[Qemu-devel] [PATCH 2/2] tap-win32: disable broken async write path

[Andrew Baumann <Andrew.Baumann@microsoft.com>]

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 overlapped parameter is
non-NULL (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 <Andrew.Baumann@microsoft.com>
---
Although this version of the patch tries to be minimally invasive and
keep the code around, I'm not sure if there is much value in fixing
this -- I haven't checked, but would expect that the write to a tap
device is just a fairly fast buffer copy. If you would prefer a patch
that simply removes the async write support entirely, that should be
easy to do.

 net/tap-win32.c | 48 +++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 37 insertions(+), 11 deletions(-)

diff --git a/net/tap-win32.c b/net/tap-win32.c
index 5e5d6db..02a3b0d 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
@@ -461,26 +466,47 @@ 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);
+                       NULL, &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) {
+        error = GetLastError();
+        if (error == ERROR_IO_PENDING) {
+            result = GetOverlappedResult(overlapped->handle,
+                                         &overlapped->write_overlapped,
+                                         &write_size, TRUE);
+        }
+    }
+#endif
 
     if (!result) {
-        switch (error = GetLastError())
-        {
-        case ERROR_IO_PENDING:
-#ifndef TUN_ASYNCHRONOUS_WRITES
-            WaitForSingleObject(overlapped->write_event, INFINITE);
+#ifdef DEBUG_TAP_WIN32
+        LPVOID msgbuf;
+        error = GetLastError();
+        FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER|FORMAT_MESSAGE_FROM_SYSTEM,
+                      NULL, error, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
+                      (LPTSTR)&msgbuf, 0, NULL);
+        fprintf(stderr, "Tap-Win32: Error WriteFile %d - %s\n", error, msgbuf);
+        LocalFree(msgbuf);
 #endif
-            break;
-        default:
-            return -1;
-        }
+        return 0;
     }
 
     return write_size;
-- 
2.5.3