bpf: Relax max_entries check for most of the inner map types

Most of the maps do not use max_entries during verification time.
Thus, those map_meta_equal() do not need to enforce max_entries
when it is inserted as an inner map during runtime.  The max_entries
check is removed from the default implementation bpf_map_meta_equal().

The prog_array_map and xsk_map are exception.  Its map_gen_lookup
uses max_entries to generate inline lookup code.  Thus, they will
implement its own map_meta_equal() to enforce max_entries.
Since there are only two cases now, the max_entries check
is not refactored and stays in its own .c file.
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200828011813.1970516-1-kafai@fb.com

kernel/bpf/arraymap.c

@@ -487,6 +487,13 @@ static int array_map_mmap(struct bpf_map *map, struct vm_area_struct *vma)
 				   vma->vm_pgoff + pgoff);
 }
 
+static bool array_map_meta_equal(const struct bpf_map *meta0,
+				 const struct bpf_map *meta1)
+{
+	return meta0->max_entries == meta1->max_entries &&
+		bpf_map_meta_equal(meta0, meta1);
+}
+
 struct bpf_iter_seq_array_map_info {
 	struct bpf_map *map;
 	void *percpu_value_buf;
@@ -625,7 +632,7 @@ static const struct bpf_iter_seq_info iter_seq_info = {
 
 static int array_map_btf_id;
 const struct bpf_map_ops array_map_ops = {
-	.map_meta_equal = bpf_map_meta_equal,
+	.map_meta_equal = array_map_meta_equal,
 	.map_alloc_check = array_map_alloc_check,
 	.map_alloc = array_map_alloc,
 	.map_free = array_map_free,

kernel/bpf/map_in_map.c

@@ -75,8 +75,7 @@ bool bpf_map_meta_equal(const struct bpf_map *meta0,
 	return meta0->map_type == meta1->map_type &&
 		meta0->key_size == meta1->key_size &&
 		meta0->value_size == meta1->value_size &&
-		meta0->map_flags == meta1->map_flags &&
-		meta0->max_entries == meta1->max_entries;
+		meta0->map_flags == meta1->map_flags;
 }
 
 void *bpf_map_fd_get_ptr(struct bpf_map *map,

net/xdp/xskmap.c

@@ -254,9 +254,16 @@ void xsk_map_try_sock_delete(struct xsk_map *map, struct xdp_sock *xs,
 	spin_unlock_bh(&map->lock);
 }
 
+static bool xsk_map_meta_equal(const struct bpf_map *meta0,
+			       const struct bpf_map *meta1)
+{
+	return meta0->max_entries == meta1->max_entries &&
+		bpf_map_meta_equal(meta0, meta1);
+}
+
 static int xsk_map_btf_id;
 const struct bpf_map_ops xsk_map_ops = {
-	.map_meta_equal = bpf_map_meta_equal,
+	.map_meta_equal = xsk_map_meta_equal,
 	.map_alloc = xsk_map_alloc,
 	.map_free = xsk_map_free,
 	.map_get_next_key = xsk_map_get_next_key,