[PATCH bpf-next 5/6] bpf: support epoll from bpf struct_ops links.

[Kui-Feng Lee <thinker.li@gmail.com>]

Add epoll support to bpf struct_ops links to trigger EPOLLHUP event upon
detachment.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
---
 include/linux/bpf.h         |  2 ++
 kernel/bpf/bpf_struct_ops.c | 14 ++++++++++++++
 kernel/bpf/syscall.c        | 15 +++++++++++++++
 3 files changed, 31 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index eeeed4b1bd32..a4550b927352 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1574,6 +1574,7 @@ struct bpf_link {
 	const struct bpf_link_ops *ops;
 	struct bpf_prog *prog;
 	struct work_struct work;
+	wait_queue_head_t wait_hup;
 };
 
 struct bpf_link_ops {
@@ -1587,6 +1588,7 @@ struct bpf_link_ops {
 			      struct bpf_link_info *info);
 	int (*update_map)(struct bpf_link *link, struct bpf_map *new_map,
 			  struct bpf_map *old_map);
+	__poll_t (*poll)(struct file *file, struct poll_table_struct *pts);
 };
 
 struct bpf_tramp_link {
diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 4a8a7e5ffc56..f19b6a76591a 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -12,6 +12,7 @@
 #include <linux/mutex.h>
 #include <linux/btf_ids.h>
 #include <linux/rcupdate_wait.h>
+#include <linux/poll.h>
 
 struct bpf_struct_ops_value {
 	struct bpf_struct_ops_common_value common;
@@ -1149,6 +1150,8 @@ bool bpf_struct_ops_kvalue_unreg(void *data)
 	 */
 	bpf_map_put(&st_map->map);
 
+	wake_up_interruptible_poll(&st_link->link.wait_hup, EPOLLHUP);
+
 	ret = true;
 
 fail_unlock:
@@ -1276,15 +1279,26 @@ static int bpf_struct_ops_map_link_detach(struct bpf_link *link)
 
 	mutex_unlock(&update_mutex);
 
+	wake_up_interruptible_poll(&st_link->link.wait_hup, EPOLLHUP);
+
 	return 0;
 }
 
+static __poll_t bpf_struct_ops_map_link_poll(struct file *file,
+					     struct poll_table_struct *ptrs)
+{
+	struct bpf_struct_ops_link *st_link = file->private_data;
+
+	return (st_link->map) ? 0 : EPOLLHUP | EPOLLERR;
+}
+
 static const struct bpf_link_ops bpf_struct_ops_map_lops = {
 	.dealloc = bpf_struct_ops_map_link_dealloc,
 	.detach = bpf_struct_ops_map_link_detach,
 	.show_fdinfo = bpf_struct_ops_map_link_show_fdinfo,
 	.fill_link_info = bpf_struct_ops_map_link_fill_link_info,
 	.update_map = bpf_struct_ops_map_link_update,
+	.poll = bpf_struct_ops_map_link_poll,
 };
 
 int bpf_struct_ops_link_create(union bpf_attr *attr)
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 4a2f95c4b2ac..b4dbca04d4f5 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2990,6 +2990,7 @@ void bpf_link_init(struct bpf_link *link, enum bpf_link_type type,
 	link->id = 0;
 	link->ops = ops;
 	link->prog = prog;
+	init_waitqueue_head(&link->wait_hup);
 }
 
 static void bpf_link_free_id(int id)
@@ -3108,6 +3109,19 @@ static void bpf_link_show_fdinfo(struct seq_file *m, struct file *filp)
 }
 #endif
 
+static __poll_t bpf_link_poll(struct file *file, struct poll_table_struct *pts)
+{
+	struct bpf_link *link = file->private_data;
+
+	if (link->ops->poll) {
+		poll_wait(file, &link->wait_hup, pts);
+
+		return link->ops->poll(file, pts);
+	}
+
+	return 0;
+}
+
 static const struct file_operations bpf_link_fops = {
 #ifdef CONFIG_PROC_FS
 	.show_fdinfo	= bpf_link_show_fdinfo,
@@ -3115,6 +3129,7 @@ static const struct file_operations bpf_link_fops = {
 	.release	= bpf_link_release,
 	.read		= bpf_dummy_read,
 	.write		= bpf_dummy_write,
+	.poll		= bpf_link_poll,
 };
 
 static int bpf_link_alloc_id(struct bpf_link *link)
-- 
2.34.1