Re: [PATCH] QueryDosDevice in handle_to_fn

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

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

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