[jira] [Commented] (LOG4NET-360) EventLogAppender can corrupt the event log on Windows Vista and higher if the string is longer than 31839 bytes

["Michael Cessna (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/LOG4NET-360?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13509369#comment-13509369 ] 

Michael Cessna commented on LOG4NET-360:
----------------------------------------

Yes, those edits look correct to me.

Thanks
                
> 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
>         Attachments: EventLogAppender.cs.patch, 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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