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Re: [Qemu-stable] [PATCH] qga: fix append file open modes for win32
From: |
Kirk Allan |
Subject: |
Re: [Qemu-stable] [PATCH] qga: fix append file open modes for win32 |
Date: |
Wed, 11 Nov 2015 08:42:54 -0700 |
>>>
> 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
>>