On Fri, May 27, 2016 at 12:49:11PM +0200, Arnd Bergmann wrote: > On Friday, May 27, 2016 10:30:52 AM CEST Catalin Marinas wrote: > > On Fri, May 27, 2016 at 10:42:59AM +0200, Arnd Bergmann wrote: > > > On Friday, May 27, 2016 8:03:57 AM CEST Heiko Carstens wrote: > > > > > > > > Cost wise, this seems like it all cancels out in the end, but what > > > > > > > > do I know? > > > > > > > > > > > > > > I think you know something, and I also think Heiko and other s390 guys > > > > > > > know something as well. So I'd like to listen their arguments here. > > > > > > > > If it comes to 64 bit arguments for compat system calls: s390 also has an > > > > x32-like ABI extension which allows user space to use full 64 bit > > > > registers. As far as I know hardly anybody ever made use of that. > > > > > > > > However even if that would be widely used, to me it wouldn't make sense to > > > > add new compat system calls which allow 64 bit arguments, simply because > > > > something like > > > > > > > > c = (u32)a | (u64)b << 32; > > > > > > > > can be done with a single 1-cycle instruction. It's just not worth the > > > > extra effort to maintain additional system call variants. > > > > > > For reference, both tile and mips also have separate 32-bit ABIs that are > > > only used on 64-bit kernels (aside from the normal 32-bit ABI). Tile > > > does it like s390 and passes 64-bit arguments as pairs, while MIPS > > > and x86 and pass them as single registers. > > > > AFAIK, x32 also requires that the upper half of a 64-bit reg is zeroed > > by the user when a 32-bit value is passed. We could require the same on > > AArch64/ILP32 but I'm a bit uneasy on trusting a multitude of C > > libraries on this. > > It's not about trusting a C library, it's about ensuring malicious code > cannot pass argumentst that the kernel code assumes will never happen. At least for pointers and sizes, we have additional checks in place already, like __access_ok(). Most of the syscalls should be safe since they either go through some compat functions taking 32-bit arguments or are routed to native functions which already need to cope with a full random 64-bit value. On arm64, I think the only risk comes from syscall handlers expecting 32-bit arguments but using 64-bit types. Apart from pointer types, I don't expect this to happen but we could enforce it via a BUILD_BUG_ON(sizeof(t) > 4 && !__TYPE_IS_PTR(t)) in __SC_DELOUSE as per the s390 implementation. With ILP32 if we go for 64-bit off_t, those syscalls would be routed directly to the native layer. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
