Re: [PATCH v2 6/7] staging: android: binder: fix alignment issues
From: Arve Hjønnevåg
Date: Wed Apr 10 2013 - 18:30:23 EST
On Wed, Apr 10, 2013 at 9:39 AM, Serban Constantinescu
> On 10/04/13 00:58, Arve Hjønnevåg wrote:
>> On Tue, Apr 9, 2013 at 3:00 AM, Serban Constantinescu
>> <serban.constantinescu@xxxxxxx> 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.
Do any 64 bit systems align pointers in a struct to 8 bytes? If so, we
should keep the start address of the struct 8 byte aligned as well.
> 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:
> [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).
Does this work on every 64 bit system?
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