Several of the structures in linux/uapi/linux/sctp.h are marked __attribute__((packed, aligned(4))). I believe this was done so that the UAPI structure was the same on both 32 and 64bit systems. The 'natural' alignment is that of 'u64' - so would differ between 32 and 64 bit x86 cpus. There are two horrible issues here: 1) I believe the natural alignment of u64 is actually 8 bytes on some 32bit architectures. So the change would have broken binary compatibility for 32bit applications compiled before the alignment was added. 2) Inside the kernel the address of the structure member is 'blindly' passed through as if it were an aligned pointer. For instance I'm pretty sure is can get passed to inet_addr_is_any() (in net/core/utils.). Here it gets passed to memcmp(). gcc will inline the memcmp() and almost certainly use 64bit accesses. These will fault on architectures (like sparc64). No amount of casting can make gcc 'forget' the alignment of a structure. Passing to an external function as 'void *' will - but even the LTO could track the alignment through. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
