Re: [PATCH] bpf/perf: Fix suspicious RCU usage in get_callchain_entry()

[Andrii Nakryiko]

On Tue, Jan 27, 2026 at 10:18 PM Qing Wang <wangqing7171@xxxxxxxxx> wrote:
>
> There is a patch intended to fix suspicious RCU usage in get_callchain_entry(),
> but it is incorrect. Specifically, rcu_read_lock()/rcu_read_unlock() is not
> called when may_fault == false.

rcu_read_lock/unlock is not called when may_fault == false because in
that case BPF program is already running within rcu_read_lock/unlock
region. So I'm not sure this patch fixes anything really. And even
with trace_in, if we cann rcu_read_lock/unlock one extra time it
shouldn't be a problem, no?

pw-bot: cr

>
> Previous discussion:
> https://lore.kernel.org/all/CAEf4BzaYL9zZN8TZyRHW3_O3vbHc7On+NSunrkDvDQx2=wwyRw@xxxxxxxxxxxxxx/#R
>
> For perf's callchain, rcu_read_lock()/rcu_read_unlock() should be called when
> trace_in == false.
>
> Fixes: d4dd9775ec24 ("bpf: wire up sleepable bpf_get_stack() and bpf_get_task_stack() helpers")
> Reported-by: syzbot+72a43cdb78469f7fbad1@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=72a43cdb78469f7fbad1
> Tested-by: syzbot+72a43cdb78469f7fbad1@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Qing Wang <wangqing7171@xxxxxxxxx>
> ---
>  kernel/bpf/stackmap.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index da3d328f5c15..f97d4aa9d038 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -460,7 +460,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>
>         max_depth = stack_map_calculate_max_depth(size, elem_size, flags);
>
> -       if (may_fault)
> +       if (!trace_in)
>                 rcu_read_lock(); /* need RCU for perf's callchain below */
>
>         if (trace_in) {
> @@ -474,7 +474,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>         }
>
>         if (unlikely(!trace) || trace->nr < skip) {
> -               if (may_fault)
> +               if (!trace_in)
>                         rcu_read_unlock();
>                 goto err_fault;
>         }
> @@ -494,7 +494,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>         }
>
>         /* trace/ips should not be dereferenced after this point */
> -       if (may_fault)
> +       if (!trace_in)
>                 rcu_read_unlock();
>
>         if (user_build_id)
> --
> 2.34.1
>