[PATCH bpf-next v4 3/7] bpf: support epoll from bpf struct_ops links.

Kui-Feng Lee
2024-05-21T22:53:05Z

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

This patch implements the "poll" of the "struct file_operations" for BPF
links and introduces a new "poll" operator in the "struct bpf_link_ops". By
implementing "poll" of "struct bpf_link_ops" for the links of struct_ops,
the file descriptor of a struct_ops link can be added to an epoll file
descriptor to receive EPOLLHUP events.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
---
include/linux/bpf.h | 1 +
kernel/bpf/bpf_struct_ops.c | 17 +++++++++++++++++
kernel/bpf/syscall.c | 11 +++++++++++
3 files changed, 29 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index b600767ebe02..5f7496ef8b7c 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1612,6 +1612,7 @@ 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 fb6e8a3190ef..794549dc9f4b 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;
@@ -56,6 +57,7 @@ struct bpf_struct_ops_map {
struct bpf_struct_ops_link {
struct bpf_link link;
struct bpf_map __rcu *map;
+ wait_queue_head_t wait_hup;
};

static DEFINE_MUTEX(update_mutex);
@@ -1167,15 +1169,28 @@ static int bpf_struct_ops_map_link_detach(struct bpf_link *link)

mutex_unlock(&update_mutex);

+ wake_up_interruptible_poll(&st_link->wait_hup, EPOLLHUP);
+
return 0;
}

+static __poll_t bpf_struct_ops_map_link_poll(struct file *file,
+ struct poll_table_struct *pts)
+{
+ struct bpf_struct_ops_link *st_link = file->private_data;
+
+ poll_wait(file, &st_link->wait_hup, pts);
+
+ return rcu_access_pointer(st_link->map) ? 0 : EPOLLHUP;
+}
+
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)
@@ -1213,6 +1228,8 @@ int bpf_struct_ops_link_create(union bpf_attr *attr)
*/
RCU_INIT_POINTER(link->map, map);

+ init_waitqueue_head(&link->wait_hup);
+
/* Once reg() is called, the object and link is already available
* to the subsystem, and it can call
* bpf_struct_ops_map_link_detach() to unreg() it. However, it is
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 13ad74ecf2cd..ad4f81ed27f0 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3150,6 +3150,16 @@ 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)
+ 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,
@@ -3157,6 +3167,7 @@ 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