From b73c1849148da1229a3c3b336311a8194970b35f Mon Sep 17 00:00:00 2001 From: Andrew Baumann Date: Wed, 18 Nov 2015 11:45:09 -0800 Subject: tap-win32: disable broken async write path The code under the TUN_ASYNCHRONOUS_WRITES path makes two incorrect assumptions about the behaviour of the WriteFile API for overlapped file handles. First, WriteFile does not update the lpNumberOfBytesWritten parameter when the write completes asynchronously (the number of bytes written is known only when the operation completes). Second, the buffer shouldn't be touched (or freed) until the operation completes. This led to at least one bug where tap_win32_write returned zero bytes written, which in turn caused further writes ("receives") to be disabled for that device. This change disables the asynchronous write path, while keeping most of the code around in case someone sees value in resurrecting it. It also adds some conditional debug output, similar to the read path. Signed-off-by: Andrew Baumann Acked-by: Stefan Weil Signed-off-by: Jason Wang --- net/tap-win32.c | 46 ++++++++++++++++++++++++++++++++++++---------- 1 file changed, 36 insertions(+), 10 deletions(-) (limited to 'net') diff --git a/net/tap-win32.c b/net/tap-win32.c index 5e5d6db..7fddb20 100644 --- a/net/tap-win32.c +++ b/net/tap-win32.c @@ -77,7 +77,12 @@ //#define DEBUG_TAP_WIN32 -#define TUN_ASYNCHRONOUS_WRITES 1 +/* FIXME: The asynch write path appears to be broken at + * present. WriteFile() ignores the lpNumberOfBytesWritten parameter + * for overlapped writes, with the result we return zero bytes sent, + * and after handling a single packet, receive is disabled for this + * interface. */ +/* #define TUN_ASYNCHRONOUS_WRITES 1 */ #define TUN_BUFFER_SIZE 1560 #define TUN_MAX_BUFFER_COUNT 32 @@ -461,27 +466,48 @@ static int tap_win32_write(tap_win32_overlapped_t *overlapped, BOOL result; DWORD error; +#ifdef TUN_ASYNCHRONOUS_WRITES result = GetOverlappedResult( overlapped->handle, &overlapped->write_overlapped, &write_size, FALSE); if (!result && GetLastError() == ERROR_IO_INCOMPLETE) WaitForSingleObject(overlapped->write_event, INFINITE); +#endif result = WriteFile(overlapped->handle, buffer, size, &write_size, &overlapped->write_overlapped); +#ifdef TUN_ASYNCHRONOUS_WRITES + /* FIXME: we can't sensibly set write_size here, without waiting + * for the IO to complete! Moreover, we can't return zero, + * because that will disable receive on this interface, and we + * also can't assume it will succeed and return the full size, + * because that will result in the buffer being reclaimed while + * the IO is in progress. */ +#error Async writes are broken. Please disable TUN_ASYNCHRONOUS_WRITES. +#else /* !TUN_ASYNCHRONOUS_WRITES */ if (!result) { - switch (error = GetLastError()) - { - case ERROR_IO_PENDING: -#ifndef TUN_ASYNCHRONOUS_WRITES - WaitForSingleObject(overlapped->write_event, INFINITE); -#endif - break; - default: - return -1; + error = GetLastError(); + if (error == ERROR_IO_PENDING) { + result = GetOverlappedResult(overlapped->handle, + &overlapped->write_overlapped, + &write_size, TRUE); } } +#endif + + if (!result) { +#ifdef DEBUG_TAP_WIN32 + LPTSTR msgbuf; + error = GetLastError(); + FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER|FORMAT_MESSAGE_FROM_SYSTEM, + NULL, error, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), + &msgbuf, 0, NULL); + fprintf(stderr, "Tap-Win32: Error WriteFile %d - %s\n", error, msgbuf); + LocalFree(msgbuf); +#endif + return 0; + } return write_size; } -- cgit v1.1