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.