On Wed, Apr 10, 2013 at 9:39 AM, Serban Constantinescu <[email protected]> wrote: > 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. >
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: > > 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). > Does this work on every 64 bit system? -- Arve Hjønnevåg -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
