Re: [PATCH bpf-next] bpf: Disable migrate when kprobe_multi attach to access bpf_prog_active

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Aug 04, 2025 at 10:15:46PM +0800, Tao Chen wrote:
> 在 2025/8/4 21:02, Jiri Olsa 写道:
> > On Mon, Aug 04, 2025 at 08:16:15PM +0800, Tao Chen wrote:
> > > The syscall link_create not protected by bpf_disable_instrumentation,
> > > accessing percpu data bpf_prog_active should use cpu local_lock when
> > > kprobe_multi program attach.
> > > 
> > > Fixes: 0dcac2725406 ("bpf: Add multi kprobe link")
> > > Signed-off-by: Tao Chen <chen.dylane@linux.dev>
> > > ---
> > >   kernel/trace/bpf_trace.c | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > > index 3ae52978cae..f6762552e8e 100644
> > > --- a/kernel/trace/bpf_trace.c
> > > +++ b/kernel/trace/bpf_trace.c
> > > @@ -2728,23 +2728,23 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
> > >   	struct pt_regs *regs;
> > >   	int err;
> > > +	migrate_disable();
> > >   	if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
> > 
> > this is called all the way from graph tracer, which disables preemption in
> > function_graph_enter_regs, so I think we can safely use __this_cpu_inc_return
> > 
> > 
> > >   		bpf_prog_inc_misses_counter(link->link.prog);
> > >   		err = 1;
> > >   		goto out;
> > >   	}
> > > -	migrate_disable();
> > 
> > hum, but now I'm not sure why we disable migration in here then
> > 
> 
> It seems that there is a cant_migrate() check in bpf_prog_run, so it should
> be disabled before run.

yes, but disabled preemption will take care of that

I think we can do change below plus some comment that Yonghong
is suggesting in the other reply

jirka


---
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 3ae52978cae6..74e8d9543c6d 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -2734,14 +2734,12 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
 		goto out;
 	}
 
-	migrate_disable();
 	rcu_read_lock();
 	regs = ftrace_partial_regs(fregs, bpf_kprobe_multi_pt_regs_ptr());
 	old_run_ctx = bpf_set_run_ctx(&run_ctx.session_ctx.run_ctx);
 	err = bpf_prog_run(link->link.prog, regs);
 	bpf_reset_run_ctx(old_run_ctx);
 	rcu_read_unlock();
-	migrate_enable();
 
  out:
 	__this_cpu_dec(bpf_prog_active);