libbpf: Add support for bpf_arena.

mmap() bpf_arena right after creation, since the kernel needs to
remember the address returned from mmap. This is user_vm_start.
LLVM will generate bpf_arena_cast_user() instructions where
necessary and JIT will add upper 32-bit of user_vm_start
to such pointers.

Fix up bpf_map_mmap_sz() to compute mmap size as
map->value_size * map->max_entries for arrays and
PAGE_SIZE * map->max_entries for arena.

Don't set BTF at arena creation time, since it doesn't support it.

Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/libbpf.c

···
 	[BPF_MAP_TYPE_BLOOM_FILTER]		= "bloom_filter",
 	[BPF_MAP_TYPE_USER_RINGBUF]             = "user_ringbuf",
 	[BPF_MAP_TYPE_CGRP_STORAGE]		= "cgrp_storage",
+	[BPF_MAP_TYPE_ARENA]			= "arena",
 };
 
 static const char * const prog_type_name[] = {
···
 	return map;
 }
 
-static size_t bpf_map_mmap_sz(unsigned int value_sz, unsigned int max_entries)
+static size_t array_map_mmap_sz(unsigned int value_sz, unsigned int max_entries)
 {
 	const long page_sz = sysconf(_SC_PAGE_SIZE);
 	size_t map_sz;
···
 	map_sz = (size_t)roundup(value_sz, 8) * max_entries;
 	map_sz = roundup(map_sz, page_sz);
 	return map_sz;
+}
+
+static size_t bpf_map_mmap_sz(const struct bpf_map *map)
+{
+	const long page_sz = sysconf(_SC_PAGE_SIZE);
+
+	switch (map->def.type) {
+	case BPF_MAP_TYPE_ARRAY:
+		return array_map_mmap_sz(map->def.value_size, map->def.max_entries);
+	case BPF_MAP_TYPE_ARENA:
+		return page_sz * map->def.max_entries;
+	default:
+		return 0; /* not supported */
+	}
 }
 
 static int bpf_map_mmap_resize(struct bpf_map *map, size_t old_sz, size_t new_sz)
···
 	pr_debug("map '%s' (global data): at sec_idx %d, offset %zu, flags %x.\n",
 		 map->name, map->sec_idx, map->sec_offset, def->map_flags);
 
-	mmap_sz = bpf_map_mmap_sz(map->def.value_size, map->def.max_entries);
+	mmap_sz = bpf_map_mmap_sz(map);
 	map->mmaped = mmap(NULL, mmap_sz, PROT_READ | PROT_WRITE,
 			   MAP_SHARED | MAP_ANONYMOUS, -1, 0);
 	if (map->mmaped == MAP_FAILED) {
···
 	case BPF_MAP_TYPE_SOCKHASH:
 	case BPF_MAP_TYPE_QUEUE:
 	case BPF_MAP_TYPE_STACK:
+	case BPF_MAP_TYPE_ARENA:
 		create_attr.btf_fd = 0;
 		create_attr.btf_key_type_id = 0;
 		create_attr.btf_value_type_id = 0;
···
 				if (err < 0)
 					goto err_out;
 			}
-
+			if (map->def.type == BPF_MAP_TYPE_ARENA) {
+				map->mmaped = mmap((void *)map->map_extra, bpf_map_mmap_sz(map),
+						   PROT_READ | PROT_WRITE,
+						   map->map_extra ? MAP_SHARED | MAP_FIXED : MAP_SHARED,
+						   map->fd, 0);
+				if (map->mmaped == MAP_FAILED) {
+					err = -errno;
+					map->mmaped = NULL;
+					pr_warn("map '%s': failed to mmap arena: %d\n",
+						map->name, err);
+					return err;
+				}
+			}
 			if (map->init_slots_sz && map->def.type != BPF_MAP_TYPE_PROG_ARRAY) {
 				err = init_map_in_map_slots(obj, map);
 				if (err < 0)
···
 	if (map->mmaped) {
 		size_t mmap_sz;
 
-		mmap_sz = bpf_map_mmap_sz(map->def.value_size, map->def.max_entries);
+		mmap_sz = bpf_map_mmap_sz(map);
 		munmap(map->mmaped, mmap_sz);
 		map->mmaped = NULL;
 	}
···
 		return libbpf_err(-EBUSY);
 
 	if (map->mmaped) {
-		int err;
 		size_t mmap_old_sz, mmap_new_sz;
+		int err;
 
-		mmap_old_sz = bpf_map_mmap_sz(map->def.value_size, map->def.max_entries);
-		mmap_new_sz = bpf_map_mmap_sz(size, map->def.max_entries);
+		if (map->def.type != BPF_MAP_TYPE_ARRAY)
+			return -EOPNOTSUPP;
+
+		mmap_old_sz = bpf_map_mmap_sz(map);
+		mmap_new_sz = array_map_mmap_sz(size, map->def.max_entries);
 		err = bpf_map_mmap_resize(map, mmap_old_sz, mmap_new_sz);
 		if (err) {
 			pr_warn("map '%s': failed to resize memory-mapped region: %d\n",
···
 
 	for (i = 0; i < s->map_cnt; i++) {
 		struct bpf_map *map = *s->maps[i].map;
-		size_t mmap_sz = bpf_map_mmap_sz(map->def.value_size, map->def.max_entries);
+		size_t mmap_sz = bpf_map_mmap_sz(map);
 		int prot, map_fd = map->fd;
 		void **mmaped = s->maps[i].mmaped;
 

tools/lib/bpf/libbpf_probes.c

···
 		key_size = 0;
 		max_entries = 1;
 		break;
+	case BPF_MAP_TYPE_ARENA:
+		key_size	= 0;
+		value_size	= 0;
+		max_entries	= 1; /* one page */
+		opts.map_extra	= 0; /* can mmap() at any address */
+		opts.map_flags	= BPF_F_MMAPABLE;
+		break;
 	case BPF_MAP_TYPE_HASH:
 	case BPF_MAP_TYPE_ARRAY:
 	case BPF_MAP_TYPE_PROG_ARRAY: