如何在进程之间共享内核对象

本文说明如何构造安全描述符，以授予给定组对内核对象的访问权限，使该对象能够在以不同用户帐户运行的进程之间共享。

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有时，服务必须创建可由运行在不同用户帐户下的进程访问的内核对象。要允许以这种方式共享对象，服务必须使用明确授权用户对象访问权限的安全描述符来创建对象。或者，安全描述符可以授予对整个用户组的访问权限。

本文中的示例代码非常特定，因为它授予经过身份验证的用户 组对互斥体的访问权限。这段代码使用了大量的安全 API 来构造互斥体的安全描述符。有关这些安全函数的完整文档，请参阅 MSDN Library 中 Platform SDK 文档的“Access Control”（访问控制）部分：

http://msdn.microsoft.com/library/en-us/security/hh/winbase/accctrl_2hf0.asp

您必须完全了解这些调用（通常情况下还包括访问控制技术），以便针对其他用户、组和内核对象改编代码。

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示例代码

以下示例代码说明了如何构造安全描述符，以授予经过身份验证的用户 组 GENERIC_READ、GENERIC_WRITE 和 GENERIC_EXECUTE 权限。新构造的安全描述符随即应用于互斥体。

出于说明目的，此示例代码使用了通用访问权限。这些通用权限利用了可用于所有内核对象的通用映射。在实际的应用程序中，创建对象时最好使用对象特定的访问权限。

给定的资源管理器在映射到对象特定的访问权限时，可能会解释给定的通用权限集，以包括其他标准访问权限。例如，为互斥体指定 GENERIC_READ、GENERIC_WRITE 和 GENERIC_EXECUTE 权限相当于指定对象特定的权限 MUTEX_MODIFY_STATE 和 SYNCHRONIZE。因此，其他线程要想获得互斥体的句柄，只能通过在 OpenMutex() 中为 dwDesiredAccess 参数指定 MUTEX_MODIFY_STATE 和/或 SYNCHRONIZE。使用 MUTEX_ALL_ACCESS 打开互斥体的尝试将失败，并显示错误代码 5 (ERROR_ACCESS_DENIED)。

#include <windows.h>
#include <stdio.h>

// The following function initializes the supplied security descriptor
// with a DACL that grants the Authenticated Users group GENERIC_READ,
// GENERIC_WRITE, and GENERIC_EXECUTE access.
// 
// The function returns NULL if any of the access control APIs fail.
// Otherwise, it returns a PVOID pointer that should be freed by calling
// FreeRestrictedSD() after the security descriptor has been used to
// create the object.

PVOID BuildRestrictedSD(PSECURITY_DESCRIPTOR pSD) {

   DWORD  dwAclLength;

   PSID   pAuthenticatedUsersSID = NULL;

   PACL   pDACL   = NULL;
   BOOL   bResult = FALSE;

   PACCESS_ALLOWED_ACE pACE = NULL;

   SID_IDENTIFIER_AUTHORITY siaNT = SECURITY_NT_AUTHORITY;
   
   SECURITY_INFORMATION si = DACL_SECURITY_INFORMATION;
   
   __try {

      // initialize the security descriptor
      if (!InitializeSecurityDescriptor(pSD, 
            SECURITY_DESCRIPTOR_REVISION)) {
         printf("InitializeSecurityDescriptor() failed with error %d\n",
               GetLastError());
         __leave;
      }

      // obtain a sid for the Authenticated Users Group
      if (!AllocateAndInitializeSid(&siaNT, 1, 
            SECURITY_AUTHENTICATED_USER_RID, 0, 0, 0, 0, 0, 0, 0, 
            &pAuthenticatedUsersSID)) {
         printf("AllocateAndInitializeSid() failed with error %d\n",
               GetLastError());
         __leave;
      }

      // NOTE:
      // 
      // The Authenticated Users group includes all user accounts that
      // have been successfully authenticated by the system. If access
      // must be restricted to a specific user or group other than 
      // Authenticated Users, the SID can be constructed using the
      // LookupAccountSid() API based on a user or group name.

      // calculate the DACL length
      dwAclLength = sizeof(ACL)
            // add space for Authenticated Users group ACE
            + sizeof(ACCESS_ALLOWED_ACE) - sizeof(DWORD)
            + GetLengthSid(pAuthenticatedUsersSID);

      // allocate memory for the DACL
      pDACL = (PACL) HeapAlloc(GetProcessHeap(), HEAP_ZERO_MEMORY, 
            dwAclLength);
      if (!pDACL) {
         printf("HeapAlloc() failed with error %d\n", GetLastError());
         __leave;
      }

      // initialize the DACL
      if (!InitializeAcl(pDACL, dwAclLength, ACL_REVISION)) {
         printf("InitializeAcl() failed with error %d\n", 
               GetLastError());
         __leave;
      }
      
      // add the Authenticated Users group ACE to the DACL with
      // GENERIC_READ, GENERIC_WRITE, and GENERIC_EXECUTE access
      if (!AddAccessAllowedAce(pDACL, ACL_REVISION,
            GENERIC_READ | GENERIC_WRITE | GENERIC_EXECUTE,
            pAuthenticatedUsersSID)) {
         printf("AddAccessAllowedAce() failed with error %d\n",
               GetLastError());
         __leave;
      }

      // set the DACL in the security descriptor
      if (!SetSecurityDescriptorDacl(pSD, TRUE, pDACL, FALSE)) {
         printf("SetSecurityDescriptorDacl() failed with error %d\n",
               GetLastError());
         __leave;
      }

      bResult = TRUE;
     
   } __finally {

      if (pAuthenticatedUsersSID) FreeSid(pAuthenticatedUsersSID);
   }

   if (bResult == FALSE) {
      if (pDACL) HeapFree(GetProcessHeap(), 0, pDACL);
      pDACL = NULL;
   }

   return (PVOID) pDACL;
}

// The following function frees memory allocated in the
// BuildRestrictedSD() function
VOID FreeRestrictedSD(PVOID ptr) {

   if (ptr) HeapFree(GetProcessHeap(), 0, ptr);

   return;
}

void main(void)
{
   SECURITY_ATTRIBUTES sa;
   SECURITY_DESCRIPTOR sd;

   PVOID  ptr;
   HANDLE hMutex;

   // build a restricted security descriptor
   ptr = BuildRestrictedSD(&sd);
   if (!ptr) {
      printf("BuildRestrictedSD() failed\n");
      return;
   }

   // create a mutex using the security descriptor
   sa.nLength = sizeof(sa);
   sa.lpSecurityDescriptor = &sd;
   sa.bInheritHandle = FALSE;

   hMutex = CreateMutex(&sa, FALSE, "RestrictedMutex");
   if (!hMutex)
      printf("CreateMutex() failed with error %d\n", GetLastError());

   // free the memory allocated by BuildRestrictedSD
   FreeRestrictedSD(ptr);

   // use the mutex ...

   printf("Press the return key to close the mutex handle...\n");
   getchar();

   // close the mutex handle
   CloseHandle(hMutex);
}