bpf: Wire up freeing of referenced kptr

A destructor kfunc can be defined as void func(type *), where type may
be void or any other pointer type as per convenience.

In this patch, we ensure that the type is sane and capture the function
pointer into off_desc of ptr_off_tab for the specific pointer offset,
with the invariant that the dtor pointer is always set when 'kptr_ref'
tag is applied to the pointer's pointee type, which is indicated by the
flag BPF_MAP_VALUE_OFF_F_REF.

Note that only BTF IDs whose destructor kfunc is registered, thus become
the allowed BTF IDs for embedding as referenced kptr. Hence it serves
the purpose of finding dtor kfunc BTF ID, as well acting as a check
against the whitelist of allowed BTF IDs for this purpose.

Finally, wire up the actual freeing of the referenced pointer if any at
all available offsets, so that no references are leaked after the BPF
map goes away and the BPF program previously moved the ownership a
referenced pointer into it.

The behavior is similar to BPF timers, where bpf_map_{update,delete}_elem
will free any existing referenced kptr. The same case is with LRU map's
bpf_lru_push_free/htab_lru_push_free functions, which are extended to
reset unreferenced and free referenced kptr.

Note that unlike BPF timers, kptr is not reset or freed when map uref
drops to zero.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220424214901.2743946-8-memxor@gmail.com

2 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 4e12b27a5de6..6141564c76c8 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -23,6 +23,7 @@
 #include <linux/slab.h>
 #include <linux/percpu-refcount.h>
 #include <linux/bpfptr.h>
+#include <linux/btf.h>
 
 struct bpf_verifier_env;
 struct bpf_verifier_log;
@@ -173,6 +174,8 @@ struct bpf_map_value_off_desc {
 	enum bpf_kptr_type type;
 	struct {
 		struct btf *btf;
+		struct module *module;
+		btf_dtor_kfunc_t dtor;
 		u32 btf_id;
 	} kptr;
 };
@@ -1447,6 +1450,7 @@ struct bpf_map_value_off_desc *bpf_map_kptr_off_contains(struct bpf_map *map, u3
 void bpf_map_free_kptr_off_tab(struct bpf_map *map);
 struct bpf_map_value_off *bpf_map_copy_kptr_off_tab(const struct bpf_map *map);
 bool bpf_map_equal_kptr_off_tab(const struct bpf_map *map_a, const struct bpf_map *map_b);
+void bpf_map_free_kptrs(struct bpf_map *map, void *map_value);
 
 struct bpf_map *bpf_map_get(u32 ufd);
 struct bpf_map *bpf_map_get_with_uref(u32 ufd);
diff --git a/include/linux/btf.h b/include/linux/btf.h
index fea424681d66..f70625dd5bb4 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -45,6 +45,8 @@ struct btf_id_dtor_kfunc {
 	u32 kfunc_btf_id;
 };
 
+typedef void (*btf_dtor_kfunc_t)(void *);
+
 extern const struct file_operations btf_fops;
 
 void btf_get(struct btf *btf);