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https://issues.apache.org/jira/browse/LOG4NET-360?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Dominik Psenner reassigned LOG4NET-360:
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Assignee: Dominik Psenner
> EventLogAppender can corrupt the event log on Windows Vista and higher if the
> string is longer than 31839 bytes
> ---------------------------------------------------------------------------------------------------------------
>
> Key: LOG4NET-360
> URL: https://issues.apache.org/jira/browse/LOG4NET-360
> Project: Log4net
> Issue Type: Bug
> Components: Appenders
> Affects Versions: 1.2.11
> Environment: Windows Vista or higher
> Reporter: Michael Cessna
> Assignee: Dominik Psenner
> Priority: Blocker
> Attachments: Event Log Corruption.txt
>
>
> Issue: EventLogAppender can corrupt the event log on Windows Vista and higher
> if the string is longer than 31839 bytes.
> The log4net EventLogAppender allows you to write up to 32000 bytes and there
> is a defect in the .NET Framework that will let you write that much data in
> Windows Vista and higher (which has a limit of 31839).
> See the attachment that shows "$exception {"The event log file is
> corrupted"} System.Exception {System.ComponentModel.Win32Exception}"
> along with the stack after a call to EventLog.WriteEntry() under Windows 7
> that used a message string that was 31,876 bytes long.
> The issue has been reported to Microsoft as well:
> https://connect.microsoft.com/VisualStudio/feedback/details/770126/eventlog-writeentry-can-corrupt-the-event-log-because-of-invalid-argument-check-in-net-framework#tabs
> The code below is my workaround for determining a max message length that
> will not corrupt the event log. I've also written our own custom
> EventLogAppender that is able to work around this issue.
> private const int MaxEventLogMsgLength_PreVista = 32766;
> private const int MaxEventLogMsgLength_VistaOrHigher = 31839;
> /// <summary>
> /// Gets the maximum allowable size of event log message for the current
> operating system.
> /// </summary>
> /// <returns></returns>
> public static int GetMaxEventLogMessageSize()
> {
> // http://msdn.microsoft.com/en-us/library/xzwc042w(v=vs.100).aspx
>
> // The 32766 documented max size is two bytes shy of 32K (I'm assuming
> 32766 may leave space for a two byte null
> // terminator of #0#0). The 32766 max length is what the .NET 4.0
> source code checks for, but this is WRONG!...
> // strings with a length > 31839 on Windows Vista or higher can CORRUPT
> the event log! See:
> // System.Diagnostics.EventLogInternal.InternalWriteEvent() for the use
> of the 32766 max size.
> var maxEventMsgSize = MaxEventLogMsgLength_PreVista;
> // Windows Vista and higher
> if (Environment.OSVersion.Platform == PlatformID.Win32NT &&
> Environment.OSVersion.Version.Major >= 6)
> {
> // See ReportEvent API:
> http://msdn.microsoft.com/en-us/library/aa363679(VS.85).aspx
> // ReportEvent's lpStrings parameter: "A pointer to a buffer
> containing an array of null-terminated strings that are
> // merged into the message before Event Viewer displays the
> string to the user. This parameter must be a valid pointer
> // (or NULL), even if wNumStrings is zero. Each string is
> limited to 31,839 characters."
> // Going beyond the size of 31839 will (at some point) corrupt
> the event log on Windows Vista or higher! It may succeed
> // for a while...but you will eventually run into the error:
> "System.ComponentModel.Win32Exception : A device attached to
> // the system is not functioning", and the event log will then
> be corrupt (I was able to corrupt an event log using a
> // length of 31877 on Windows 7).
> // The max size for Windows Vista or higher is documented here:
> http://msdn.microsoft.com/en-us/library/xzwc042w(v=vs.100).aspx.
> // Going over this size may succeed a few times but the buffer
> will overrun and eventually corrupt the log (based on testing).
> // Log4net's own EventLogAppender will write up to 32000 bytes
> (0x7D00), which can corrupt the event log.
> // The maxEventMsgSize size is based on the max buffer size of
> the lpStrings parameter of the ReportEvent API.
> // The documented max size for EventLog.WriteEntry for Windows
> Vista and higher is 31839, but I'm leaving room for a
> // terminator of #0#0, as we cannot see the source of
> ReportEvent (though we could use an API monitor to examine the
> // buffer, given enough time).
> // TODO: Use an API monitor to examine how the ReportEvent API
> allocates a buffer for the event log message strings.
> const int terminatorLength = 2; // Safety for now.
> maxEventMsgSize = MaxEventLogMsgLength_VistaOrHigher -
> terminatorLength;
> }
> return maxEventMsgSize;
> }
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