    printf("SetEnvironmentStringsW ... ");
    DWORD result = SetEnvironmentStringsW(env);
    if (result) {
        printf("OK\n");
    } else {
        printf("Error %u\n", GetLastError());
    }
}

void ShowGetEnvironmentVariableW(WCHAR* name) {
    WCHAR value[1023+1] = L"NOP";
    DWORD size = sizeof(value);
    SetLastError(ERROR_SUCCESS);
    DWORD written = GetEnvironmentVariableW(name, value, size);
    if (written && (written < size)) {
        wprintf(L"%ls: '%ls' (%u)\n", name, value, written);
    } else {
        DWORD error_code = GetLastError();
        if (error_code == ERROR_ENVVAR_NOT_FOUND) {
            wprintf(L"Variable '%ls' was not found.\n", name);
        } else {
            wprintf(L"Error reading %ls: %u\n", name, error_code);
        }
        wprintf(L"%ls: '%ls' (%u)\n", name, value, written);
    }
}

void ShowSetEnvironmentVariableW(WCHAR* name, WCHAR* value) {
    wprintf(L"Set var '%ls' to value '%ls' ... ", name, value);
    DWORD result = SetEnvironmentVariableW(name, value);
    if (result) {
        printf("OK\n");
    } else {
        DWORD error_code = GetLastError();
        if (error_code == ERROR_INVALID_PARAMETER) {
            printf("Error: %u (ERROR_INVALID_PARAMETER)\n", error_code);
        } else {
            printf("Error: %u\n", error_code);
        }
    }
}

int restart_self(WCHAR* cmd, WCHAR* env) {
    STARTUPINFO si;
    ZeroMemory( &si, sizeof(si) );
    si.cb = sizeof(si);
    PROCESS_INFORMATION pi;
    ZeroMemory( &pi, sizeof(pi) );

    if (!CreateProcessW( NULL, cmd, NULL, NULL, FALSE, CREATE_UNICODE_ENVIRONMENT, env, NULL, &si, &pi )) {
        printf( "CreateProcess failed (%u).\n", GetLastError() );
        return 1;
    }

    // Wait until child process exits.
    WaitForSingleObject( pi.hProcess, INFINITE );
    DWORD code;
    GetExitCodeProcess(pi.hProcess, &code);

    CloseHandle(pi.hProcess);
    CloseHandle(pi.hThread);

    return code;
}

int wmain(int argc, wchar_t* argv[]) {
    if ((argc == 2) && (0==wcscmp(argv[1], L"0"))) {
        printf("arg 0\n");
        return restart_self(argv[0], L"\0");
    } else if ((argc == 2) && (0==wcscmp(argv[1], L"1"))) {
        printf("arg 1\n");
        return restart_self(argv[0], L"a=1\0");
    } else {
        WCHAR* old_env = GetEnvironmentStringsW();

        ShowGetEnvironmentStringsW();

        ShowSetEnvironmentVariableW(L"TEST", L"test");
        ShowGetEnvironmentVariableW(L"TEST");
        ShowGetEnvironmentStringsW();

        ShowSetEnvironmentVariableW(L"TEST", NULL);
        ShowGetEnvironmentVariableW(L"TEST");
        ShowGetEnvironmentStringsW();

        ShowSetEnvironmentStringsW(old_env);
        ShowGetEnvironmentStringsW();

        FreeEnvironmentStringsW(old_env);
    }
    return 0;
}

编译命令 #

$ gcc passenv.c -municode -o passenv

测试结果 #

情况1：子进程有正常环境（passenv 1）

这种情况下，SetEnvironmentStringsW能正常工作，输出如下：

$> passenv 1
arg 1
--- get environment ---
a=1
-----------------------
Set var 'TEST' to value 'test' ... OK
TEST: 'test' (4)
--- get environment ---
a=1
TEST=test
-----------------------
Set var 'TEST' to value '(null)' ... OK
Variable 'TEST' was not found.
TEST: 'NOP' (0)
--- get environment ---
a=1
-----------------------
SetEnvironmentStringsW ... OK
--- get environment ---
a=1
-----------------------

情况2：子进程是空环境（passenv 0）

这种情况下，最后一步调用SetEnvironmentStringsW会失败，返回ERROR_INVALID_PARAMETER（错误码87），输出如下：

$> passenv 0
arg 0
--- get environment ---
-----------------------
Set var 'TEST' to value 'test' ... OK
TEST: 'test' (4)
--- get environment ---
TEST=test
-----------------------
Set var 'TEST' to value '(null)' ... OK
Variable 'TEST' was not found.
TEST: 'NOP' (0)
--- get environment ---
-----------------------
SetEnvironmentStringsW ... Error 87
--- get environment ---
-----------------------

我想知道这是SetEnvironmentStringsW的已知限制吗？官方文档里好像没提到这个情况，有没有什么workaround可以在空环境的进程里正常恢复环境变量块？

内容来源于stack exchange