[PATCH bpf-next 2/4] bpf: Add bpf command to get number of map entries

[Charalampos Stylianopoulos <charalampos.stylianopoulos@xxxxxxxxx>]

Introduces a new command to the main bpf syscall to return the number
of entries in a map. Returns EOPNOTSUPP if the relevant map operation
is not implemented.

Co-developed-by: Nick Zavaritsky <mejedi@xxxxxxxxx>
Signed-off-by: Nick Zavaritsky <mejedi@xxxxxxxxx>
Signed-off-by: Charalampos Stylianopoulos <charalampos.stylianopoulos@xxxxxxxxx>
---
 include/uapi/linux/bpf.h | 17 +++++++++++++++++
 kernel/bpf/syscall.c     | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 2acf9b336371..f5c7adea1387 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -903,6 +903,17 @@ union bpf_iter_link_info {
  *		A new file descriptor (a nonnegative integer), or -1 if an
  *		error occurred (in which case, *errno* is set appropriately).
  *
+ * BPF_MAP_GET_NUM_ENTRIES
+ *	Description
+ *		Get the number of entries currently present in the map.
+ *
+ *		This operation is supported only for a subset of maps. If the
+ *		map is not supported, the operation returns **EOPNOTSUPP**.
+ *
+ *	Return
+ *		Returns zero on success. On error, -1 is returned and *errno*
+ *		is set appropriately.
+ *
  * NOTES
  *	eBPF objects (maps and programs) can be shared between processes.
  *
@@ -958,6 +969,7 @@ enum bpf_cmd {
 	BPF_LINK_DETACH,
 	BPF_PROG_BIND_MAP,
 	BPF_TOKEN_CREATE,
+	BPF_MAP_GET_NUM_ENTRIES,
 	__MAX_BPF_CMD,
 };
 
@@ -1837,6 +1849,11 @@ union bpf_attr {
 		__u32		bpffs_fd;
 	} token_create;
 
+	struct { /* struct used by BPF_MAP_GET_NUM_ENTRIES command*/
+		__u32 map_fd;
+		__u32 num_entries;
+	} map_get_num_entries;
+
 } __attribute__((aligned(8)));
 
 /* The description below is an attempt at providing documentation to eBPF
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 4e88797fdbeb..a31a63a4aa5d 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -5748,6 +5748,35 @@ static int token_create(union bpf_attr *attr)
 	return bpf_token_create(attr);
 }
 
+#define BPF_MAP_GET_NUM_ENTRIES_LAST_FIELD map_get_num_entries.num_entries
+
+static int bpf_get_num_entries(union bpf_attr *attr, union bpf_attr __user *uattr)
+{
+	__u32 num_entries = 0;
+	struct bpf_map *map;
+
+	if (CHECK_ATTR(BPF_MAP_GET_NUM_ENTRIES))
+		return -EINVAL;
+
+
+	CLASS(fd, f)(attr->map_fd);
+	map = __bpf_map_get(f);
+	if (IS_ERR(map))
+		return PTR_ERR(map);
+
+	if (!map->ops->map_num_entries)
+		return -EOPNOTSUPP;
+
+	num_entries = map->ops->map_num_entries(map);
+	if (num_entries < 0)
+		return num_entries;
+
+	if (put_user(num_entries, &uattr->map_get_num_entries.num_entries))
+		return -EFAULT;
+
+	return 0;
+}
+
 static int __sys_bpf(enum bpf_cmd cmd, bpfptr_t uattr, unsigned int size)
 {
 	union bpf_attr attr;
@@ -5884,6 +5913,9 @@ static int __sys_bpf(enum bpf_cmd cmd, bpfptr_t uattr, unsigned int size)
 	case BPF_TOKEN_CREATE:
 		err = token_create(&attr);
 		break;
+	case BPF_MAP_GET_NUM_ENTRIES:
+		err = bpf_get_num_entries(&attr, uattr.user);
+		break;
 	default:
 		err = -EINVAL;
 		break;
-- 
2.43.0