bpf/verifier: allow kfunc to return an allocated mem

For drivers (outside of network), the incoming data is not statically
defined in a struct. Most of the time the data buffer is kzalloc-ed
and thus we can not rely on eBPF and BTF to explore the data.

This commit allows to return an arbitrary memory, previously allocated by
the driver.
An interesting extra point is that the kfunc can mark the exported
memory region as read only or read/write.

So, when a kfunc is not returning a pointer to a struct but to a plain
type, we can consider it is a valid allocated memory assuming that:
- one of the arguments is either called rdonly_buf_size or
  rdwr_buf_size
- and this argument is a const from the caller point of view

We can then use this parameter as the size of the allocated memory.

The memory is either read-only or read-write based on the name
of the size parameter.

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Link: https://lore.kernel.org/r/20220906151303.2780789-7-benjamin.tissoires@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

3 files changed, 20 insertions, 1 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 3cf161cfd396..79883f883ff3 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1944,6 +1944,13 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
 			   const char *func_name,
 			   struct btf_func_model *m);
 
+struct bpf_kfunc_arg_meta {
+	u64 r0_size;
+	bool r0_rdonly;
+	int ref_obj_id;
+	u32 flags;
+};
+
 struct bpf_reg_state;
 int btf_check_subprog_arg_match(struct bpf_verifier_env *env, int subprog,
 				struct bpf_reg_state *regs);
@@ -1952,7 +1959,7 @@ int btf_check_subprog_call(struct bpf_verifier_env *env, int subprog,
 int btf_check_kfunc_arg_match(struct bpf_verifier_env *env,
 			      const struct btf *btf, u32 func_id,
 			      struct bpf_reg_state *regs,
-			      u32 kfunc_flags);
+			      struct bpf_kfunc_arg_meta *meta);
 int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog,
 			  struct bpf_reg_state *reg);
 int btf_check_type_match(struct bpf_verifier_log *log, const struct bpf_prog *prog,
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 1fdddbf3546b..8fbc1d05281e 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -598,6 +598,8 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 			    struct bpf_attach_target_info *tgt_info);
 void bpf_free_kfunc_btf_tab(struct bpf_kfunc_btf_tab *tab);
 
+int mark_chain_precision(struct bpf_verifier_env *env, int regno);
+
 #define BPF_BASE_TYPE_MASK	GENMASK(BPF_BASE_TYPE_BITS - 1, 0)
 
 /* extract base type from bpf_{arg, return, reg}_type. */
diff --git a/include/linux/btf.h b/include/linux/btf.h
index ad93c2d9cc1c..1fcc833a8690 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -441,4 +441,14 @@ static inline int register_btf_id_dtor_kfuncs(const struct btf_id_dtor_kfunc *dt
 }
 #endif
 
+static inline bool btf_type_is_struct_ptr(struct btf *btf, const struct btf_type *t)
+{
+	if (!btf_type_is_ptr(t))
+		return false;
+
+	t = btf_type_skip_modifiers(btf, t->type, NULL);
+
+	return btf_type_is_struct(t);
+}
+
 #endif