bpf: support epoll from bpf struct_ops links.

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>
Link: https://lore.kernel.org/r/20240530065946.979330-4-thinker.li@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 19f8836..5eb6112 100644 (file)
--- 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 58314b1..a2cf31b 100644 (file)
--- 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)
@@ -1208,6 +1223,8 @@ int bpf_struct_ops_link_create(union bpf_attr *attr)
        if (err)
                goto err_out;
 
+       init_waitqueue_head(&link->wait_hup);
+
        /* Hold the update_mutex such that the subsystem cannot
         * do link->ops->detach() before the link is fully initialized.
         */

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 2222c3f..81efa19 100644 (file)
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3150,6 +3150,13 @@ 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;
+
+       return link->ops->poll(file, pts);
+}
+
 static const struct file_operations bpf_link_fops = {
 #ifdef CONFIG_PROC_FS
        .show_fdinfo    = bpf_link_show_fdinfo,
@@ -3159,6 +3166,16 @@ static const struct file_operations bpf_link_fops = {
        .write          = bpf_dummy_write,
 };
 
+static const struct file_operations bpf_link_fops_poll = {
+#ifdef CONFIG_PROC_FS
+       .show_fdinfo    = bpf_link_show_fdinfo,
+#endif
+       .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)
 {
        int id;
@@ -3201,7 +3218,9 @@ int bpf_link_prime(struct bpf_link *link, struct bpf_link_primer *primer)
                return id;
        }
 
-       file = anon_inode_getfile("bpf_link", &bpf_link_fops, link, O_CLOEXEC);
+       file = anon_inode_getfile("bpf_link",
+                                 link->ops->poll ? &bpf_link_fops_poll : &bpf_link_fops,
+                                 link, O_CLOEXEC);
        if (IS_ERR(file)) {
                bpf_link_free_id(id);
                put_unused_fd(fd);
@@ -3229,7 +3248,9 @@ int bpf_link_settle(struct bpf_link_primer *primer)
 
 int bpf_link_new_fd(struct bpf_link *link)
 {
-       return anon_inode_getfd("bpf-link", &bpf_link_fops, link, O_CLOEXEC);
+       return anon_inode_getfd("bpf-link",
+                               link->ops->poll ? &bpf_link_fops_poll : &bpf_link_fops,
+                               link, O_CLOEXEC);
 }
 
 struct bpf_link *bpf_link_get_from_fd(u32 ufd)
@@ -3239,7 +3260,7 @@ struct bpf_link *bpf_link_get_from_fd(u32 ufd)
 
        if (!f.file)
                return ERR_PTR(-EBADF);
-       if (f.file->f_op != &bpf_link_fops) {
+       if (f.file->f_op != &bpf_link_fops && f.file->f_op != &bpf_link_fops_poll) {
                fdput(f);
                return ERR_PTR(-EINVAL);
        }
@@ -4971,7 +4992,7 @@ static int bpf_obj_get_info_by_fd(const union bpf_attr *attr,
                                             uattr);
        else if (f.file->f_op == &btf_fops)
                err = bpf_btf_get_info_by_fd(f.file, f.file->private_data, attr, uattr);
-       else if (f.file->f_op == &bpf_link_fops)
+       else if (f.file->f_op == &bpf_link_fops || f.file->f_op == &bpf_link_fops_poll)
                err = bpf_link_get_info_by_fd(f.file, f.file->private_data,
                                              attr, uattr);
        else
@@ -5106,7 +5127,7 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
        if (!file)
                return -EBADF;
 
-       if (file->f_op == &bpf_link_fops) {
+       if (file->f_op == &bpf_link_fops || file->f_op == &bpf_link_fops_poll) {
                struct bpf_link *link = file->private_data;
 
                if (link->ops == &bpf_raw_tp_link_lops) {