Re: [PATCH v3 tip/perf/core 2/2] uprobes: SRCU-protect uretprobe lifetime (with timeout)

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Feb 25, 2025 at 3:46 AM Breno Leitao <leitao@debian.org> wrote:
>
> Hello Andrii,
>
> On Mon, Feb 24, 2025 at 02:23:51PM -0800, Andrii Nakryiko wrote:
> > On Mon, Feb 24, 2025 at 4:23 AM Breno Leitao <leitao@debian.org> wrote:
> > >
> > > Hello Andrii,
> > >
> > > On Wed, Oct 23, 2024 at 09:41:59PM -0700, Andrii Nakryiko wrote:
> > > >
> > > > +static struct uprobe* hprobe_expire(struct hprobe *hprobe, bool get)
> > > > +{
> > > > +     enum hprobe_state hstate;
> > > > +
> > > > +     /*
> > > > +      * return_instance's hprobe is protected by RCU.
> > > > +      * Underlying uprobe is itself protected from reuse by SRCU.
> > > > +      */
> > > > +     lockdep_assert(rcu_read_lock_held() && srcu_read_lock_held(&uretprobes_srcu));
> > >
> > > I am hitting this warning in d082ecbc71e9e ("Linux 6.14-rc4") on
> > > aarch64. I suppose this might happen on x86 as well, but I haven't
> > > tested.
> > >
> > >         WARNING: CPU: 28 PID: 158906 at kernel/events/uprobes.c:768 hprobe_expire (kernel/events/uprobes.c:825)
> > >
> > >         Call trace:
> > >         hprobe_expire (kernel/events/uprobes.c:825) (P)
> > >         uprobe_copy_process (kernel/events/uprobes.c:691 kernel/events/uprobes.c:2103 kernel/events/uprobes.c:2142)
> > >         copy_process (kernel/fork.c:2636)
> > >         kernel_clone (kernel/fork.c:2815)
> > >         __arm64_sys_clone (kernel/fork.c:? kernel/fork.c:2926 kernel/fork.c:2926)
> > >         invoke_syscall (arch/arm64/kernel/syscall.c:35 arch/arm64/kernel/syscall.c:49)
> > >         do_el0_svc (arch/arm64/kernel/syscall.c:139 arch/arm64/kernel/syscall.c:151)
> > >         el0_svc (arch/arm64/kernel/entry-common.c:165 arch/arm64/kernel/entry-common.c:178 arch/arm64/kernel/entry-common.c:745)
> > >         el0t_64_sync_handler (arch/arm64/kernel/entry-common.c:797)
> > >         el0t_64_sync (arch/arm64/kernel/entry.S:600)
> > >
> > > I broke down that warning, and the problem is on related to
> > > rcu_read_lock_held(), since RCU read lock does not seem to be held in
> > > this path.
> > >
> > > Reading this code, RCU read lock seems to protect old hprobe, which
> > > doesn't seem so.
> > >
> > > I am wondering if we need to protect it properly, something as:
> > >
> > >         @@ -2089,7 +2092,9 @@ static int dup_utask(struct task_struct *t, struct uprobe_task *o_utask)
> > >                                 return -ENOMEM;
> > >
> > >                         /* if uprobe is non-NULL, we'll have an extra refcount for uprobe */
> > >         +               rcu_read_lock();
> > >                         uprobe = hprobe_expire(&o->hprobe, true);
> > >         +               rcu_write_lock();
> > >
> >
> > I think this is not good enough. rcu_read_lock/unlock should be around
> > the entire for loop, because, technically, that return_instance can be
> > freed before we even get to hprobe_expire.
>
> re you suggesting that we should use an RCU read lock to protect the
> "traversal" of return_instances? In other words, is it currently being
> traversed unsafely, given that return_instance can be freed at any time?

Yes, that's what I was suggesting initially.

But reading through all this again and paging in all the context I had
when I was implementing this, I think this
lockdep_assert(rcu_read_lock_held()) is a false positive.

For timer thread, yes, we need to keep rcu_read_lock(), because timer
thread doesn't own return_instance, so it has to guarantee that memory
won't go away.

But dup_utask() is called for current thread's return_instances, so
they can't go anywhere and RCU read lock is not necessary here.

So I'm going to send a patch removing part of this lockdep_assert()
and will expand the comment with the actual lifetime rules.

>
> > So, just like we have guard(srcu)(&uretprobes_srcu); we should have
> > guard(rcu)();
> >
> > Except, there is that kmemdup() hidden inside dup_return_instance(),
> > so we can't really do that.
>
> Right. kmemdup() is using GFP_KERNEL, which might sleep, so, it cannot
> be called using rcu read lock.
>
> Thanks
> --breno