Re: [PATCH v3 tip/perf/core 2/2] uprobes: SRCU-protect uretprobe lifetime (with timeout)

Hello Andrii,

On Wed, Oct 23, 2024 at 09:41:59PM -0700, Andrii Nakryiko wrote:

+static struct uprobe* hprobe_expire(struct hprobe *hprobe, bool get)
+{
+	enum hprobe_state hstate;
+
+	/*
+	 * return_instance's hprobe is protected by RCU.
+	 * Underlying uprobe is itself protected from reuse by SRCU.
+	 */
+	lockdep_assert(rcu_read_lock_held() && srcu_read_lock_held(&uretprobes_srcu));

I am hitting this warning in d082ecbc71e9e ("Linux 6.14-rc4") on
aarch64. I suppose this might happen on x86 as well, but I haven't
tested.

	WARNING: CPU: 28 PID: 158906 at kernel/events/uprobes.c:768 hprobe_expire (kernel/events/uprobes.c:825)

	Call trace:
	hprobe_expire (kernel/events/uprobes.c:825) (P)
	uprobe_copy_process (kernel/events/uprobes.c:691 kernel/events/uprobes.c:2103 kernel/events/uprobes.c:2142)
	copy_process (kernel/fork.c:2636)
	kernel_clone (kernel/fork.c:2815)
	__arm64_sys_clone (kernel/fork.c:? kernel/fork.c:2926 kernel/fork.c:2926)
	invoke_syscall (arch/arm64/kernel/syscall.c:35 arch/arm64/kernel/syscall.c:49)
	do_el0_svc (arch/arm64/kernel/syscall.c:139 arch/arm64/kernel/syscall.c:151)
	el0_svc (arch/arm64/kernel/entry-common.c:165 arch/arm64/kernel/entry-common.c:178 arch/arm64/kernel/entry-common.c:745)
	el0t_64_sync_handler (arch/arm64/kernel/entry-common.c:797)
	el0t_64_sync (arch/arm64/kernel/entry.S:600)

I broke down that warning, and the problem is on related to
rcu_read_lock_held(), since RCU read lock does not seem to be held in
this path.

Reading this code, RCU read lock seems to protect old hprobe, which
doesn't seem so.

I am wondering if we need to protect it properly, something as:

	@@ -2089,7 +2092,9 @@ static int dup_utask(struct task_struct *t, struct uprobe_task *o_utask)
				return -ENOMEM;

			/* if uprobe is non-NULL, we'll have an extra refcount for uprobe */
	+               rcu_read_lock();
			uprobe = hprobe_expire(&o->hprobe, true);
	+               rcu_write_lock();

			/*
			* New utask will have stable properly refcounted uprobe or