Re: [PATCH 5/5] perf/x86: Relax privilege filter restriction on AMD IBS

[Namhyung Kim <namhyung@kernel.org>]

Hello,

On Mon, Sep 23, 2024 at 04:03:47PM +0530, Ravi Bangoria wrote:
> Hi Namhyung,
> 
> > While IBS is available for per-thread profiling, still regular users
> > cannot open an event due to the default paranoid setting (2) which
> > doesn't allow unprivileged users to get kernel samples.  That means
> > it needs to set exclude_kernel bit in the attribute but IBS driver
> > would reject it since it has PERF_PMU_CAP_NO_EXCLUDE.  This is not what
> > we want and I've been getting requests to fix this issue.
> 
> I'm working on some IBS improvements that impacts this change as well.
> Is it be possible to hold off this patch for some time. I'll try to
> post my patches soon.
> 
> > @@ -1111,6 +1127,12 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs)
> >  		regs.flags |= PERF_EFLAGS_EXACT;
> >  	}
> >  
> > +	if ((event->attr.config2 & IBS_SW_FILTER_MASK) &&
> > +	    perf_exclude_event(event, &regs)) {
> > +		throttle = perf_event_account_interrupt(event);
> > +		goto out;
> > +	}
> 
> Throttling can give surprises when the sample period is very small.
> For ex,
> 
>   $ ./perf record -e cycles:uh -c 192 -- make
>   [ perf record: Woken up 52 times to write data ]
>   [ perf record: Captured and wrote 23.016 MB perf.data (705634 samples) ]
> 
>   $ ./perf record -e ibs_op/swfilt=1/uh -c 192 -- make
>   [ perf record: Woken up 1 times to write data ]
>   [ perf record: Captured and wrote 1.608 MB perf.data (19 samples) ]
> 
> It seems like the IBS event gets throttled (and disabled) before the
> cpu get a chance to go back to userspace), hence we end up with very
> few samples.

Thanks for raising this issue.  This indeed looks like a surprising
result.  Not sure what we can do here other than adding a documentation
to refrain from using such a small period.  I don't think we want to
skip the throttling logic for the filtered samples.  Otherwise it can be
used for DoS-like attacks IMHO.

Thanks,
Namhyung