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

[Tao Chen]

在 2025/8/5 17:07, Jiri Olsa 写道:

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.dyl...@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 see, you are right, preempt will pass the check, thanks.

void __cant_migrate(const char *file, int line)
{
        static unsigned long prev_jiffy;

        if (irqs_disabled())
                return;

        if (is_migration_disabled(current))
                return;

        if (!IS_ENABLED(CONFIG_PREEMPT_COUNT))
                return;

        if (preempt_count() > 0)
                return;
...

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

Yes, i will remove the migrate_disable and add some comment as you
and Yonghong suggested.

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);

--
Best Regards
Tao Chen