[PATCH v3 02/13] epoll: introduce user structures for polling from userspace

[Roman Penyaev <rpenyaev@suse.de>]

This one introduces structures of user items array:

struct epoll_uheader -
    describes inserted epoll items.

struct epoll_uitem -
    single epoll item visible to userspace.

Signed-off-by: Roman Penyaev <rpenyaev@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 2cc183e86a29..f598442512f3 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -9,6 +9,8 @@
  *
  *  Davide Libenzi <davidel@xmailserver.org>
  *
+ *  Polling from userspace support by Roman Penyaev <rpenyaev@suse.de>
+ *  (C) Copyright 2019 SUSE, All Rights Reserved
  */
 
 #include <linux/init.h>
@@ -109,6 +111,13 @@
 
 #define EP_ITEM_COST (sizeof(struct epitem) + sizeof(struct eppoll_entry))
 
+/*
+ * That is around 1.3mb of allocated memory for one epfd.  What is more
+ * important is ->index_length, which should be ^2, so do not increase
+ * max items number to avoid size doubling of user index.
+ */
+#define EP_USERPOLL_MAX_ITEMS_NR 65536
+
 struct epoll_filefd {
 	struct file *file;
 	int fd;
diff --git a/include/uapi/linux/eventpoll.h b/include/uapi/linux/eventpoll.h
index 39dfc29f0f52..161dfcbcf3b5 100644
--- a/include/uapi/linux/eventpoll.h
+++ b/include/uapi/linux/eventpoll.h
@@ -79,4 +79,32 @@ struct epoll_event {
 	__u64 data;
 } EPOLL_PACKED;
 
+#define EPOLL_USERPOLL_HEADER_MAGIC 0xeb01eb01
+#define EPOLL_USERPOLL_HEADER_SIZE  128
+
+/*
+ * Item, shared with userspace.  Unfortunately we can't embed epoll_event
+ * structure, because it is badly aligned on all 64-bit archs, except
+ * x86-64 (see EPOLL_PACKED).  sizeof(epoll_uitem) == 16
+ */
+struct epoll_uitem {
+	__poll_t ready_events;
+	__poll_t events;
+	__u64 data;
+};
+
+/*
+ * Header, shared with userspace. sizeof(epoll_uheader) == 128
+ */
+struct epoll_uheader {
+	u32 magic;          /* epoll user header magic */
+	u32 header_length;  /* length of the header + items */
+	u32 index_length;   /* length of the index ring, always pow2 */
+	u32 max_items_nr;   /* max number of items */
+	u32 head;           /* updated by userland */
+	u32 tail;           /* updated by kernel */
+
+	struct epoll_uitem items[] __aligned(EPOLL_USERPOLL_HEADER_SIZE);
+};
+
 #endif /* _UAPI_LINUX_EVENTPOLL_H */
-- 
2.21.0