On 10/04/13 00:58, Arve Hjønnevåg wrote:
On Tue, Apr 9, 2013 at 3:00 AM, Serban Constantinescu
<[email protected]> 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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