* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > On Wed, Jul 12, 2017 at 09:27:50PM +0200, Ingo Molnar wrote: > > Maybe we could offer a menu of unwinders - i.e. make the whole Kconfig interface a > > bit nicer: > > > > CONFIG_UNWINDER_FRAME_POINTER > > CONFIG_UNWINDER_ORC > > CONFIG_UNWINDER_GUESS > > > > ... or so? > > So far I haven't been able to figure out how to make the above three > options into a multiple choice selection, such that allnoconfig selects > CONFIG_UNWINDER_GUESS and alldefconfig selects > CONFIG_UNWINDER_FRAME_POINTER. I don't think that's a problem: the scheduler preemption model Kconfig setup has similar behavior - allyesconfig does not enable CONFIG_PREEMPT=y. The new x86 default will eventually be the Orc unwinder, but not initially. > > I wouldn't mind making CONFIG_UNWINDER_ORC the new default either, due to the > > non-trivial speedup it offers - but maybe folks would object? > > Personally I wouldn't have an objection to making ORC the default, though we > should probably wait to give it some burn-in time first. Sure, that's what testing is for. > If we *do* decide to eventually make it the default, we could flip the switch at > the same time we introduced the multiple-choice config and rename above. That > way, users of "make oldconfig" would see the change and would be encouraged to > switch ORC. I disagree, as the current Kconfig layout actively hinders the 'more testing' part: you can only enable Orc if you knew how to do it, and 99% of our testers won't bother. In practice that's a testing coverage that is close to not testing it at all ... > > > > CONFIG_FRAME_POINTERS et al would be left for architectures where it has a meaning > > > > beyond backtrace generation. (Not sure whether there's any such architectures.) > > > > > > Well, on x86, hardened usercopy relies on frame pointers, but not the > > > unwinder. It does the frame pointer walk manually to avoid the full > > > unwinder overhead. See arch_within_stack_frames(). BTW., I think this aspect of the hardened user-copy is crazy stuff - there can be many stack frames, and this adds a serious amount of overhead even with frame pointers... I think the current behavior is fine: if frame pointers are disabled then arch_within_stack_frames() returns NOT_STACK. Maybe it could do a few sanity checks: we do know the kernel stack range and we could check alignment as well. Thanks, Ingo -- 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
