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

[Kirk Allan]

 >>>
> Quoting Paolo Bonzini (2015-11-11 08:49:57)
>> 
>> 
>> 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).as
> px
>> 
>> 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
> 
> Yah, both are more proper actually (I was relying on FILE_GENERIC_READ
> already containing the other flags from FILE_GENERIC_WRITE, but that's
> more likely to break in the future).
> 
> I think I prefer the former since it avoids confusion on GENERIC_READ vs.
> FILE_GENERIC_READ differences.
> 
> Kirk, I'm planning on squashing this into your v2 series, so no need to
> resubmit.

Sounds great.  Thanks
- Kirk

> 
> Thanks!
> 
>> 
>> ?
>> 
>> Paolo
>>