• The Old New Thing

    Why does the Win32 Time service require the date to be correct before it will set the time?


    Public Service Announcement: Daylight Saving Time ends in most parts of the United States this weekend.

    Andy points out that if you attempt to synchronize your clock when the date is set incorrectly, the operation fails with the error message "An error occurred while Windows was synchronizing with time.windows.com. For security reasons, Windows cannot synchronize with the server because your date does not match. Please fix the date and try again." He wonders what the security risk is.

    First of all, for people who are trying to solve the problem, the solution is to follow the steps in the error message. Set your date to the correct date, then try again. If that doesn't help, also set the time to something close to the correct time. Once your time gets close, the time server can nudge it the rest of the way.

    Back to the original question: What is the security risk being defended against, here?

    At first glance, you might think that the server is attempting to defend itself against a client whose time is set incorrectly, but actually the potential attack is in reverse: Your computer is protecting itself against a rogue time server.

    The Kerberos authentication protocol relies heavily on all participants agreeing on what time it is (with some slop tolerance). If somebody manages to fool the client into synchronizing its time against a rogue server (for example, by using a DNS poisoning attack), the attacker can use that invalid date (typically a backdate) as a foothold for the next level of attacks.

    The default configuration for the Windows Time service is to reject attempts to change the clock on domain-joined machines by more than 15 hours. You can change the configuration settings by following the instructions in this KB article (which happens also to have been the source material for most of this article).

  • The Old New Thing

    The story of MUI, as told by others (with some commentary)


    First, the story as told by others:

    Now the question you're all going to ask so I may as well answer it: Why is this information kept in a desktop.ini file instead of being attached to the file itself (say, in an alternate stream)? If it were in an alternate stream, then it would track the file when it was moved or copied.

    First answer: Because alternate streams require NTFS. Localized names were introduced in Windows 2000, and Windows 2000 gave you the option of formatting your drive as FAT or NTFS. (It wasn't until Windows Vista that NTFS became mandatory.) Therefore the mechanism for localized names needed to work when your drive was formatted as FAT.

    Okay, fine, maybe you tell people, "Sorry, this feature requires NTFS." All those multinational corporations who use FAT-formatted drives in the year 2000 are screwed.

    Well, placing the information in an alternate data stream means that each file would have to be accessed just to get its name. Remember, it's more efficient when you buy in bulk. Consider a directory with 500 files. A simple directory listing like one provided by cmd.exe takes, say, seven I/O requests (open directory, five "give me the next 100 files", close directory). If each file had to be opened in order to probe for an alternate stream, you now have 507 I/O requests: open directory, five "give me the next 100 files", close directory; and then 500 "open alternate stream on this file" calls that fail. And that was the optimistic case where the localized name doesn't exist. For the pessimistic case where every file has a localized name, the I/O count balloons to 1507: open directory, five "give me the next 100 files", close directory; and then 500 × (open alternate stream, read localized name, close alternate stream).

    You increased the number of I/O requests by a factor of over 200. And when you are looking at files on a slow network (hello, multinational company with servers all over the world), that factor can be deadly. Suppose the ping time to the server is 500ms. The cmd.exe program gets you the directory listing in three and a half seconds. The alternate data stream localized name version takes twelve minutes.

    On the other hand, if all the localized names are placed in a single file, then the I/O count is only 10: open directory, five "give me the next 100 files", close directory; open desktop.ini, read contents, close desktop.ini. We're down to five seconds. Not as good as three and a half seconds, but way better than twelve minutes. And if, in the common case, the directory contains no localized names, the open desktop.ini step fails, and we save two I/O's, bringing our time down to four seconds. And then you can be clever and say, "Wait a second, I already have the directory listing; I can just look at the results to see if desktop.ini is on the list. If not, then I don't even have to bother trying to open it!" Now you are back to three and a half seconds.

    Okay, maybe you tell people, "Sorry, this feature requires a high-speed network." All those multinational corporations with servers around the world are screwed. The theoretically highest-speed network connection between New York and Tokyo has a 72ms ping time because that's how fast it takes light itself to travel that distance and back. Your 500-file directory takes nearly two minutes to display.

    Seeing as multinational corporations were the initial target audience for the MUI feature (since they're the ones who are most likely to have users with different language preferences), designing your feature so that your target audience can't use it seems like a pretty bad execution plan.

    A file's name is traditionally considered metadata, and traditionally, metadata is visible without requiring access to the file. A file can show up in a directory listing even though you don't have access to it. But if the file's localized name is stored in the file itself, you now have the situation where you don't even know the name of the file because you can't access it. This is even worse than I'm sorry, you don't have permission to know where this shortcut file should show up in the Explorer window. This would be I'm sorry, you don't have permission to know the name of the file in this directory listing.

    Remember that reading from an alternate data stream triggers a recall if the file had been archived to tape. You don't want to have to restore an entire directory from tape just because somebody opened the folder in Explorer.

    And finally, alternate data streams are very fragile. Any tool that processes a file has a decent chance of inadvertently destroying the alternate data streams. (And good luck if your backup program doesn't preserve the alternate data streams.)

  • The Old New Thing

    The quiet fading away of the CtlPanelClass


    If you search MSDN for CtlPanelClass, you'll find a few really old Knowledge Base articles that include it in a list of "class names of common Windows applications." I'm not sure why the Knowledge Base articles bothered to list those classes; there is no technical reason for applications to need to know this, and including the information merely encourages programmers to rely on the window class name in strange undocumented ways. (This is another one of those cases where a Knowledge Base article written to assist in troubleshooting becomes interpreted as formal documentation.)

    Windows Vista finally got rid of that window class. It took only ten years.

    The window class was used by the old Windows 3.1 Control Panel application. Back in Windows 3.1, the Control Panel was a standalone application and was not integrated into the shell namespace (in large part because there was no such thing as a shell namespace back then for it to be a part of).

    There was one program which not only searched for a window of that specific class name, but it also sent the window WM_COMMAND messages, since of course it knew what the menu IDs were for the Control Panel application, and it knew that the Windows developers would never change the IDs or replace the standalone Control Panel application with anything else.

    When the standalone Control Panel application was converted to a virtual folder, it also came with some decoys in order to maintain compatibility with older programs that relied on the old behavior in strange undocumented ways.

    One of those decoys, the CtlPanelClass was removed for Windows Vista. A crash was traced back to a bug in the decoy window which manifested itself if a control panel took more than ten seconds to launch itself. To fix the bug, we just removed the decoy window, but we were prepared to write a compatibility shim in case there were people still running that ancient application from 1993. We didn't actually turn the compatibility shim on, but we were ready just in case.

    We removed the decoy and crossed our fingers.

    We got lucky. Nobody noticed.

  • The Old New Thing

    The wisdom of seventh graders: Being President


    Today is Election Day in the United States. Some years ago, seventh grade students (age 12) were asked to imagine they had just been elected president and to give an address to the nation on one thing they would change.

    Remember, these are only the most entertaining ideas. Do not assume all student ideas are like these.

    • Ban "all types of non-purpose car racing."
    • Defend against zombie attack.
    • Withdraw from Iraq and use the money to fund electric cars and ban smoking.
    • Repeal all taxes.
    • Name Arnold Schwarzenegger as Vice President.
    • Shorten all school days to increase learning.
    • Build a holographic wall at the border. For the immigrants who don't fall for it and run right through, station border patrol just behind it.
    • Shut down all cigarette factories. "If you somehow manage to get cigarettes, we don't care."
    • Make chocolate milk the official drink.

    And this sentence came from a student destined for greatness as a politician: "Something must be done, and I will make it happen."

  • The Old New Thing

    Saying that your case is different doesn't make it so


    A customer wanted to do one of those user-hostile things that Windows doesn't make easy to do (the sort of thing I tend to call out on this Web site). After receiving an explanation that Windows doesn't provide a way of doing what they want because it abuses the component in question and goes against user expectations, the customer countered, "Yes, we understand that, but our case is different."

    The customer then proceeded to explain how they intended to use this newfound power (if only they could figure out how to do it) and under what circumstances they intend to invoke it. Their explanation was interesting in that the description could be applied to any other program on the planet.

    Yes, we understand that, but our case is different. We would do this only after the user installs the program or reconfigures it from the Add or Remove Programs control panel. After a few days, we would stop doing it, unless triggered by a reinstall or a reconfiguration.

    So far, there's nothing here that explains why your program should be able to do this, but not, say, Photoshop. There is no evidence that this program is any different from the tens of thousands of other programs out there, many of which probably want to do that very same thing this program wants to do.

    Just because you say that your case is different doesn't make it so.

  • The Old New Thing

    Why doesn't the End Task button end my task immediately?


    Commenter littleguru asks, "Why does the End Now button not kill the process immediately?"

    When you click the End Now button, the process really does end now, but not before a brief message from our sponsor.

    When you kill a hung application, Windows Error Reporting steps in to record the state of the hung application so it can be submitted to the mother ship (with your permission). If you are running Windows XP or Windows Vista, you can briefly see a process called dumprep.exe or WerFault.exe; these are the guys who are doing the data collection.

    After being uploaded to Microsoft, these failure reports are studied to determine why the application stopped responding and what could be done to fix it. I've been asked to do quite a few of these analyses myself, and sometimes it's something pretty mundane (an application sends a cross-thread message while holding a critical section, and the thread can't receive the message because it's stuck waiting for the critical section that the sender is holding—classic deadlock), and sometimes it's something pretty weird (application has a bug if the number of sound output devices is not equal to one). Whatever the reason, I write up my analysis, and the people who are in charge of such things make arrangements for the information to be sent back to the vendors who wrote the application (assuming the vendors are registered with Winqual).

    If you don't want Windows Error Reporting to collect application crash and hang reports, you can disable it from the Group Policy Editor under Windows Error Reporting. Of course, if you do this, then you don't get to vote on which program crashes and failures Microsoft should work on fixing.

    Note: This entry is an experiment: I mentioned Windows Error Reporting and WHQL. If people complain about digital certificate authorities, that'll just confirm my bias against returning to those old debugging stories.

    Update: Experimental results obtained. No more stories involving Windows Error Reporting and WHQL.

  • The Old New Thing

    It's the Great Pumpkin, Charlie Brown: The world of competitive pumpkin-growing


    Bill Littlefield of NPR's sports program Only a Game interviews Susan Warren about competitive pumpkin-growing. [Direct link - Real format] An excerpt from her book Backyard Giants was printed in The Wall Street Journal: The Race to Break the Squash Barrier, the quest to grow a one-ton pumpkin. I'm fascinated by these subcultures of people obsessed with one thing.

  • The Old New Thing

    Debugging walkthrough: Diagnosing a __purecall failure


    Prerequisite: Understanding what __purecall means.

    I was asked to help diagnose an issue in which a program managed to stumble into the __purecall function.

    00a14509 a100000000      mov     eax,dword ptr ds:[00000000h] ds:0023:00000000=????????

    The stack at the point of failure looked like this:


    The line at XYZ!CViewFrame::SetFrame that called the mystic __purecall was a simple AddRef:

      pSomething->AddRef(); // crashes in __purecall

    From what we know of __purecall, this means that somebody called into a virtual method on a derived class after the derived class's destructor has run. Okay, well, let's see if we can find the object in question. Since the method being called is a COM method, the __stdcall calling convention applies, which means that the this pointer is on the stack.

    0:023> dd esp+4 l1
    0529f76c  06a88d58

    Using our knowledge of the layout of a COM object, we can navigate through memory to find the vtable.

    0:023> dps 06a88d58
    06a88d58  009b2eac XYZ!CRegistrationSink::`vftable'
    06a88d5c  06b20058
    06a88d60  00000002 
    06a88d64  00998930 XYZ!CObjectWithBrush::`vftable'
    06a88d68  00000000 
    06a88d6c  009c9c80 XYZ!CBrowseSite::`vftable'
    06a88d70  009c9c70 XYZ!CBrowseSite::`vftable'
    06a88d74  00000000
    0:023> dps 009b2eac
    009b2eac  00a14509 XYZ!_purecall // virtual QueryInterface() = 0
    009b2eb0  00a14509 XYZ!_purecall // virtual AddRef() = 0
    009b2eb4  00a14509 XYZ!_purecall // virtual Release() = 0
    009b2eb8  009cb1e4 XYZ!CRegistrationSink::Register
    009b2ebc  009b3d2d XYZ!CRegistrationSink::Unregister

    We see that the object has been destructed down to the CRegistrationSink base class, and the attempt to increment its reference count has led us into the abyss of __purecall.

    But what was this object before it descended into madness?

    Well, we know that the object was something derived from CRegistrationSink. And the other values in memory tell us that the object most likely also derived from CObjectWithBrush and CBrowseSite. Just for fun, here's the CObjectWithBrush vtable, to confirm that we destructed down to that point:

    00998930  00a14509 XYZ!_purecall // virtual QueryInterface() = 0
    00998934  00a14509 XYZ!_purecall // virtual AddRef() = 0
    00998938  00a14509 XYZ!_purecall // virtual Release() = 0
    0099893c  0099880d XYZ!CObjectWithBrush::SetBrush
    00998940  00a319ee XYZ!CObjectWithBrush::GetBrush
    00998944  00a13fd9 XYZ!CObjectWithBrush::`scalar deleting destructor'

    Ooh, it looks like CObjectWithBrush has a virtual destructor. Probably to destroy the brush.

    A check of the source code tells us that nobody derives from CBrowseSite, so that is almost certainly the original object type.

    As a cross-check, we check whether what we have matches the memory layout of a CBrowseSite:

    0:023> dt XYZ!CBrowseSite 06a88d58
       +0x000 __VFN_table : 0x009b2eac
       +0x004 m_prgreg         : 0x06a88d58 Registration
       +0x008 m_creg           : 2
       +0x00c __VFN_table : 0x00998930
       +0x010 m_hbr            : (null)
       +0x014 __VFN_table : 0x009c9c80
       +0x018 __VFN_table : 0x009c9c70
       +0x01c m_cRef           : 0

    Looks not unreasonable. (Well, aside from the fact that we have a bug...) The object has most likely begun its destruction because its reference count (_cRef) went to zero.

    At this point, there was enough information to ask the developers responsible for CViewFrame and CBrowseSite to work out how the CViewFrame ended up running around with a pointer to an object that has already been destructed.

  • The Old New Thing

    Why is there an LVN_ODSTATECHANGED notification when there's already a perfectly good LVN_ITEMCHANGED notification?


    If you work with owner-data listviews, you take the responsibility for managing the data associated with each item in the list view. The list view control itself only knows how many items there are; when it needs information about an item, it asks you for it. It's the fancy name for a "virtual list view" control.

    When you use an ownerdata list view, you will receive a new notification, LVN_ODSTATECHANGED. The OD stands for ownerdata, so this is an "owner data state changed" notification. The list view sends this notification when the state of one or more items in an owner data list view control change simultaneously. Mind you, the list view control can also send the LVN_ITEMCHANGED notification if the state of an item changes, so you need to be on the lookout for both.

    If there is a notification LVN_ITEMCHANGED, then what's the purpose of the LVN_ODSTATECHANGED message? It's redundant, after all.

    Well yes, it's redundant, but it's faster, too. The LVN_ODSTATECHANGED notification tells you that the state of all items in the specified range has changed. It's a shorthand for sending an individual LVN_ITEMCHANGED for all items in the range [iFrom..iTo]. If you have an ownerdata list view with 500,000 items and somebody does a select-all, you'll be glad that you get a single LVN_ODSTATECHANGED notification with iFrom=0 and iTo=499999 instead of a half million individual little LVN_ITEMCHANGED notifications.

  • The Old New Thing

    How do I programmatically invoke Aero Peek on a window?


    A customer wanted to know if there was a way for their application to invoke the Aero Peek feature so that their window appeared and all the other windows on the system turned transparent.

    No, there is no such programmatic interface exposed. Aero Peek is a feature for the user to invoke, not a feature for applications to invoke so they can draw attention to themselves.

    Yes, I realize you wrote a program so awesome that all other programs pale in comparison, and that part of your mission is to make all the other programs literally pale in comparison to your program.


    Maybe you can meet up with that other program that is the most awesome program in the history of the universe and share your sorrows over a beer.

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