[PATCH bpf-next v1 20/25] bpf: Introduce bpf_kptr_drop

Kumar Kartikeya Dwivedi
2022-10-11T01:27:55Z

Introduce bpf_kptr_drop, which is the kfunc used to free local kptrs
allocated using bpf_kptr_new. Similar to bpf_kptr_new, it implicitly
destructs the fields part of the local kptr automatically without user
intervention.

Just like the previous patch, btf_struct_meta that is needed to free up
the special fields is passed as a hidden argument to the kfunc.

For the user, a convenience macro hides over the kernel side kfunc which
is named bpf_kptr_drop_impl.

Continuing the previous example:

void prog(void) {
struct foo *f;

f = bpf_kptr_new(typeof(*f));
if (!f)
return;
bpf_kptr_drop(f);
}

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
kernel/bpf/helpers.c | 11 ++++
kernel/bpf/verifier.c | 66 +++++++++++++++----
.../testing/selftests/bpf/bpf_experimental.h | 13 ++++
3 files changed, 79 insertions(+), 11 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index c7c3c049e647..90e3000d1c3a 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1755,6 +1755,16 @@ void *bpf_kptr_new_impl(u64 local_type_id__k, u64 flags, void *meta__ign)
return p;
}

+void bpf_kptr_drop_impl(void *p__lkptr, void *meta__ign)
+{
+ struct btf_struct_meta *meta = meta__ign;
+ void *p = p__lkptr;
+
+ if (meta)
+ bpf_obj_free_fields(meta->fields_tab, p);
+ bpf_mem_free(&bpf_global_ma, p);
+}
+
__diag_pop();

BTF_SET8_START(generic_btf_ids)
@@ -1762,6 +1772,7 @@ BTF_SET8_START(generic_btf_ids)
BTF_ID_FLAGS(func, crash_kexec, KF_DESTRUCTIVE)
#endif
BTF_ID_FLAGS(func, bpf_kptr_new_impl, KF_ACQUIRE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_kptr_drop_impl, KF_RELEASE)
BTF_SET8_END(generic_btf_ids)

static const struct btf_kfunc_id_set generic_kfunc_set = {
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 21d71db16699..cda854715981 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7693,6 +7693,10 @@ struct bpf_kfunc_call_arg_meta {
u64 value;
bool found;
} arg_constant;
+ struct {
+ struct btf *btf;
+ u32 btf_id;
+ } arg_kptr_drop;
};

static bool is_kfunc_acquire(struct bpf_kfunc_call_arg_meta *meta)
@@ -7771,6 +7775,11 @@ static bool is_kfunc_arg_sfx_ignore(const struct btf *btf, const struct btf_para
return __kfunc_param_match_suffix(btf, arg, "__ign");
}

+static bool is_kfunc_arg_local_kptr(const struct btf *btf, const struct btf_param *arg)
+{
+ return __kfunc_param_match_suffix(btf, arg, "__lkptr");
+}
+
static bool is_kfunc_arg_ret_buf_size(const struct btf *btf,
const struct btf_param *arg,
const struct bpf_reg_state *reg,
@@ -7871,6 +7880,7 @@ static u32 *reg2btf_ids[__BPF_REG_TYPE_MAX] = {

enum kfunc_ptr_arg_type {
KF_ARG_PTR_TO_CTX,
+ KF_ARG_PTR_TO_LOCAL_BTF_ID, /* Local kptr */
KF_ARG_PTR_TO_BTF_ID, /* Also covers reg2btf_ids conversions */
KF_ARG_PTR_TO_KPTR_STRONG, /* PTR_TO_KPTR but type specific */
KF_ARG_PTR_TO_DYNPTR,
@@ -7878,6 +7888,20 @@ enum kfunc_ptr_arg_type {
KF_ARG_PTR_TO_MEM_SIZE, /* Size derived from next argument, skip it */
};

+enum special_kfunc_type {
+ KF_bpf_kptr_new_impl,
+ KF_bpf_kptr_drop_impl,
+};
+
+BTF_SET_START(special_kfunc_set)
+BTF_ID(func, bpf_kptr_new_impl)
+BTF_ID(func, bpf_kptr_drop_impl)
+BTF_SET_END(special_kfunc_set)
+
+BTF_ID_LIST(special_kfunc_list)
+BTF_ID(func, bpf_kptr_new_impl)
+BTF_ID(func, bpf_kptr_drop_impl)
+
enum kfunc_ptr_arg_type get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
struct bpf_kfunc_call_arg_meta *meta,
const struct btf_type *t,
@@ -7899,6 +7923,9 @@ enum kfunc_ptr_arg_type get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
if (btf_get_prog_ctx_type(&env->log, meta->btf, t, resolve_prog_type(env->prog), argno))
return KF_ARG_PTR_TO_CTX;

+ if (is_kfunc_arg_local_kptr(meta->btf, &args[argno]))
+ return KF_ARG_PTR_TO_LOCAL_BTF_ID;
+
if ((base_type(reg->type) == PTR_TO_BTF_ID || reg2btf_ids[base_type(reg->type)])) {
if (!btf_type_is_struct(ref_t)) {
verbose(env, "kernel function %s args#%d pointer type %s %s is not supported\n",
@@ -8117,6 +8144,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
return kf_arg_type;

switch (kf_arg_type) {
+ case KF_ARG_PTR_TO_LOCAL_BTF_ID:
case KF_ARG_PTR_TO_BTF_ID:
if (is_kfunc_trusted_args(meta) && !reg->ref_obj_id) {
verbose(env, "R%d must be referenced\n", regno);
@@ -8151,6 +8179,21 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
return -EINVAL;
}
break;
+ case KF_ARG_PTR_TO_LOCAL_BTF_ID:
+ if (reg->type != (PTR_TO_BTF_ID | MEM_TYPE_LOCAL)) {
+ verbose(env, "arg#%d expected point to local kptr\n", i);
+ return -EINVAL;
+ }
+ if (!reg->ref_obj_id) {
+ verbose(env, "local kptr must be referenced\n");
+ return -EINVAL;
+ }
+ if (meta->btf == btf_vmlinux &&
+ meta->func_id == special_kfunc_list[KF_bpf_kptr_drop_impl]) {
+ meta->arg_kptr_drop.btf = reg->btf;
+ meta->arg_kptr_drop.btf_id = reg->btf_id;
+ }
+ break;
case KF_ARG_PTR_TO_BTF_ID:
/* Only base_type is checked, further checks are done here */
if (reg->type != PTR_TO_BTF_ID &&
@@ -8221,17 +8264,6 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
return 0;
}

-enum special_kfunc_type {
- KF_bpf_kptr_new_impl,
-};
-
-BTF_SET_START(special_kfunc_set)
-BTF_ID(func, bpf_kptr_new_impl)
-BTF_SET_END(special_kfunc_set)
-
-BTF_ID_LIST(special_kfunc_list)
-BTF_ID(func, bpf_kptr_new_impl)
-
static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
int *insn_idx_p)
{
@@ -8358,6 +8390,10 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
env->insn_aux_data[insn_idx].kptr_new_size = ret_t->size;
env->insn_aux_data[insn_idx].kptr_struct_meta =
btf_find_struct_meta(ret_btf, ret_btf_id);
+ } else if (meta.func_id == special_kfunc_list[KF_bpf_kptr_drop_impl]) {
+ env->insn_aux_data[insn_idx].kptr_struct_meta =
+ btf_find_struct_meta(meta.arg_kptr_drop.btf,
+ meta.arg_kptr_drop.btf_id);
} else {
verbose(env, "kernel function %s unhandled dynamic return type\n",
meta.func_name);
@@ -14557,6 +14593,14 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
insn_buf[2] = addr[1];
insn_buf[3] = *insn;
*cnt = 4;
+ } else if (desc->func_id == special_kfunc_list[KF_bpf_kptr_drop_impl]) {
+ struct btf_struct_meta *kptr_struct_meta = env->insn_aux_data[insn_idx].kptr_struct_meta;
+ struct bpf_insn addr[2] = { BPF_LD_IMM64(BPF_REG_2, (long)kptr_struct_meta) };
+
+ insn_buf[0] = addr[0];
+ insn_buf[1] = addr[1];
+ insn_buf[2] = *insn;
+ *cnt = 3;
}
return 0;
}
diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
index 9c7d0badb02e..c47d16f3e817 100644
--- a/tools/testing/selftests/bpf/bpf_experimental.h
+++ b/tools/testing/selftests/bpf/bpf_experimental.h
@@ -39,4 +39,17 @@ extern void *bpf_kptr_new_impl(__u64 local_type_id, __u64 flags, void *meta__ign
/* Convenience macro to wrap over bpf_kptr_new_impl */
#define bpf_kptr_new(type) bpf_kptr_new_impl(bpf_core_type_id_local(type), 0, NULL)

+/* Description
+ * Free a local kptr. All fields of local kptr that require destruction
+ * will be destructed before the storage is freed.
+ *
+ * The 'meta__ign' parameter is a hidden argument that is ignored.
+ * Returns
+ * Void.
+ */
+extern void bpf_kptr_drop_impl(void *kptr, void *meta__ign) __ksym;
+
+/* Convenience macro to wrap over bpf_kptr_drop_impl */
+#define bpf_kptr_drop(kptr) bpf_kptr_drop_impl(kptr, NULL)
+
#endif
--
2.34.1