Re: [PATCH 01/23] all: syscall wrappers: add documentation

[Catalin Marinas <catalin.marinas@arm.com>]

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