Re: [PATCH v2] tracing/uprobe: Add missing PID filter for uretprobe

Jiri Olsa
2024-08-27T10:08:41Z

On Mon, Aug 26, 2024 at 01:57:52PM +0200, Oleg Nesterov wrote:
> On 08/26, Jiri Olsa wrote:
> >
> > On Mon, Aug 26, 2024 at 12:40:18AM +0200, Oleg Nesterov wrote:
> > > $ ./test &
> > > $ bpftrace -p $! -e 'uprobe:./test:func { printf("%d\n", pid); }'
> > >
> > > I hope that the syntax of the 2nd command is correct...
> > >
> > > I _think_ that it will print 2 pids too.
> >
> > yes.. but with CLONE_VM both processes share 'mm'
>
> Yes sure,
>
> > so they are threads,
>
> Well this depends on definition ;) but the CLONE_VM child is not a sub-thread,
> it has another TGID. See below.
>
> > and at least uprobe_multi filters by process [1] now.. ;-)
>
> OK, if you say that this behaviour is fine I won't argue, I simply do not know.
> But see below.
>
> > > But "perf-record -p" works as expected.
> >
> > I wonder it's because there's the perf layer that schedules each
> > uprobe event only when its process (PID1/2) is scheduled in and will
> > receive events only from that cpu while the process is running on it
>
> Not sure I understand... The task which hits the breakpoint is always
> current, it is always scheduled in.
>
> The main purpose of uprobe_perf_func()->uprobe_perf_filter() is NOT that
> we want to avoid __uprobe_perf_func() although this makes sense.
>
> The main purpose is that we want to remove the breakpoints in current->mm
> when uprobe_perf_filter() returns false, that is why UPROBE_HANDLER_REMOVE.
> IOW, the main purpose is not penalise user-space unnecessarily.
>
> IIUC (but I am not sure), perf-record -p will work "correctly" even if we
> remove uprobe_perf_filter() altogether. IIRC the perf layer does its own
> filtering but I forgot everything.
>
> And this makes me think that perhaps BPF can't rely on uprobe_perf_filter()
> either, even we forget about ret-probes.
>
> > [1] 46ba0e49b642 bpf: fix multi-uprobe PID filtering logic
>
> Looks obviously wrong... get_pid_task(PIDTYPE_TGID) can return a zombie
> leader with ->mm == NULL while other threads and thus the whole process
> is still alive.
>
> And again, the changelog says "the intent for PID filtering it to filter by
> *process*", but clone(CLONE_VM) creates another process, not a thread.
>
> So perhaps we need
>
> - if (link->task && current->mm != link->task->mm)
> + if (link->task && !same_thread_group(current, link->task))
>
> in uprobe_prog_run() to make "filter by *process*" true, but this won't
> fix the problem with link->task->mm == NULL in uprobe_multi_link_filter().

would the same_thread_group(current, link->task) work in such case?
(zombie leader with other alive threads)

jirka

>
>
> Does bpftrace use bpf_uprobe_multi_link_attach/etc ? I guess not...
> Then which userspace tool uses this code? ;)
>
> Oleg.
>