Re: llist code relies on undefined behaviour, upsets llvm/clang

Peter Zijlstra
2017-01-16T09:06:11Z

On Mon, Jan 16, 2017 at 08:36:00AM +1100, Anton Blanchard wrote:
> Hi,
>
> I was debugging a hang on a ppc64le kernel built with clang, and it
> looks to be undefined behaviour with pointer wrapping in the llist code.
>
> A test case is below. llist_for_each_entry() does container_of() on a
> NULL pointer, which wraps our pointer negative, then adds the same
> offset back in and expects to get back to NULL. Unfortunately clang
> decides that this can never be NULL and optimises it into an infinite
> loop.
>
> Build with -DFIX, such that the llist_node has a zero offset from the
> start of the struct, and things work.
>
> Is anyone other than ppc64le building kernels with llvm/clang these
> days? This should reproduce on ARM64 and x86-64.

Last I checked I couldn't build a x86_64 kernel with llvm. So no, not
something I've ever ran into.

Also, I would argue that this is broken in llvm, the kernel very much
relies on things like this all over the place. Sure, we're way outside
of what the C language spec says, but who bloody cares ;-)

If llvm wants to compile the kernel, it needs to learn the C dialect the
kernel uses.