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

Andrii Nakryiko
2024-09-03T18:10:07Z

On Mon, Sep 2, 2024 at 2:11 AM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Fri, Aug 30, 2024 at 08:51:12AM -0700, Andrii Nakryiko wrote:
> > On Fri, Aug 30, 2024 at 6:34 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> > >
> > > On Fri, Aug 30, 2024 at 12:12:09PM +0200, Oleg Nesterov wrote:
> > > > The whole discussion was very confusing (yes, I too contributed to the
> > > > confusion ;), let me try to summarise.
> > > >
> > > > > U(ret)probes are designed to be filterable using the PID, which is the
> > > > > second parameter in the perf_event_open syscall. Currently, uprobe works
> > > > > well with the filtering, but uretprobe is not affected by it.
> > > >
> > > > And this is correct. But the CONFIG_BPF_EVENTS code in __uprobe_perf_func()
> > > > misunderstands the purpose of uprobe_perf_filter().
> > > >
> > > > Lets forget about BPF for the moment. It is not that uprobe_perf_filter()
> > > > does the filtering by the PID, it doesn't. We can simply kill this function
> > > > and perf will work correctly. The perf layer in __uprobe_perf_func() does
> > > > the filtering when perf_event->hw.target != NULL.
> > > >
> > > > So why does uprobe_perf_filter() call uprobe_perf_filter()? Not to avoid
> > > > the __uprobe_perf_func() call (as the BPF code assumes), but to trigger
> > > > unapply_uprobe() in handler_chain().
> > > >
> > > > Suppose you do, say,
> > > >
> > > > $ perf probe -x /path/to/libc some_hot_function
> > > > or
> > > > $ perf probe -x /path/to/libc some_hot_function%return
> > > > then
> > > > $perf record -e ... -p 1
> > > >
> > > > to trace the usage of some_hot_function() in the init process. Everything
> > > > will work just fine if we kill uprobe_perf_filter()->uprobe_perf_filter().
> > > >
> > > > But. If INIT forks a child C, dup_mm() will copy int3 installed by perf.
> > > > So the child C will hit this breakpoint and cal handle_swbp/etc for no
> > > > reason every time it calls some_hot_function(), not good.
> > > >
> > > > That is why uprobe_perf_func() calls uprobe_perf_filter() which returns
> > > > UPROBE_HANDLER_REMOVE when C hits the breakpoint. handler_chain() will
> > > > call unapply_uprobe() which will remove this breakpoint from C->mm.
> > >
> > > thanks for the info, I wasn't aware this was the intention
> > >
> > > uprobe_multi does not have perf event mechanism/check, so it's using
> > > the filter function to do the process filtering.. which is not working
> > > properly as you pointed out earlier
> >
> > So this part I don't completely get. I get that using task->mm
> > comparison is wrong due to CLONE_VM, but why same_thread_group() check
> > is wrong? I.e., why task->signal comparison is wrong?
>
> the way I understand it is that we take the group leader task and
> store it in bpf_uprobe_multi_link::task
>
> but it can exit while the rest of the threads is still running so
> the uprobe_multi_link_filter won't match them (leader->mm is NULL)

Aren't we conflating two things here? Yes, from what Oleg explained,
it's clear that using task->mm is wrong. So that is what I feel is the
main issue. We shouldn't use task->mm at all, only task->signal should
be used instead. We should fix that (in bpf tree, please).

But I don't get the concern about linux->mm or linux->signal becoming
NULL because of a task existing. Look at put_task_struct(), it WILL
NOT call __put_task_struct() (which then calls put_signal_struct()),
so task->signal at least will be there and valid until multi-uprobe is
detached and we call put_task().

So. Can you please send fixes against the bpf tree, switching to
task->signal? And maybe also include the fix to prevent
UPROBE_HANDLER_REMOVE to be returned from the BPF program?

This thread is almost 50 emails deep now, we should break out of it.
We can argue on your actual fixes. :)

>
> Oleg suggested change below (in addition to same_thread_group change)
> to take that in account
>
> jirka
>
>
> ---
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 98e395f1baae..9e6b390aa6da 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -3235,9 +3235,23 @@ uprobe_multi_link_filter(struct uprobe_consumer *con, enum uprobe_filter_ctx ctx
> struct mm_struct *mm)
> {
> struct bpf_uprobe *uprobe;
> + struct task_struct *task, *t;
> + bool ret = false;
>
> uprobe = container_of(con, struct bpf_uprobe, consumer);
> - return uprobe->link->task->mm == mm;
> + task = uprobe->link->task;
> +
> + rcu_read_lock();
> + for_each_thread(task, t) {
> + struct mm_struct *mm = READ_ONCE(t->mm);
> + if (mm) {
> + ret = t->mm == mm;
> + break;
> + }
> + }
> + rcu_read_unlock();
> +
> + return ret;
> }
>
> static int