bpf(): allow creating nested maps (#3968)

* bpf(): allow creating nested maps

Add support for creating nested maps via the bpf() syscall wrapper.

Updates #3729

* Address feedback from shpalani

include/linux/bpf.h

@@ -92,7 +92,8 @@ typedef struct
     uint32_t key_size;          ///< Size in bytes of keys.
     uint32_t value_size;        ///< Size in bytes of values.
     uint32_t max_entries;       ///< Maximum number of entries in the map.
-    uint32_t map_flags;         ///< Flags (currently 0).
+    uint32_t map_flags;         ///< Not supported, must be zero.
+    uint32_t inner_map_fd;      ///< File descriptor of inner map.
 } sys_bpf_map_create_attr_t;
 
 typedef struct

libs/api/bpf_syscall.cpp

@@ -93,7 +93,12 @@ bpf(int cmd, union bpf_attr* attr, unsigned int size)
         }
         case BPF_MAP_CREATE: {
             ExtensibleStruct<sys_bpf_map_create_attr_t> map_create_attr((void*)attr, (size_t)size);
-            struct bpf_map_create_opts opts = {.map_flags = map_create_attr->map_flags};
+
+            struct bpf_map_create_opts opts = {
+                .inner_map_fd = map_create_attr->inner_map_fd,
+                .map_flags = map_create_attr->map_flags,
+            };
+
             return bpf_map_create(
                 map_create_attr->map_type,
                 nullptr,

tests/unit/libbpf_test.cpp

@@ -3006,6 +3006,64 @@ TEST_CASE("BPF_PROG_BIND_MAP etc.", "[libbpf]")
     attr.prog_bind_map.flags = 0;
     REQUIRE(bpf(BPF_PROG_BIND_MAP, &attr, sizeof(attr)) == 0);
 
+    // Verify we can create a nested array map.
+    uint32_t key = 0;
+    fd_t value = map_fd;
+    attr.map_create = {};
+    attr.map_create.map_type = BPF_MAP_TYPE_ARRAY_OF_MAPS;
+    attr.map_create.key_size = sizeof(key);
+    attr.map_create.value_size = sizeof(value);
+    attr.map_create.max_entries = 1;
+    attr.map_create.inner_map_fd = map_fd;
+    int nested_map_fd = bpf(BPF_MAP_CREATE, &attr, sizeof(attr.map_create));
+    REQUIRE(nested_map_fd >= 0);
+
+    // Ensure we can insert map_fd into the outer map.
+    attr.map_update = {};
+    attr.map_update.map_fd = nested_map_fd;
+    attr.map_update.key = (uintptr_t)&key;
+    attr.map_update.value = (uintptr_t)&value;
+    REQUIRE(bpf(BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr.map_update)) == 0);
+
+    // Ensure that looking up the same key gives the ID of the inner map.
+    ebpf_id_t value_id = 0;
+    attr.map_lookup = {};
+    attr.map_lookup.map_fd = nested_map_fd;
+    attr.map_lookup.key = (uintptr_t)&key;
+    attr.map_lookup.value = (uintptr_t)&value_id;
+    REQUIRE(bpf(BPF_MAP_LOOKUP_ELEM, &attr, sizeof(attr.map_lookup)) == 0);
+    REQUIRE(value_id == map_id);
+
+    Platform::_close(nested_map_fd);
+
+    // Verify we can create a nested hash map.
+    attr.map_create = {};
+    attr.map_create.map_type = BPF_MAP_TYPE_HASH_OF_MAPS;
+    attr.map_create.key_size = sizeof(key);
+    attr.map_create.value_size = sizeof(value);
+    attr.map_create.max_entries = 1;
+    attr.map_create.inner_map_fd = map_fd;
+    nested_map_fd = bpf(BPF_MAP_CREATE, &attr, sizeof(attr.map_create));
+    REQUIRE(nested_map_fd >= 0);
+
+    // Ensure we can insert map_fd into the outer map.
+    attr.map_update = {};
+    attr.map_update.map_fd = nested_map_fd;
+    attr.map_update.key = (uintptr_t)&key;
+    attr.map_update.value = (uintptr_t)&value;
+    REQUIRE(bpf(BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr.map_update)) == 0);
+
+    // Ensure that looking up the same key gives the ID of the inner map.
+    value_id = 0;
+    attr.map_lookup = {};
+    attr.map_lookup.map_fd = nested_map_fd;
+    attr.map_lookup.key = (uintptr_t)&key;
+    attr.map_lookup.value = (uintptr_t)&value_id;
+    REQUIRE(bpf(BPF_MAP_LOOKUP_ELEM, &attr, sizeof(attr.map_lookup)) == 0);
+    REQUIRE(value_id == map_id);
+
+    Platform::_close(nested_map_fd);
+
     // Release our own references on the map and program.
     Platform::_close(map_fd);
     Platform::_close(program_fd);