Re: [FIX bpf,perf] bpf,perf: return EOPNOTSUPP for bpf handler on PERF_COUNT_SW_DUMMY

[Peter Zijlstra]

On Mon, Nov 16, 2020 at 01:02:09PM -0800, Martin KaFai Lau wrote:
> On Mon, Nov 16, 2020 at 07:37:52PM +0100, Florian Lehner wrote:
> > bpf handlers for perf events other than tracepoints, kprobes or uprobes
> > are attached to the overflow_handler of the perf event.
> > 
> > Perf events of type software/dummy are placeholder events. So when
> > attaching a bpf handle to an overflow_handler of such an event, the bpf
> > handler will not be triggered.
> > 
> > This fix returns the error EOPNOTSUPP to indicate that attaching a bpf
> > handler to a perf event of type software/dummy is not supported.
> > 
> > Signed-off-by: Florian Lehner <dev@xxxxxxxxxxx>
> It is missing a Fixes tag.

I don't think it actually fixes anything. worse it could break things.

Atatching a bpf filter to a dummy event is pointless, but harmless. We
allow it now, disallowing it will break whatever programs out there are
doing harmless silly things.

I really don't see the point of this patch. It grows the kernel code for
absolutely no distinguishable benefit.