[PATCH] QueryDosDevice in handle_to_fn

Corinna Vinschen corinna-cygwin@cygwin.com
Sun Mar 16 15:22:00 GMT 2008

On Mar 16 03:14, Brian Dessent wrote:
>   I debugged this and found the
> strangest thing, when you call QueryDosDevice (NULL, fnbuf, len) to get
> the list of all DOS devices and len >= 65536, Win32 always returns 0
> with GetLastError set to ERROR_MORE_DATA.  Since len was being set as
> "sizeof (OBJECT_NAME_INFORMATION) + NT_MAX_PATH * sizeof (WCHAR)" this
> always happened, causing handle_to_fn() to simply give up and copy the
> Win32 name into the POSIX name and return.  The attached patch fixes the
> problem by just clamping the size of the buffer to under 64k.

<insert lament here>

> Another observation that I had while debugging this is that calling
> strncasematch() in this function is probably wrong -- it expands to
> cygwin_strncasecmp(), which is a wrapper that first converts both
> arguments to temporary UNICODE strings and then calls
> RtlCompareUnicodeString() -- we're doing this on strings that we had
> just converted *out* of UNICODE.  I think ascii_strncasematch() is
> probably what we want here instead, either that or try to stay in
> unicode throughout.

Using ascii_strncasematch here is right because the DEVICE_PREFIX is
plain ascii anyway.  But, yes, the function should be converted to
do everything in WCHAR/UNICODE_STRING and only convert to char *
when creating the final posix_fn.

> +  /* For some reason QueryDosDevice will fail with Win32 errno 234
> +     (ERROR_MORE_DATA) if you try to pass a buffer larger than 64k  */
> +  size_t qddlen = len < 65536 ? len : 65535;

len is a const value.  Checking len for being < 65536 is a constant
expression which always results in qddlen being 65535 so the ?: is
a noop, more or less.

Did you test if QueryDosDeviceW has the same problem as QueryDosDeviceA?
If not, we should use that function.


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

More information about the Cygwin-patches mailing list