Revert "bpf: remove unnecessary rcu_read_{lock,unlock}() in multi-uprobe attach logic"

This reverts commit 4a8f635.

Althought get_pid_task() internally already calls rcu_read_lock() and
rcu_read_unlock(), the find_vpid() was not.

The documentation for find_vpid() clearly states:

  "Must be called with the tasklist_lock or rcu_read_lock() held."

Add proper rcu_read_lock/unlock() to protect the find_vpid().

Reported-by: Xuewen Yan <[email protected]>
Signed-off-by: Di Shen <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>

Author: Di Shen

kernel/trace/bpf_trace.c

@@ -3318,7 +3318,9 @@ int bpf_uprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
 	}
 
 	if (pid) {
+		rcu_read_lock();
 		task = get_pid_task(find_vpid(pid), PIDTYPE_TGID);
+		rcu_read_unlock();
 		if (!task) {
 			err = -ESRCH;
 			goto error_path_put;