Petr Mladek
2023-Jan-26 11:16 UTC
[PATCH 2/2] vhost: check for pending livepatches from vhost worker kthreads
On Wed 2023-01-25 10:57:30, Seth Forshee wrote:> On Wed, Jan 25, 2023 at 12:34:26PM +0100, Petr Mladek wrote: > > On Tue 2023-01-24 11:21:39, Seth Forshee wrote: > > > On Tue, Jan 24, 2023 at 03:17:43PM +0100, Petr Mladek wrote: > > > > On Fri 2023-01-20 16:12:22, Seth Forshee (DigitalOcean) wrote: > > > > > Livepatch relies on stack checking of sleeping tasks to switch kthreads, > > > > > so a busy kthread can block a livepatch transition indefinitely. We've > > > > > seen this happen fairly often with busy vhost kthreads. > > > > > > > > > --- a/drivers/vhost/vhost.c > > > > > +++ b/drivers/vhost/vhost.c > > > > > @@ -366,6 +367,9 @@ static int vhost_worker(void *data) > > > > > if (need_resched()) > > > > > schedule(); > > > > > } > > > > > + > > > > > + if (unlikely(klp_patch_pending(current))) > > > > > + klp_switch_current(); > > > > > > > > I suggest to use the following intead: > > > > > > > > if (unlikely(klp_patch_pending(current))) > > > > klp_update_patch_state(current); > > > > > > > > We already use this in do_idle(). The reason is basically the same. > > > > It is almost impossible to livepatch the idle task when a CPU is > > > > very idle. > > > > > > > > klp_update_patch_state(current) does not check the stack. > > > > It switches the task immediately. > > > > > > > > It should be safe because the kthread never leaves vhost_worker(). > > > > It means that the same kthread could never re-enter this function > > > > and use the new code. > > > > > > My knowledge of livepatching internals is fairly limited, so I'll accept > > > it if you say that it's safe to do it this way. But let me ask about one > > > scenario. > > > > > > Let's say that a livepatch is loaded which replaces vhost_worker(). New > > > vhost worker threads are started which use the replacement function. Now > > > if the patch is disabled, these new worker threads would be switched > > > despite still running the code from the patch module, correct? Could the > > > module then be unloaded, freeing the memory containing the code these > > > kthreads are executing? > > > > The above scenario would require calling klp_update_patch_state() from > > the code in the livepatch module. It is not possible at the moment because > > this function is not exported for modules. > > vhost can be built as a module, so in order to call > klp_update_patch_state() from vhost_worker() it would have to be > exported to modules.I see.> > Hmm, the same problem might be when we livepatch a function that calls > > another function that calls klp_update_patch_state(). But in this case > > it would be kthread() from kernel/kthread.c. It would affect any > > running kthread. I doubt that anyone would seriously think about > > livepatching this function. > > Yes, there are clearly certain functions that are not safe/practical to > patch, and authors need to know what they are doing. Most kthread main() > functions probably qualify as impractical at best, at least without a > strategy to restart relevant kthreads. > > But a livepatch transition will normally stall if patching these > functions when a relevant kthread is running (unless the patch is > forced), so a patch author who made a mistake should quickly notice. > vhost_worker() would behave differently.Another crazy idea: /** * klp_update_patch_state_safe() - do not update the path state when * called from a livepatch. * @task: task_struct to be updated * @calller_addr: address of the function which calls this one * * Do not update the patch set when called from a livepatch. * It would allow to remove the livepatch module even when * the code still might be in use. */ void klp_update_patch_state_safe(struct task_struct *task, void *caller_addr) { static bool checked; static bool safe; if (unlikely(!checked)) { struct module *mod; preempt_disable(); mod = __module_address(caller_addr); if (!mod || !is_livepatch_module(mod)) safe = true; checked = true; preempt_enable(); } if (safe) klp_update_patch_state(task); } and use in vhost_worker() if (unlikely(klp_patch_pending(current))) klp_update_patch_state_safe(current, vhost_worker); Even better might be to get the caller address using some compiler macro. I guess that it should be possible. And even better would be to detect this at the compile time. But I do not know how to do so.> > A good enough solution might be to document this. Livepatches could > > not be created blindly. There are more situations where the > > livepatch is tricky or not possible at all. > > I can add this if you like. Is Documentation/livepatch/livepatch.rst the > right place for this?Yes, the best place probably would be "7. Limitations" section in Documentation/livepatch/livepatch.rst. Even better would be to add a document about the best practices. We have dreamed about it for years ;-)> > Crazy idea. We could prevent this problem even technically. A solution > > would be to increment a per-process counter in klp_ftrace_handler() when a > > function is redirected(). And klp_update_patch_state() might refuse > > the migration when this counter is not zero. But it would require > > to use a trampoline on return that would decrement the counter. > > I am not sure if this is worth the complexity. > > > > One the other hand, this counter might actually remove the need > > of the reliable backtrace. It is possible that I miss something > > or that it is not easy/possible to implement the return trampoline. > > I agree this should work for unpatching, and even for patching a > function which is already patched. > > Maybe I'm misunderstanding, but this would only work for unpatching or > patching an already-patched function, wouldn't it? Because the original > functions would not increment the counter so you would not know if tasks > still had those on their call stacks.Right. I knew that it could not be that easy. Otherwise, we would have used it. I just did not spent enough cycles on the idea yesterday.> > Back to the original problem. I still consider calling > > klp_update_patch_state(current) in vhost_worker() safe. > > Okay, I can send a v2 which does this, so long as it's okay to export > klp_update_patch_state() to modules.It would be acceptable for me if we added a warning above the function definition and into the livepatch documentation. But I would prefer klp_update_patch_state_safe() if it worked. It is possible that I have missed something. Best Regards, Petr
Petr Mladek
2023-Jan-26 11:49 UTC
[PATCH 2/2] vhost: check for pending livepatches from vhost worker kthreads
On Thu 2023-01-26 12:16:36, Petr Mladek wrote:> On Wed 2023-01-25 10:57:30, Seth Forshee wrote: > > On Wed, Jan 25, 2023 at 12:34:26PM +0100, Petr Mladek wrote: > > > On Tue 2023-01-24 11:21:39, Seth Forshee wrote: > > > > On Tue, Jan 24, 2023 at 03:17:43PM +0100, Petr Mladek wrote: > > > > > On Fri 2023-01-20 16:12:22, Seth Forshee (DigitalOcean) wrote: > > > > > > Livepatch relies on stack checking of sleeping tasks to switch kthreads, > > > > > > so a busy kthread can block a livepatch transition indefinitely. We've > > > > > > seen this happen fairly often with busy vhost kthreads. > > > > > > > > > > > --- a/drivers/vhost/vhost.c > > > > > > +++ b/drivers/vhost/vhost.c > > > > > > @@ -366,6 +367,9 @@ static int vhost_worker(void *data) > > > > > > if (need_resched()) > > > > > > schedule(); > > > > > > } > > > > > > + > > > > > > + if (unlikely(klp_patch_pending(current))) > > > > > > + klp_switch_current(); > > > > > > > > > > I suggest to use the following intead: > > > > > > > > > > if (unlikely(klp_patch_pending(current))) > > > > > klp_update_patch_state(current); > > > > > > > > > > We already use this in do_idle(). The reason is basically the same. > > > > > It is almost impossible to livepatch the idle task when a CPU is > > > > > very idle. > > > > > > > > > Let's say that a livepatch is loaded which replaces vhost_worker(). New > > > > vhost worker threads are started which use the replacement function. Now > > > > if the patch is disabled, these new worker threads would be switched > > > > despite still running the code from the patch module, correct? Could the > > > > module then be unloaded, freeing the memory containing the code these > > > > kthreads are executing? > > > > > > Hmm, the same problem might be when we livepatch a function that calls > > > another function that calls klp_update_patch_state(). But in this case > > > it would be kthread() from kernel/kthread.c. It would affect any > > > running kthread. I doubt that anyone would seriously think about > > > livepatching this function.And I missed something. klp_update_patch_state_safe(), proposed below, would not cover the above scenario. It might be possible to add something similar to kthread() function. I think that it is the only "livepatchable" function that might call vhost_worker(). We could block klp_update_patch_state() for the entire kthread when the kthread() function is called from a livepatch. Well, it is all just the best effort. The reference counting in the ftrace handler would be more reliable. But it would require adding the trampoline on the return.> /** > * klp_update_patch_state_safe() - do not update the path state when > * called from a livepatch. > * @task: task_struct to be updated > * @calller_addr: address of the function which calls this one > * > * Do not update the patch set when called from a livepatch. > * It would allow to remove the livepatch module even when > * the code still might be in use. > */ > void klp_update_patch_state_safe(struct task_struct *task, void *caller_addr) > { > static bool checked; > static bool safe; > > if (unlikely(!checked)) { > struct module *mod; > > preempt_disable(); > mod = __module_address(caller_addr); > if (!mod || !is_livepatch_module(mod)) > safe = true; > checked = true; > preempt_enable(); > } > > if (safe) > klp_update_patch_state(task); > } > > and use in vhost_worker() > > if (unlikely(klp_patch_pending(current))) > klp_update_patch_state_safe(current, vhost_worker); > > Even better might be to get the caller address using some compiler > macro. I guess that it should be possible. > > And even better would be to detect this at the compile time. But > I do not know how to do so. > > > Okay, I can send a v2 which does this, so long as it's okay to export > > klp_update_patch_state() to modules. > > It would be acceptable for me if we added a warning above the function > definition and into the livepatch documentation.I would probably go this way after all. Still thinking... Best Regards, Petr
Reasonably Related Threads
- [PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads
- [PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads
- [PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads
- [PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads
- apple gmux