bpf: Recognize lock and list fields in allocated objects

Allow specifying bpf_spin_lock, bpf_list_head, bpf_list_node fields in a
allocated object.

Also update btf_struct_access to reject direct access to these special
fields.

A bpf_list_head allows implementing map-in-map style use cases, where an
allocated object with bpf_list_head is linked into a list in a map
value. This would require embedding a bpf_list_node, support for which
is also included. The bpf_spin_lock is used to protect the bpf_list_head
and other data.

While we strictly don't require to hold a bpf_spin_lock while touching
the bpf_list_head in such objects, as when have access to it, we have
complete ownership of the object, the locking constraint is still kept
and may be conditionally lifted in the future.

Note that the specification of such types can be done just like map
values, e.g.:

struct bar {
	struct bpf_list_node node;
};

struct foo {
	struct bpf_spin_lock lock;
	struct bpf_list_head head __contains(bar, node);
	struct bpf_list_node node;
};

struct map_value {
	struct bpf_spin_lock lock;
	struct bpf_list_head head __contains(foo, node);
};

To recognize such types in user BTF, we build a btf_struct_metas array
of metadata items corresponding to each BTF ID. This is done once during
the btf_parse stage to avoid having to do it each time during the
verification process's requirement to inspect the metadata.

Moreover, the computed metadata needs to be passed to some helpers in
future patches which requires allocating them and storing them in the
BTF that is pinned by the program itself, so that valid access can be
assumed to such data during program runtime.

A key thing to note is that once a btf_struct_meta is available for a
type, both the btf_record and btf_field_offs should be available. It is
critical that btf_field_offs is available in case special fields are
present, as we extensively rely on special fields being zeroed out in
map values and allocated objects in later patches. The code ensures that
by bailing out in case of errors and ensuring both are available
together. If the record is not available, the special fields won't be
recognized, so not having both is also fine (in terms of being a
verification error and not a runtime bug).

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20221118015614.2013203-7-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 35 insertions, 0 deletions

diff --git a/include/linux/btf.h b/include/linux/btf.h
index d80345fa566b..a01a8da20021 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -6,6 +6,8 @@
 
 #include <linux/types.h>
 #include <linux/bpfptr.h>
+#include <linux/bsearch.h>
+#include <linux/btf_ids.h>
 #include <uapi/linux/btf.h>
 #include <uapi/linux/bpf.h>
 
@@ -78,6 +80,17 @@ struct btf_id_dtor_kfunc {
 	u32 kfunc_btf_id;
 };
 
+struct btf_struct_meta {
+	u32 btf_id;
+	struct btf_record *record;
+	struct btf_field_offs *field_offs;
+};
+
+struct btf_struct_metas {
+	u32 cnt;
+	struct btf_struct_meta types[];
+};
+
 typedef void (*btf_dtor_kfunc_t)(void *);
 
 extern const struct file_operations btf_fops;
@@ -408,6 +421,23 @@ static inline struct btf_param *btf_params(const struct btf_type *t)
 	return (struct btf_param *)(t + 1);
 }
 
+static inline int btf_id_cmp_func(const void *a, const void *b)
+{
+	const int *pa = a, *pb = b;
+
+	return *pa - *pb;
+}
+
+static inline bool btf_id_set_contains(const struct btf_id_set *set, u32 id)
+{
+	return bsearch(&id, set->ids, set->cnt, sizeof(u32), btf_id_cmp_func) != NULL;
+}
+
+static inline void *btf_id_set8_contains(const struct btf_id_set8 *set, u32 id)
+{
+	return bsearch(&id, set->pairs, set->cnt, sizeof(set->pairs[0]), btf_id_cmp_func);
+}
+
 #ifdef CONFIG_BPF_SYSCALL
 struct bpf_prog;
 
@@ -423,6 +453,7 @@ int register_btf_kfunc_id_set(enum bpf_prog_type prog_type,
 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);
+struct btf_struct_meta *btf_find_struct_meta(const struct btf *btf, u32 btf_id);
 #else
 static inline const struct btf_type *btf_type_by_id(const struct btf *btf,
 						    u32 type_id)
@@ -454,6 +485,10 @@ static inline int register_btf_id_dtor_kfuncs(const struct btf_id_dtor_kfunc *dt
 {
 	return 0;
 }
+static inline struct btf_struct_meta *btf_find_struct_meta(const struct btf *btf, u32 btf_id)
+{
+	return NULL;
+}
 #endif
 
 static inline bool btf_type_is_struct_ptr(struct btf *btf, const struct btf_type *t)