Re: [PATCH bpf-next v2 1/2] bpf: Add lookup_and_delete_elem for BPF_MAP_STACK_TRACE

[Tao Chen]

在 2025/9/19 10:01, Alexei Starovoitov 写道:
> On Thu, Sep 18, 2025 at 6:35 AM Tao Chen <chen.dylane@xxxxxxxxx> wrote:
> > 在 2025/9/18 09:35, Alexei Starovoitov 写道:
> > > On Wed, Sep 17, 2025 at 3:16 PM Andrii Nakryiko
> > > <andrii.nakryiko@xxxxxxxxx> wrote:
> > > > P.S. It seems like a good idea to switch STACKMAP to open addressing
> > > > instead of the current kind-of-bucket-chain-but-not-really
> > > > implementation. It's fixed size and pre-allocated already, so open
> > > > addressing seems like a great approach here, IMO.
> > >
> > > That makes sense. It won't have backward compat issues.
> > > Just more reliable stack_id.
> > >
> > > Fixed value_size is another footgun there.
> > > Especially for collecting user stack traces.
> > > We can switch the whole stackmap to bpf_mem_alloc()
> > > or wait for kmalloc_nolock().
> > > But it's probably a diminishing return.
> > >
> > > bpf_get_stack() also isn't great with a copy into
> > > perf_callchain_entry, then 2nd copy into on stack/percpu buf/ringbuf,
> > > and 3rd copy of correct size into ringbuf (optional).
> > >
> > > Also, I just realized we have another nasty race there.
> > > In the past bpf progs were run in preempt disabled context,
> > > but we forgot to adjust bpf_get_stack[id]() helpers when everything
> > > switched to migrate disable.
> > >
> > > The return value from get_perf_callchain() may be reused
> > > if another task preempts and requests the stack.
> > > We have partially incorrect comment in __bpf_get_stack() too:
> > >           if (may_fault)
> > >                   rcu_read_lock(); /* need RCU for perf's callchain below */
> > >
> > > rcu can be preemptable. so rcu_read_lock() makes
> > > trace = get_perf_callchain(...)
> > > accessible, but that per-cpu trace buffer can be overwritten.
> > > It's not an issue for CONFIG_PREEMPT_NONE=y, but that doesn't
> > > give much comfort.
> >
> > Hi Alexei,
> >
> > Can we fix it like this?
> >
> > -       if (may_fault)
> > -               rcu_read_lock(); /* need RCU for perf's callchain below */
> > +       preempt_diable();
> >
> >           if (trace_in)
> >                   trace = trace_in;
> > @@ -455,8 +454,7 @@ static long __bpf_get_stack(struct pt_regs *regs,
> > struct task_struct *task,
> >                                              crosstask, false);
> >
> >           if (unlikely(!trace) || trace->nr < skip) {
> > -               if (may_fault)
> > -                       rcu_read_unlock();
> > +               preempt_enable();
> >                   goto err_fault;
> >           }
> >
> > @@ -475,9 +473,7 @@ static long __bpf_get_stack(struct pt_regs *regs,
> > struct task_struct *task,
> >                   memcpy(buf, ips, copy_len);
> >           }
> >
> > -       /* trace/ips should not be dereferenced after this point */
> > -       if (may_fault)
> > -               rcu_read_unlock();
> > +       preempt_enable();
>
> That should do it. Don't see an issue at first glance.

Ok, i will send a patch later, thanks.

--
Best Regards
Tao Chen