On Mon 2020-01-06 17:56:54, Peter Zijlstra wrote: > On Mon, Jan 06, 2020 at 05:39:30PM +0100, Jann Horn wrote: > > Hi! > > > > I was chatting with kpsingh about BPF trampolines, and I noticed that > > it looks like BPF trampolines (as of current bpf-next/master) seem to > > be missing synchronization between trampoline code updates and > > trampoline execution. Or maybe I'm missing something? > > > > If I understand correctly, trampolines are executed directly from the > > fentry placeholders at the start of arbitrary kernel functions, so > > they can run without any locks held. So for example, if task A starts > > executing a trampoline on entry to sys_open(), then gets preempted in > > the middle of the trampoline, and then task B quickly calls > > BPF_RAW_TRACEPOINT_OPEN twice, and then task A continues execution, > > task A will end up executing the middle of newly-written machine code, > > which can probably end up crashing the kernel somehow? > > > > I think that at least to synchronize trampoline text freeing with > > concurrent trampoline execution, it is necessary to do something > > similar to what the livepatching code does with klp_check_stack(), and > > then either use a callback from the scheduler to periodically re-check > > tasks that were in the trampoline or let the trampoline tail-call into > > a cleanup helper that is part of normal kernel text. And you'd > > probably have to gate BPF trampolines on > > CONFIG_HAVE_RELIABLE_STACKTRACE. > > ftrace uses synchronize_rcu_tasks() to flip between trampolines iirc. ftrace calls also schedule_on_each_cpu(ftrace_sync) to handle situations where RCU is not watching, see rcu_is_watching(). The following is called in ftrace_shutdown(): schedule_on_each_cpu(ftrace_sync); if (IS_ENABLED(CONFIG_PREEMPTION)) synchronize_rcu_tasks(); arch_ftrace_trampoline_free(ops); Best Regards, Petr
