linux-kernel - Re: [PATCH v2 6/7] staging: android: binder: fix alignment issues

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Date:	Wed, 10 Apr 2013 17:39:56 +0100
From:	Serban Constantinescu <Serban.Constantinescu@....com>
To:	Arve Hjønnevåg <arve@...roid.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	John Stultz <john.stultz@...aro.org>,
	Dave Butcher <Dave.Butcher@....com>
Subject: Re: [PATCH v2 6/7] staging: android: binder: fix alignment issues

On 10/04/13 00:58, Arve Hjønnevåg wrote:
> On Tue, Apr 9, 2013 at 3:00 AM, Serban Constantinescu
> <serban.constantinescu@....com> wrote:
>> The Android userspace aligns the data written to the binder buffers to
>> 4bytes. Thus for 32bit platforms or 64bit platforms running an 32bit
>> Android userspace we can have a buffer looking like this:
>>
>> platform    buffer(binder_cmd   pointer)      size
>> 32/32                 32b         32b          8B
>> 64/32                 32b         64b          12B
>> 64/64                 32b         64b          12B
>>
>> Thus the kernel needs to check that the buffer size is aligned to 4bytes
>> not to (void *) that will be 8bytes on 64bit machines.
>>
>> The change does not affect existing 32bit ABI.
>>
>
> Do we not want the pointers to be 8 byte aligned on 64bit platforms?

No since here we do not align pointers we align binder_buffers and 
offsets in a buffer.

Let's assume that from the userspace we receive a sequence of BC_INCREFS 
and BC_FREE_BUFFER. According to their definitions the buffer would look 
like:

Buffer:
[addr]		[element]
0		BC_INCREFS
4		__u32
8		BC_FREE_BUFFER
12		void *        //(8 bytes for 64bit or 4 bytes for 32bit)

Thus the data_size(sizeof(Buffer)) will be 20 bytes for 64bit 
systems(4bytes aligned). Same explanation for offp where it represents 
the offset form the start of the buffer to a flat_binder_object(for 
example here the offset to void* - 12bytes).


Thanks,
Serban

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