From: Marcelo Ricardo Leitner > Sent: 21 July 2020 03:55 > > On Mon, Jul 20, 2020 at 03:50:16PM +0000, David Laight wrote: > > Several of the structures in linux/uapi/linux/sctp.h are > > marked __attribute__((packed, aligned(4))). > > I don't think we can change that by now. It's bad, yes, but it's > exposed and, well, for a long time (since 2005). > > > > > 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. > > Not sure which? Try arm for starters. > > So the change would have broken binary compatibility > > for 32bit applications compiled before the alignment > > was added. > > If nobody complained in 15 years, that's probably not a problem. ;-) > > > > > 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). > > For 2) here we should fix it by copying the data into a different > buffer, or something like that. At least on some architectures. I did wonder if the buffer could be read to 8n+4 aligned memory, but there are aligned 64bit items elsewhere. > That is happening on structs sctp_setpeerprim sctp_prim > sctp_paddrparams sctp_paddrinfo, right? > As they all use the pattern of having a sockaddr_storage after a s32. Not no mention sctp_assoc_stats.... Which is broken for 32bit binaries on x86 and sparc 64bit kernels. I think there is (there should be) a kernel type on 64bit systems that is 8 bytes with the alignment it would have on the corresponding 32bit architecture. If nothing else using alignof() on a structure containing a member of that type will give the 4 or 8 required to fix the code. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
