Consider a pointer to a type that would normally need acquire / release
semantics to be safely held. There may be scenarios where such a pointer
can be safely held without the need to acquire a reference.
For example, although a PTR_TO_BTF_ID for a rbtree_map node is released
via bpf_rbtree_add helper, the helper doesn't change the address of the
node and must be called with the rbtree_map's spinlock held. Since the
only way to remove a node from the rbtree - bpf_rbtree_remove helper -
requires the same lock, the newly-added node cannot be removed by a
concurrently-running program until the lock is released. Therefore it is
safe to hold a reference to this node until bpf_rbtree_unlock is called.
This patch introduces a new type flag and associated verifier logic to
handle such "non-owning" references.
Currently the only usecase I have is the rbtree example above, so the
verifier logic is straightforward:
* Tag return types of bpf_rbtree_{add,find} with OBJ_NON_OWNING_REF
* These both require the rbtree lock to be held to return anything
non-NULL
* Since ret type for both is PTR_TO_BTF_ID_OR_NULL, if lock is not
held and NULL is returned, existing mark_ptr_or_null_reg logic
will clear reg type.
* So if mark_ptr_or_null_reg logic turns the returned reg into a
PTR_TO_BTF_ID | OBJ_NON_OWNING_REF, verifier knows lock is held.
* When the lock is released the verifier invalidates any regs holding
non owning refs similarly to existing release_reference logic - but no
need to clear ref_obj_id as an 'owning' reference was never acquired.
[ TODO: Currently the invalidation logic in
clear_rbtree_node_non_owning_refs is not parametrized by map so
unlocking any rbtree lock will invalidate all non-owning refs ]
Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx>
---
include/linux/bpf.h | 1 +
kernel/bpf/rbtree.c | 4 +--
kernel/bpf/verifier.c | 65 +++++++++++++++++++++++++++++++++++++++----
3 files changed, 62 insertions(+), 8 deletions(-)
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index b762c6b3dcfb..f164bd6e2f3a 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -415,6 +415,7 @@ enum bpf_type_flag {
/* Size is known at compile time. */
MEM_FIXED_SIZE = BIT(10 + BPF_BASE_TYPE_BITS),
+ OBJ_NON_OWNING_REF = BIT(11 + BPF_BASE_TYPE_BITS),
__BPF_TYPE_FLAG_MAX,
__BPF_TYPE_LAST_FLAG = __BPF_TYPE_FLAG_MAX - 1,
};
diff --git a/kernel/bpf/rbtree.c b/kernel/bpf/rbtree.c
index b5d158254de6..cc89639df8a2 100644
--- a/kernel/bpf/rbtree.c
+++ b/kernel/bpf/rbtree.c
@@ -144,7 +144,7 @@ BPF_CALL_3(bpf_rbtree_add, struct bpf_map *, map, void *, value, void *, cb)
const struct bpf_func_proto bpf_rbtree_add_proto = {
.func = bpf_rbtree_add,
.gpl_only = true,
- .ret_type = RET_PTR_TO_BTF_ID_OR_NULL,
+ .ret_type = RET_PTR_TO_BTF_ID_OR_NULL | OBJ_NON_OWNING_REF,
.ret_btf_id = BPF_PTR_POISON,
.arg1_type = ARG_CONST_MAP_PTR,
.arg2_type = ARG_PTR_TO_BTF_ID | OBJ_RELEASE,
@@ -167,7 +167,7 @@ BPF_CALL_3(bpf_rbtree_find, struct bpf_map *, map, void *, key, void *, cb)
const struct bpf_func_proto bpf_rbtree_find_proto = {
.func = bpf_rbtree_find,
.gpl_only = true,
- .ret_type = RET_PTR_TO_BTF_ID_OR_NULL,
+ .ret_type = RET_PTR_TO_BTF_ID_OR_NULL | OBJ_NON_OWNING_REF,
.ret_btf_id = BPF_PTR_POISON,
.arg1_type = ARG_CONST_MAP_PTR,
.arg2_type = ARG_ANYTHING,
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3c9af1047d80..26aa228fa860 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -469,6 +469,11 @@ static bool type_is_rdonly_mem(u32 type)
return type & MEM_RDONLY;
}
+static bool type_is_non_owning_ref(u32 type)
+{
+ return type & OBJ_NON_OWNING_REF;
+}
+
static bool type_may_be_null(u32 type)
{
return type & PTR_MAYBE_NULL;
@@ -595,7 +600,9 @@ static bool function_returns_rbtree_node(enum bpf_func_id func_id)
static const char *reg_type_str(struct bpf_verifier_env *env,
enum bpf_reg_type type)
{
- char postfix[16] = {0}, prefix[32] = {0};
+ char postfix[32] = {0}, prefix[32] = {0};
+ unsigned int postfix_idx = 0;
+
static const char * const str[] = {
[NOT_INIT] = "?",
[SCALAR_VALUE] = "scalar",
@@ -620,11 +627,18 @@ static const char *reg_type_str(struct bpf_verifier_env *env,
[PTR_TO_SPIN_LOCK] = "spin_lock",
};
- if (type & PTR_MAYBE_NULL) {
+ if (type_may_be_null(type)) {
if (base_type(type) == PTR_TO_BTF_ID)
- strncpy(postfix, "or_null_", 16);
+ postfix_idx += strlcpy(postfix + postfix_idx, "or_null_", 32 - postfix_idx);
else
- strncpy(postfix, "_or_null", 16);
+ postfix_idx += strlcpy(postfix + postfix_idx, "_or_null", 32 - postfix_idx);
+ }
+
+ if (type_is_non_owning_ref(type)) {
+ if (base_type(type) == PTR_TO_BTF_ID)
+ postfix_idx += strlcpy(postfix + postfix_idx, "non_own_", 32 - postfix_idx);
+ else
+ postfix_idx += strlcpy(postfix + postfix_idx, "_non_own", 32 - postfix_idx);
}
if (type & MEM_RDONLY)
@@ -5758,7 +5772,14 @@ static const struct bpf_reg_types int_ptr_types = {
static const struct bpf_reg_types spin_lock_types = {
.types = {
PTR_TO_MAP_VALUE,
- PTR_TO_SPIN_LOCK
+ PTR_TO_SPIN_LOCK,
+ },
+};
+
+static const struct bpf_reg_types btf_ptr_types = {
+ .types = {
+ PTR_TO_BTF_ID,
+ PTR_TO_BTF_ID | OBJ_NON_OWNING_REF,
},
};
@@ -5767,7 +5788,6 @@ static const struct bpf_reg_types scalar_types = { .types = { SCALAR_VALUE } };
static const struct bpf_reg_types context_types = { .types = { PTR_TO_CTX } };
static const struct bpf_reg_types alloc_mem_types = { .types = { PTR_TO_MEM | MEM_ALLOC } };
static const struct bpf_reg_types const_map_ptr_types = { .types = { CONST_PTR_TO_MAP } };
-static const struct bpf_reg_types spin_lock_types = { .types = { PTR_TO_MAP_VALUE } };
static const struct bpf_reg_types percpu_btf_ptr_types = { .types = { PTR_TO_BTF_ID | MEM_PERCPU } };
static const struct bpf_reg_types func_ptr_types = { .types = { PTR_TO_FUNC } };
static const struct bpf_reg_types stack_ptr_types = { .types = { PTR_TO_STACK } };
@@ -6723,6 +6743,33 @@ static int release_reference(struct bpf_verifier_env *env,
return 0;
}
+static void clear_non_owning_ref_regs(struct bpf_verifier_env *env,
+ struct bpf_func_state *state)
+{
+ struct bpf_reg_state *regs = state->regs, *reg;
+ int i;
+
+ for (i = 0; i < MAX_BPF_REG; i++)
+ if (type_is_non_owning_ref(regs[i].type))
+ mark_reg_unknown(env, regs, i);
+
+ bpf_for_each_spilled_reg(i, state, reg) {
+ if (!reg)
+ continue;
+ if (type_is_non_owning_ref(reg->type))
+ __mark_reg_unknown(env, reg);
+ }
+}
+
+static void clear_rbtree_node_non_owning_refs(struct bpf_verifier_env *env)
+{
+ struct bpf_verifier_state *vstate = env->cur_state;
+ int i;
+
+ for (i = 0; i <= vstate->curframe; i++)
+ clear_non_owning_ref_regs(env, vstate->frame[i]);
+}
+
static void clear_caller_saved_regs(struct bpf_verifier_env *env,
struct bpf_reg_state *regs)
{
@@ -7584,6 +7631,12 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
return -EFAULT;
}
break;
+ case BPF_FUNC_rbtree_unlock:
+ /* TODO clear_rbtree_node_non_owning_refs calls should be
+ * parametrized by base_type or ideally by owning map
+ */
+ clear_rbtree_node_non_owning_refs(env);
+ break;
}
if (err)
--
2.30.2
