Michael Cessna created LOG4NET-360:
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Summary: 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
Priority: Blocker
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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