bpf: Populate pairs of btf_id and destructor kfunc in btf

To support storing referenced PTR_TO_BTF_ID in maps, we require
associating a specific BTF ID with a 'destructor' kfunc. This is because
we need to release a live referenced pointer at a certain offset in map
value from the map destruction path, otherwise we end up leaking
resources.

Hence, introduce support for passing an array of btf_id, kfunc_btf_id
pairs that denote a BTF ID and its associated release function. Then,
add an accessor 'btf_find_dtor_kfunc' which can be used to look up the
destructor kfunc of a certain BTF ID. If found, we can use it to free
the object from the map free path.

The registration of these pairs also serve as a whitelist of structures
which are allowed as referenced PTR_TO_BTF_ID in a BPF map, because
without finding the destructor kfunc, we will bail and return an error.
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220424214901.2743946-7-memxor@gmail.com

include/linux/btf.h

@@ -40,6 +40,11 @@ struct btf_kfunc_id_set {
 	};
 };
 
+struct btf_id_dtor_kfunc {
+	u32 btf_id;
+	u32 kfunc_btf_id;
+};
+
 extern const struct file_operations btf_fops;
 
 void btf_get(struct btf *btf);
@@ -346,6 +351,9 @@ bool btf_kfunc_id_set_contains(const struct btf *btf,
 			       enum btf_kfunc_type type, u32 kfunc_btf_id);
 int register_btf_kfunc_id_set(enum bpf_prog_type prog_type,
 			      const struct btf_kfunc_id_set *s);
+s32 btf_find_dtor_kfunc(struct btf *btf, u32 btf_id);
+int register_btf_id_dtor_kfuncs(const struct btf_id_dtor_kfunc *dtors, u32 add_cnt,
+				struct module *owner);
 #else
 static inline const struct btf_type *btf_type_by_id(const struct btf *btf,
 						    u32 type_id)
@@ -369,6 +377,15 @@ static inline int register_btf_kfunc_id_set(enum bpf_prog_type prog_type,
 {
 	return 0;
 }
+static inline s32 btf_find_dtor_kfunc(struct btf *btf, u32 btf_id)
+{
+	return -ENOENT;
+}
+static inline int register_btf_id_dtor_kfuncs(const struct btf_id_dtor_kfunc *dtors,
+					      u32 add_cnt, struct module *owner)
+{
+	return 0;
+}
 #endif
 
 #endif

kernel/bpf/btf.c

@@ -207,12 +207,18 @@ enum btf_kfunc_hook {
 
 enum {
 	BTF_KFUNC_SET_MAX_CNT = 32,
+	BTF_DTOR_KFUNC_MAX_CNT = 256,
 };
 
 struct btf_kfunc_set_tab {
 	struct btf_id_set *sets[BTF_KFUNC_HOOK_MAX][BTF_KFUNC_TYPE_MAX];
 };
 
+struct btf_id_dtor_kfunc_tab {
+	u32 cnt;
+	struct btf_id_dtor_kfunc dtors[];
+};
+
 struct btf {
 	void *data;
 	struct btf_type **types;
@@ -228,6 +234,7 @@ struct btf {
 	u32 id;
 	struct rcu_head rcu;
 	struct btf_kfunc_set_tab *kfunc_set_tab;
+	struct btf_id_dtor_kfunc_tab *dtor_kfunc_tab;
 
 	/* split BTF support */
 	struct btf *base_btf;
@@ -1616,8 +1623,19 @@ static void btf_free_kfunc_set_tab(struct btf *btf)
 	btf->kfunc_set_tab = NULL;
 }
 
+static void btf_free_dtor_kfunc_tab(struct btf *btf)
+{
+	struct btf_id_dtor_kfunc_tab *tab = btf->dtor_kfunc_tab;
+
+	if (!tab)
+		return;
+	kfree(tab);
+	btf->dtor_kfunc_tab = NULL;
+}
+
 static void btf_free(struct btf *btf)
 {
+	btf_free_dtor_kfunc_tab(btf);
 	btf_free_kfunc_set_tab(btf);
 	kvfree(btf->types);
 	kvfree(btf->resolved_sizes);
@@ -7076,6 +7094,96 @@ int register_btf_kfunc_id_set(enum bpf_prog_type prog_type,
 }
 EXPORT_SYMBOL_GPL(register_btf_kfunc_id_set);
 
+s32 btf_find_dtor_kfunc(struct btf *btf, u32 btf_id)
+{
+	struct btf_id_dtor_kfunc_tab *tab = btf->dtor_kfunc_tab;
+	struct btf_id_dtor_kfunc *dtor;
+
+	if (!tab)
+		return -ENOENT;
+	/* Even though the size of tab->dtors[0] is > sizeof(u32), we only need
+	 * to compare the first u32 with btf_id, so we can reuse btf_id_cmp_func.
+	 */
+	BUILD_BUG_ON(offsetof(struct btf_id_dtor_kfunc, btf_id) != 0);
+	dtor = bsearch(&btf_id, tab->dtors, tab->cnt, sizeof(tab->dtors[0]), btf_id_cmp_func);
+	if (!dtor)
+		return -ENOENT;
+	return dtor->kfunc_btf_id;
+}
+
+/* This function must be invoked only from initcalls/module init functions */
+int register_btf_id_dtor_kfuncs(const struct btf_id_dtor_kfunc *dtors, u32 add_cnt,
+				struct module *owner)
+{
+	struct btf_id_dtor_kfunc_tab *tab;
+	struct btf *btf;
+	u32 tab_cnt;
+	int ret;
+
+	btf = btf_get_module_btf(owner);
+	if (!btf) {
+		if (!owner && IS_ENABLED(CONFIG_DEBUG_INFO_BTF)) {
+			pr_err("missing vmlinux BTF, cannot register dtor kfuncs\n");
+			return -ENOENT;
+		}
+		if (owner && IS_ENABLED(CONFIG_DEBUG_INFO_BTF_MODULES)) {
+			pr_err("missing module BTF, cannot register dtor kfuncs\n");
+			return -ENOENT;
+		}
+		return 0;
+	}
+	if (IS_ERR(btf))
+		return PTR_ERR(btf);
+
+	if (add_cnt >= BTF_DTOR_KFUNC_MAX_CNT) {
+		pr_err("cannot register more than %d kfunc destructors\n", BTF_DTOR_KFUNC_MAX_CNT);
+		ret = -E2BIG;
+		goto end;
+	}
+
+	tab = btf->dtor_kfunc_tab;
+	/* Only one call allowed for modules */
+	if (WARN_ON_ONCE(tab && btf_is_module(btf))) {
+		ret = -EINVAL;
+		goto end;
+	}
+
+	tab_cnt = tab ? tab->cnt : 0;
+	if (tab_cnt > U32_MAX - add_cnt) {
+		ret = -EOVERFLOW;
+		goto end;
+	}
+	if (tab_cnt + add_cnt >= BTF_DTOR_KFUNC_MAX_CNT) {
+		pr_err("cannot register more than %d kfunc destructors\n", BTF_DTOR_KFUNC_MAX_CNT);
+		ret = -E2BIG;
+		goto end;
+	}
+
+	tab = krealloc(btf->dtor_kfunc_tab,
+		       offsetof(struct btf_id_dtor_kfunc_tab, dtors[tab_cnt + add_cnt]),
+		       GFP_KERNEL | __GFP_NOWARN);
+	if (!tab) {
+		ret = -ENOMEM;
+		goto end;
+	}
+
+	if (!btf->dtor_kfunc_tab)
+		tab->cnt = 0;
+	btf->dtor_kfunc_tab = tab;
+
+	memcpy(tab->dtors + tab->cnt, dtors, add_cnt * sizeof(tab->dtors[0]));
+	tab->cnt += add_cnt;
+
+	sort(tab->dtors, tab->cnt, sizeof(tab->dtors[0]), btf_id_cmp_func, NULL);
+
+	return 0;
+end:
+	btf_free_dtor_kfunc_tab(btf);
+	btf_put(btf);
+	return ret;
+}
+EXPORT_SYMBOL_GPL(register_btf_id_dtor_kfuncs);
+
 #define MAX_TYPES_ARE_COMPAT_DEPTH 2
 
 static