Re: [Qemu-devel] [PATCH] qga: fix append file open modes for win32

[Paolo Bonzini <pbonzini@redhat.com>]

On 11/11/2015 15:02, Michael Roth wrote:
>> GENERIC_READ for files
>>         = FILE_READ_DATA
>>         + FILE_READ_ATTRIBUTES
>>         + FILE_READ_EA
>>         + SYNCHRONIZE
>>         + STANDARD_RIGHTS_READ (which is READ_CONTROL)
>>
>> GENERIC_WRITE for files
>>         = FILE_APPEND_DATA
>>         + FILE_WRITE_DATA
>>         + FILE_WRITE_ATTRIBUTES
>>         + FILE_WRITE_EA
>>         + SYNCHRONIZE
>>         + STANDARD_RIGHTS_WRITE (which is READ_CONTROL too!)
>>
>> Of these of course qemu-ga only needs FILE_*_DATA and possibly SYNCHRONIZE.
>>
>> The above description doesn't say what happens if you specify
>> FILE_READ_DATA and FILE_APPEND_DATA together, but I guess you can expect
>> it to just work.
> 
> Thanks, this is very helpful. This seems to coincide with
> FILE_GENERIC_WRITE / FILE_GENERIC_READ if you want to access the
> bitmasks directly (though it looks like you can still add flags
> to GENERIC_WRITE / GENERIC_READ):
> 
>   https://msdn.microsoft.com/en-us/library/windows/desktop/aa364399(v=vs.85).aspx

Yes, I had missed the FILE_GENERIC_* definitions.  I found them now in
mingw as well, and they are exactly what you would expect (an | of the
various flags).

> Looks like the crux of it is that FILE_WRITE_DATA causes us not to ignore
> the start offset (0) for writes, so we end up writing data at the
> beginning of the file instead of the end.
> 
> So I think the following
> should work:
> 
>   a: FILE_GENERIC_WRITE & ~FILE_WRITE_DATA
>   a+: FILE_GENERIC_READ | FILE_APPEND_DATA
> 
> FILE_APPEND_DATA by itself might work for a:, but for consistency I
> think I'd prefer sticking with the generic set and masking out
> FILE_WRITE_DATA.

For a+ I would use any of

	(FILE_GENERIC_READ | FILE_GENERIC_WRITE) & ~FILE_WRITE_DATA
	GENERIC_READ | (FILE_GENERIC_WRITE & ~FILE_WRITE_DATA)

Perhaps you can define this:

#define FILE_GENERIC_APPEND	(FILE_GENERIC_WRITE & ~FILE_WRITE_DATA)

and then use

	a: FILE_GENERIC_APPEND
	a+: GENERIC_READ|FILE_GENERIC_APPEND

or

	a: FILE_GENERIC_APPEND
	a+: FILE_GENERIC_READ|FILE_GENERIC_APPEND

?

Paolo