On Sun, Feb 28, 2016 at 07:57:29AM +0100, Ingo Molnar wrote: > > Another thing I've noticed are _thousands_ of these warnings: > > objtool: kernel/sched/core.o: preempt_schedule_irq()+0x20: function has unreachable instruction > objtool: kernel/sched/core.o: preempt_schedule_irq()+0x27: function has unreachable instruction > objtool: kernel/sched/core.o: preempt_schedule_irq()+0x2c: function has unreachable instruction > objtool: kernel/sched/core.o: preempt_schedule_irq()+0x33: function has unreachable instruction > objtool: net/ceph/pagevec.o: ceph_put_page_vector()+0x171: function has unreachable instruction > objtool: net/ceph/pagevec.o: ceph_put_page_vector()+0x178: function has unreachable instruction > objtool: net/ceph/pagevec.o: ceph_put_page_vector()+0x17d: function has unreachable instruction > objtool: net/ceph/pagevec.o: ceph_put_page_vector()+0x184: function has unreachable instruction > objtool: net/ceph/pagevec.o: ceph_get_direct_page_vector()+0xe2: function has unreachable instruction > objtool: net/ceph/pagevec.o: ceph_get_direct_page_vector()+0xe9: function has unreachable instruction > objtool: net/ceph/pagevec.o: ceph_get_direct_page_vector()+0xee: function has unreachable instruction > objtool: net/ceph/pagevec.o: ceph_get_direct_page_vector()+0xf5: function has unreachable instruction > objtool: net/ceph/pagevec.o: zero_user_segment()+0x120: function has unreachable instruction > objtool: net/ceph/pagevec.o: zero_user_segment()+0x127: function has unreachable instruction > objtool: net/ceph/pagevec.o: zero_user_segment()+0x12c: function has unreachable instruction > objtool: net/ceph/pagevec.o: zero_user_segment()+0x133: function has unreachable instruction > > that's on an x86-64 allyesconfig kernel, with objtool merged to tip:master, using > GCC 4.9.2: > > gcc version 4.9.2 20150212 (Red Hat 4.9.2-6) (GCC) Those are caused by CONFIG_KASAN, CONFIG_UBSAN, and CONFIG_GCOV_KERNEL, which can all add unreachable instructions. Technically, an unreachable instruction isn't really a problem, but objtool warns about it because it often means there's something going on in the control flow which it doesn't understand. I guess we could make CONFIG_STACK_VALIDATION conflict with those options, but I think that would disable it on allyesconfig. I'll see if I can make objtool smarter so that it detects these special cases of unreachable instructions and ignores them. > Also, please prefix such warnings with the standard compiler prefix, something > like this: > > kernel/sched/core.c: warning: objtool: preempt_schedule_irq()+0x20: function has unreachable instruction > > so that scripts/tools monitoring new build warnings can pick them up > automatically? Yeah, good idea. -- Josh -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
