On Sun, Dec 11, 2016 at 01:08:33PM +1100, Balbir Singh wrote: > > > On 11/12/16 04:17, Josh Poimboeuf wrote: > > On Sat, Dec 10, 2016 at 04:46:17PM +1100, Balbir Singh wrote: > >> On Thu, 2016-12-08 at 12:08 -0600, Josh Poimboeuf wrote: > >>> Dusting the cobwebs off the consistency model again. This is based on > >>> linux-next/master. > >>> > >>> v1 was posted on 2015-02-09: > >>> > >>> https://lkml.kernel.org/r/cover.1423499826.git.jpoimboe@xxxxxxxxxx > >>> > >>> v2 was posted on 2016-04-28: > >>> > >>> https://lkml.kernel.org/r/cover.1461875890.git.jpoimboe@xxxxxxxxxx > >>> > >>> The biggest issue from v2 was finding a decent way to detect preemption > >>> and page faults on the stack of a sleeping task. > >> > >> Could you please elaborate on this? Preemption of a sleeping task and > >> faults as in the future (time) preemption and faults? > > > > The normal way for a task to go to sleep is to call schedule(). objtool > > ensures the stack trace is reliable in that case, by making sure that > > all functions save the frame pointer on the stack before calling out to > > another function. > > > > But a task can also go to sleep in a few other ways. One way is by > > preemption, where an interrupt handler interrupts the task and calls > > preempt_schedule_irq(). > > It's preempted, not sleeping. It's on_rq but not on_cpu. You're right, I used the word "sleeping" when I meant "not currently executing on a CPU". (Peter Z also pointed that out.) > Another way is by a page fault exception. In > > both cases, there's no guarantee that the interrupted function saved the > > frame pointer on the stack beforehand. So the stack trace might be > > unreliable. Fortunately, interrupts and exceptions leave evidence > > behind on the stack. So when walking the stack of a sleeping task, we > > can detect when an IRQ or exception occurred, and consider such a stack > > unreliable. > > > > Thanks for the explanation. I presume a whole lot of this is arch specific > code? I'll look at the patches as well Most of the new livepatch code is arch-independent, but the consistency model part of it (i.e., !klp_patch.immediate) is currently only supported by x86_64. For adding support for other architectures, there are a few options: 1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and for non-DWARF unwinders, also making sure there's a way for the stack tracing code to detect interrupts on the stack. 2) Alternatively, figure out a way to patch kthreads without stack checking. If all kthreads sleep in the same place, then we can designate that place as a patching point. I think Petr M has been working on that? In that case, arches without HAVE_RELIABLE_STACKTRACE would still be able to use the non-stack-checking parts of the consistency model: a) patching user tasks when they cross the kernel/user space boundary; and b) patching kthreads and idle tasks at their designated patch points. This option isn't as good as option 1 because it requires signaling most of the tasks to patch them. But it could still be a good backup option for those architectures which don't have reliable stack traces yet. In the meantime, other architectures can keep today's behavior by setting klp_patch.immediate to true. -- 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
