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. -- Steve
