Re: [PATCH v1 00/10] Move uid filtering to BPF filters

[Namhyung Kim <namhyung@kernel.org>]

On Wed, Feb 12, 2025 at 11:27:38PM -0800, Ian Rogers wrote:
> On Wed, Feb 12, 2025 at 5:44 PM Namhyung Kim <namhyung@kernel.org> wrote:
> >
> > On Wed, Feb 12, 2025 at 03:17:35PM -0800, Ian Rogers wrote:
> > > On Wed, Feb 12, 2025 at 2:56 PM Namhyung Kim <namhyung@kernel.org> wrote:
> > > >
> > > > On Wed, Feb 12, 2025 at 12:00:42PM -0800, Ian Rogers wrote:
> > > > > On Wed, Feb 12, 2025 at 10:46 AM Namhyung Kim <namhyung@kernel.org> wrote:
> > > > > > It's not completely broken and works sometimes.
> > > > >
> > > > > No this is the definition of completely broken. If it only works
> > > > > sometimes then you can't use it, we can't put a test on it, there is
> > > > > no point in it. Even when it doesn't fail on perf_event_open, does it
> > > > > work for processes that start after /proc is scanned? No, it is
> > > > > completely broken.
> > > >
> > > > Ok, we have a different definition for it.  Let's ignore the imaginary
> > > > users of the broken features.  Have you added a test for this change?
> > > >
> > > > Anyway I've tested your change and found some issues:
> > > >
> > > > 1. It silently exited when BPF-skel is not built.  Better to put some
> > > >    messages at least.
> > > >
> > > >   $ sudo ./perf record -u $(id -u) -- sleep 1
> > > >
> > > > 2. Even with BPF-skel, perf record doesn't work well.  It did something
> > > >    but failed to get sample data for some reason.
> > > >
> > > >   $ sudo ./perf record -u $(id -u) -- sleep 1
> > > >   [ perf record: Woken up 1 times to write data ]
> > > >   [ perf record: Captured and wrote 0.045 MB perf.data ]
> > > >
> > > >    Oh, I think you now need to pass -a because it now works in
> > > >    system-wide mode and drops samples for other users.
> > >
> > > This is a pre-existing problem, no?
> >
> > No, it worked without -a in the past.  Please see my previous reply.
> > I think -u/--uid is one of the supported target in the perf tool (not
> > for the system call) and it used to disable system-wide mode if -u is
> > used at the same time.
> 
> As I keep repeating the '-u' option has never worked in the past, it
> either early exited or missed recording new processes.

I agree the early exit of existing process is a problem of '-u'.  But
I'm not sure it would miss all new processes.  IIUC '-u' is a shortcut
for '-p' with all processes belong to the user.  That means it'll have
perf_event with attr.inherit set then it can track new processes from
them.  Of course, there's a race between /proc scanning and process
creation so it will miss some.  But even '-p' has the same race.

> 
> The documentation for `perf record` is:
> ```
> ...
>        -a, --all-cpus
>           System-wide collection from all CPUs (default if no target
> is specified).
> ...
>        -u, --uid=
>           Record events in threads owned by uid. Name or number.
> ...
> ```
> So `-a` is implied without a target/workload but you are specifying a
> workload and `-u`. So do you want samples from the workload or from
> the user's processes? I can tell from your command line being sleep
> that you want it to imply `-a` but would that be true if it were a
> benchmark? For the benchmark case you probably don't want `-a`
> implied. If you specify `-p` and `-u` should the processes that don't
> match -u be sampled or are you expecting implicit system wide
> profiling now?

I don't follow.  You cannot specify two targets at the same time.  If
you use -a and -u together currently, you will see:

  $ perf record -a -u $(id -u)
  Warning:
  UID switch overriding SYSTEM
  
And for -p and -u:

  $ perf record -p 1 -u $(id -u)
  Warning:
  PID/TID switch overriding UID

And specifying a target and a workload means it will profile the target
while the workload is running.

  $ perf record -u $(id -u) -- sleep 1

So the above command will profile all processes belong to me for 1
second.  As you're doing system wide for 1 second with below command.

  $ perf record -a -- sleep 1

You can change sleep with any other workload but it's not the target of
the profile unless it matches to the target specifically.

> 
> I agree the behavior in the patch series doesn't match the existing
> behavior, but I'm not sure the existing behavior agrees with the
> documentation nor working as expected. Having the `-u` not select
> system wide, as in the patch, seems to agree with the documentation
> better. You have specified a target workload/process/eventual pid and
> you want samples owned by the uid, why should you now start getting
> samples from other processes? With `-p` you wouldn't expect `-a` to
> suddenly get implied, I'm not sure it makes any more sense with any
> target/workload.

As I said, the existing behavior doesn't imply system wide.  But your
change now requires it.

Thanks,
Namhyung

> 
> > > > 3. With BPF-skel, non-root users will see this.
> > > >
> > > >   $ ./perf record -u $(id -u) -- sleep 1
> > > >   cannot get fd for 'filters' map
> > > >   failed to set filter "BPF" on event cycles:P with 13 (Permission denied)
> > > >
> > > >    I think it's confusing and better to tell user that you need to run
> > > >    'perf record --setup-filter pin' as root first.  But maybe due to the
> > > >    issue #2, you still need to run it as root.
> 
> I think that is an okay addition to BPF filters in general. I'm wary
> of having patch series feature crept into requiring the entire tool
> being reimplemented.
> 
> Thanks,
> Ian