On Tue, Apr 2, 2024 at 5:52 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > On Wed, 3 Apr 2024 09:40:48 +0900 > Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> wrote: > > > OK, for me, this last sentence is preferred for the help message. That explains > > what this is for. > > > > All callbacks that attach to the function tracing have some sort > > of protection against recursion. This option is only to verify that > > ftrace (and other users of ftrace_test_recursion_trylock()) are not > > called outside of RCU, as if they are, it can cause a race. > > But it also has a noticeable overhead when enabled. Sounds good to me, I can add this to the description of the Kconfig option. > > > > BTW, how much overhead does this introduce? and the race case a kernel crash? I just checked our fleet-wide production data for the last 24 hours. Within the kprobe/kretprobe code path (ftrace_trampoline and everything called from it), rcu_is_watching (both calls, see below) cause 0.484% CPU cycles usage, which isn't nothing. So definitely we'd prefer to be able to avoid that in production use cases. > > or just messed up the ftrace buffer? > > There's a hypothetical race where it can cause a use after free. > > That is, after you shutdown ftrace, you need to call synchronize_rcu_tasks(), > which requires RCU to be watching. There's a theoretical case where that > task calls the trampoline and misses the synchronization. Note, these > locations are with preemption disabled, as rcu is always watching when > preemption is enabled. Thus it would be extremely fast where as the > synchronize_rcu_tasks() is rather slow. > > We also have synchronize_rcu_tasks_rude() which would actually keep the > trace from happening, as it would schedule on each CPU forcing all CPUs to > have RCU watching. > > I have never heard of this race being hit. I guess it could happen on a VM > where a vCPU gets preempted at the right moment for a long time and the > other CPUs synchronize. > > But like lockdep, where deadlocks can crash the kernel, we don't enable it > for production. > > The overhead is another function call within the function tracer. I had > numbers before, but I guess I could run tests again and get new numbers. > I just noticed another rcu_is_watching() call, in rethook_try_get(), which seems to be a similar and complementary validation check to the one we are putting under CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING option in this patch. It feels like both of them should be controlled by the same settings. WDYT? Can I add CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING guard around rcu_is_watching() check in rethook_try_get() as well? > Thanks, > > -- Steve
