On Fri, 29 Jan 2021 14:01:03 -0500 Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > On Fri, 29 Jan 2021 18:59:43 +0100 > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > On Fri, Jan 29, 2021 at 09:45:48AM -0800, Alexei Starovoitov wrote: > > > Same things apply to bpf side. We can statically prove safety for > > > ftrace and kprobe attaching whereas to deal with NMI situation we > > > have to use run-time checks for recursion prevention, etc. > > > > I have no idea what you're saying. You can attach to functions that are > > called with random locks held, you can create kprobes in some very > > sensitive places. > > > > What can you staticlly prove about that? > > I think the main difference is, if you attach a kprobe or ftrace function, > you can theoretically analyze the location before you do the attachment. > > Does, the NMI context mean "in_nmi()" returns true? Because there's cases > in ftrace callbacks where that is checked (like the stack tracer). And > having ftrace return true for "in_nmi()" will break a lot of existing > utilities. Specifically, kprobe and ftrace callbacks may have this: if (in_nmi()) return; raw_spin_lock_irqsave(&lock, flags); [..] raw_spin_unlock_irqrestore(&lock, flags); Which is totally fine to have, but the above only works if "in_nmi()" returns true only if you are in a real NMI. The stack tracer code does exactly the above. -- Steve
