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SubjectRe: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()")
On Sat, Jan 30, 2021 at 11:02:49AM +0900, Masami Hiramatsu wrote:
> On Fri, 29 Jan 2021 18:59:43 +0100
> Peter Zijlstra <peterz@infradead.org> 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?
>
> For the bpf and the kprobe tracer, if a probe hits in the NMI context,
> it can call the handler with another handler processing events.
>
> kprobes is carefully avoiding the deadlock by checking recursion
> with per-cpu variable. But if the handler is shared with the other events
> like tracepoints, it needs to its own recursion cheker too.
>
> So, Alexei, maybe you need something like this instead of in_nmi() check.
>
> DEFINE_PER_CPU(bool, under_running_bpf);
>
> common_handler()
> {
> if (__this_cpu_read(under_running_bpf))
> return;
> __this_cpu_write(under_running_bpf, true);
> /* execute bpf prog */
> __this_cpu_write(under_running_bpf, false);
> }
>
> Does this work for you?

This exactly check is already in trace_call_bpf.
Right after if (in_nmi()).
See bpf_prog_active. It serves different purpose though.
Simply removing if (in_nmi()) from trace_call_bpf is a bit scary.
I need to analyze all code paths first.

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