Re: [PATCH v1 4/8] tracing/bpf: guard syscall probe with preempt_notrace

[Mathieu Desnoyers]

On 2024-10-04 02:30, Mathieu Desnoyers wrote:
> On 2024-10-04 01:05, Alexei Starovoitov wrote:
> > On Thu, Oct 3, 2024 at 3:25 PM Steven Rostedt <rostedt@xxxxxxxxxxx> 
> > wrote:
> > > On Thu,  3 Oct 2024 11:16:34 -0400
> > > Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> > >
> > > > In preparation for allowing system call enter/exit instrumentation to
> > > > handle page faults, make sure that bpf can handle this change by
> > > > explicitly disabling preemption within the bpf system call tracepoint
> > > > probes to respect the current expectations within bpf tracing code.
> > > >
> > > > This change does not yet allow bpf to take page faults per se within 
> > > > its
> > > > probe, but allows its existing probes to adapt to the upcoming change.
> > >
> > > I guess the BPF folks should state if this is needed or not?
> > >
> > > Does the BPF hooks into the tracepoints expect preemption to be disabled
> > > when called?
> >
> > Andrii pointed it out already.
> > bpf doesn't need preemption to be disabled.
> > Only migration needs to be disabled.
>
> I'm well aware of this. Feel free to relax those constraints in
> follow up patches in your own tracers. I'm simply not introducing
> any behavior change in the "big switch" patch introducing faultable
> syscall tracepoints. It's just too easy to overlook a dependency on
> preempt off deep inside some tracer code for me to make assumptions
> at the tracepoint level.
>
> If a regression happens, it will be caused by the tracer-specific
> patch that relaxes the constraints, not by the tracepoint change
> that affects multiple tracers at once.

I also notice that the bpf verifier checks a "active_preempt_lock"
state to make sure sleepable functions are not called while within
preempt off region. So I would expect that the verifier has some
knowledge about the fact that tracepoint probes are called with
preempt off already.

Likewise in reverse for functions which deal with per-cpu data: those
would expect to be used with preempt off if multiple functions need to
touch the same cpu's data.

So if we make the syscall tracepoint constraints more relax (migrate
off rather than preempt off), I suspect we may have to update the
verifier.

This contributes to my uneasiness towards introducing this kind of
side-effect in a tracepoint change that affects all tracers.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com