Token query types and performance optimizations

[dmex]

Hello,

The WIL library defines some token classes without fixed sizes. For example:

wil/include/wil/token_helpers.h

Lines 38 to 57 in a8cd266

 template<typename T> struct MapTokenStructToInfoClass; 
 template<> struct MapTokenStructToInfoClass<TOKEN_ACCESS_INFORMATION> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenAccessInformation; static constexpr bool FixedSize = false; }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_APPCONTAINER_INFORMATION> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenAppContainerSid; static constexpr bool FixedSize = false; }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_DEFAULT_DACL> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenDefaultDacl; static constexpr bool FixedSize = false; }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_GROUPS_AND_PRIVILEGES> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenGroupsAndPrivileges; static constexpr bool FixedSize = false; }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_MANDATORY_LABEL> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenIntegrityLevel; static constexpr bool FixedSize = false; }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_OWNER> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenOwner; static constexpr bool FixedSize = false;  }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_PRIMARY_GROUP> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenPrimaryGroup; static constexpr bool FixedSize = false;  }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_PRIVILEGES> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenPrivileges; static constexpr bool FixedSize = false;  }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_USER> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenUser; static constexpr bool FixedSize = false;  }; 
  
 // fixed size cases 
 template<> struct MapTokenStructToInfoClass<TOKEN_ELEVATION_TYPE> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenElevationType; static constexpr bool FixedSize = true; }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_MANDATORY_POLICY> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenMandatoryPolicy; static constexpr bool FixedSize = true; }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_ORIGIN> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenOrigin; static constexpr bool FixedSize = true; }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_SOURCE> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenSource; static constexpr bool FixedSize = true; }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_STATISTICS> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenStatistics; static constexpr bool FixedSize = true; }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_TYPE> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenType; static constexpr bool FixedSize = true; }; 
 template<> struct MapTokenStructToInfoClass<SECURITY_IMPERSONATION_LEVEL> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenImpersonationLevel;  static constexpr bool FixedSize = true; }; 
 template<> struct MapTokenStructToInfoClass<TOKEN_ELEVATION> { static constexpr TOKEN_INFORMATION_CLASS infoClass = TokenElevation; static constexpr bool FixedSize = true; }; 

TOKEN_USER was defined FixedSize = false when it has a fixed size defined in the Windows SDK:

#define TOKEN_USER_MAX_SIZE (sizeof(TOKEN_USER) + SECURITY_MAX_SID_SIZE)

The others also have types in the Windows SDK:

#define TOKEN_OWNER_MAX_SIZE (sizeof(TOKEN_OWNER) + SECURITY_MAX_SID_SIZE)
#define TOKEN_PRIMARY_GROUP_MAX_SIZE (sizeof(TOKEN_PRIMARY_GROUP) + SECURITY_MAX_SID_SIZE)
#define TOKEN_TOKEN_DEFAULT_DACL_MAX_SIZE (sizeof(TOKEN_DEFAULT_DACL) + SECURITY_MAX_SID_SIZE)

As an example this WIL code:

wil::unique_tokeninfo_ptr<TOKEN_USER> user;
RETURN_IF_FAILED(wil::get_token_information_nothrow(user, GetCurrentProcessToken()));

Is expanded into:

wil/include/wil/token_helpers.h

Lines 177 to 188 in a8cd266

 tokenInfo.reset(); 
 tokenHandle = GetCurrentThreadEffectiveTokenWithOverride(tokenHandle); 
  
 DWORD tokenInfoSize = 0; 
 const auto infoClass = details::MapTokenStructToInfoClass<T>::infoClass; 
 RETURN_LAST_ERROR_IF(!((!GetTokenInformation(tokenHandle, infoClass, nullptr, 0, &tokenInfoSize)) && 
     (::GetLastError() == ERROR_INSUFFICIENT_BUFFER))); 
 unique_tokeninfo_ptr<T> tokenInfoClose{ static_cast<T*>(::operator new(tokenInfoSize, std::nothrow)) }; 
 RETURN_IF_NULL_ALLOC(tokenInfoClose); 
 RETURN_IF_WIN32_BOOL_FALSE(GetTokenInformation(tokenHandle, infoClass, tokenInfoClose.get(), tokenInfoSize, &tokenInfoSize)); 
 tokenInfo = wistd::move(tokenInfoClose); 
  

We query TOKEN_USER and TOKEN_OWNER hundreds of times a day and since WIL doesn't use the fixed sizes for these types it's instead using multiple system calls to query the size, then multiple system calls to allocate virtual memory (also waiting/blocking the process heap lock) which is a significantly more slower and larger overhead than necessary.

The WIL templates should be doing something like this instead:

    ULONG returnLength;
    UCHAR tokenUserBuffer[TOKEN_USER_MAX_SIZE];
    PTOKEN_USER tokenUser = (PTOKEN_USER)tokenUserBuffer;

    GetTokenInformation(
        TokenHandle,
        TokenUser,
        tokenUser,
        sizeof(tokenUserBuffer),
        &returnLength
        );

[Jon Wiswall]

When I first added these helpers, templatizing on the type seemed right; instead, I should have templatized on the info class instead, and used that to imply the output type. I'll look into adding a variants that do that.When C++20 constexpr is enabled it should also make the code denser.

At the time, the goal was to avoid allocating more than necessary in the common case. The OS had lots of the "dual call" sequences, so I just lifted them all into this method.

However, I can see a parallel set of types that have a fixed buffer inside them that provide the T result on a block of bytes, appropriately aligned.