Re: [PATCH v2 04/12] uprobes: revamp uprobe refcounting and lifetime management

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

On Sun, Jul 7, 2024 at 7:48 AM Oleg Nesterov <oleg@redhat.com> wrote:
>
> And I forgot to mention...
>
> In any case __uprobe_unregister() can't ignore the error code from
> register_for_each_vma(). If it fails to restore the original insn,
> we should not remove this uprobe from uprobes_tree.
>
> Otherwise the next handle_swbp() will send SIGTRAP to the (no longer)
> probed application.

Yep, that would be unfortunate (just like SIGILL sent when uretprobe
detects "improper" stack pointer progression, for example), but from
what I gather it's not really expected to fail on unregistration given
we successfully registered uprobe. I guess it's a decision between
leaking memory with an uprobe stuck in the tree or killing process due
to some very rare (or buggy) condition?


>
> On 07/05, Oleg Nesterov wrote:
> >
> > Tried to read this patch, but I fail to understand it. It looks
> > obvioulsy wrong to me, see below.
> >
> > I tend to agree with the comments from Peter, but lets ignore them
> > for the moment.
> >
> > On 07/01, Andrii Nakryiko wrote:
> > >
> > >  static void put_uprobe(struct uprobe *uprobe)
> > >  {
> > > -   if (refcount_dec_and_test(&uprobe->ref)) {
> > > +   s64 v;
> > > +
> > > +   /*
> > > +    * here uprobe instance is guaranteed to be alive, so we use Tasks
> > > +    * Trace RCU to guarantee that uprobe won't be freed from under us, if
> > > +    * we end up being a losing "destructor" inside uprobe_treelock'ed
> > > +    * section double-checking uprobe->ref value below.
> > > +    * Note call_rcu_tasks_trace() + uprobe_free_rcu below.
> > > +    */
> > > +   rcu_read_lock_trace();
> > > +
> > > +   v = atomic64_add_return(UPROBE_REFCNT_PUT, &uprobe->ref);
> > > +
> > > +   if (unlikely((u32)v == 0)) {
> >
> > I must have missed something, but how can this ever happen?
> >
> > Suppose uprobe_register(inode) is called the 1st time. To simplify, suppose
> > that this binary is not used, so _register() doesn't install breakpoints/etc.
> >
> > IIUC, with this change (u32)uprobe->ref == 1 when uprobe_register() succeeds.
> >
> > Now suppose that uprobe_unregister() is called right after that. It does
> >
> >       uprobe = find_uprobe(inode, offset);
> >
> > this increments the counter, (u32)uprobe->ref == 2
> >
> >       __uprobe_unregister(...);
> >
> > this wont't change the counter,
> >
> >       put_uprobe(uprobe);
> >
> > this drops the reference added by find_uprobe(), (u32)uprobe->ref == 1.
> >
> > Where should the "final" put_uprobe() come from?
> >
> > IIUC, this patch lacks another put_uprobe() after consumer_del(), no?
> >
> > Oleg.
>