Re: [PATCH] perf: Fix probing for PERF_FLAG_FD_CLOEXEC flag

[Adrian Hunter]

On 19/02/2015 4:55 p.m., David Ahern wrote:
> On 2/19/15 12:06 AM, Adrian Hunter wrote:
> > >       /* not supported, confirm error related to PERF_FLAG_FD_CLOEXEC */
> > > -    fd = sys_perf_event_open(&attr, pid, cpu, -1, 0);
> > > +    fd = sys_perf_event_open(&attr, 0, cpu, -1, 0);
> >
> > I would prefer to avoid pid = 0 unless necessary and so just do the same
> > thing again i.e.
> >
> >     while (1) {
> >         fd = sys_perf_event_open(&attr, pid, cpu, -1, 0);
> >         if (fd < 0 && pid == -1 && errno == EACCES) {
> >             pid = 0;
> >             continue;
> >         }
> >         break;
> >     }
>
> The probing is getting of hand. In this case the intent is a probe for a flag
> and flags are the first thing checked kernel side. Given that the parameters
> passed to sys_perf_event_open should be as simple and known safe as possible.
> pid = -1 has known limitations. Why can't pid just be getpid() in both cases?
> Simplifies this function a lot and removes the need for sched_getcpu(). So
>      pid = getpid();
>
>      fd = sys_perf_event_open(&attr, pid, -1, -1, PERF_FLAG_FD_CLOEXEC);
>
> and if that fails
>
>      fd = sys_perf_event_open(&attr, pid, -1, -1, 0);
>
> Why is anything more complicated needed?

Yes, I am sorry it is a pain. I don't know why I didn't add a comment
to the code :-(. Using -1 for the pid is a workaround to avoid gratuitous
jump label changes. If pid=0 is used and then a system-wide trace is done
with Intel PT, there will be a jump label change shortly after the tracing
starts. That means the running code gets changed, but Intel PT decoding
has to walk the code to reconstruct the trace - so errors result. There
will always be occasional jump label changes, but this avoids one that
would otherwise always happen.
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