[PATCH bpf-next v3 6/7] bpf: Let bpf_iter_task_new accept null task ptr

[Chuyi Zhou]

When using task_iter to iterate all threads of a specific task, we enforce
that the user must pass a valid task pointer to ensure safety. However,
when iterating all threads/process in the system, BPF verifier still
require a valid ptr instead of "nullable" pointer, even though it's
pointless, which is a kind of surprising from usability standpoint. It
would be nice if we could let that kfunc accept a explicit null pointer
when we are using BPF_TASK_ITER_ALL/BPF_TASK_ITER_PROC and a valid pointer
when using BPF_TASK_ITER_THREAD.

Given a trival kfunc:
	__bpf_kfunc void FN(struct TYPE_A *obj)

BPF Prog would reject a nullptr for obj. The error info is:
"arg#x pointer type xx xx must point to scalar, or struct with scalar"
reported by get_kfunc_ptr_arg_type(). The reg->type is SCALAR_VALUE and
the btf type of ref_t is not scalar or scalar_struct which leads to the
rejection of get_kfunc_ptr_arg_type.

This patch reuse the __opt annotation which was used to indicate that
the buffer associated with an __sz or __szk argument may be null:
	__bpf_kfunc void FN(struct TYPE_A *obj__opt)
Here __opt indicates obj can be optional, user can pass a explicit nullptr
or a normal TYPE_A pointer. In get_kfunc_ptr_arg_type(), we will detect
whether the current arg is optional and register is null, If so, return
a new kfunc_ptr_arg_type KF_ARG_PTR_TO_NULL and skip to the next arg in
check_kfunc_args().

Signed-off-by: Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx>
---
 kernel/bpf/task_iter.c |  7 +++++--
 kernel/bpf/verifier.c  | 13 ++++++++++++-
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index 9bcd3f9922b1..7ac007f161cc 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -867,7 +867,7 @@ struct bpf_iter_task_kern {
 	unsigned int type;
 } __attribute__((aligned(8)));
 
-__bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *task, unsigned int type)
+__bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *task__opt, unsigned int type)
 {
 	struct bpf_iter_task_kern *kit = (void *)it;
 	BUILD_BUG_ON(sizeof(struct bpf_iter_task_kern) != sizeof(struct bpf_iter_task));
@@ -877,14 +877,17 @@ __bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it, struct task_struct *
 	switch (type) {
 	case BPF_TASK_ITER_ALL:
 	case BPF_TASK_ITER_PROC:
+		break;
 	case BPF_TASK_ITER_THREAD:
+		if (!task__opt)
+			return -EINVAL;
 		break;
 	default:
 		return -EINVAL;
 	}
 
 	if (type == BPF_TASK_ITER_THREAD)
-		kit->task = task;
+		kit->task = task__opt;
 	else
 		kit->task = &init_task;
 	kit->pos = kit->task;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a065e18a0b3a..a79204c75a90 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10331,6 +10331,7 @@ enum kfunc_ptr_arg_type {
 	KF_ARG_PTR_TO_CALLBACK,
 	KF_ARG_PTR_TO_RB_ROOT,
 	KF_ARG_PTR_TO_RB_NODE,
+	KF_ARG_PTR_TO_NULL,
 };
 
 enum special_kfunc_type {
@@ -10425,6 +10426,12 @@ static bool is_kfunc_bpf_rcu_read_unlock(struct bpf_kfunc_call_arg_meta *meta)
 	return meta->func_id == special_kfunc_list[KF_bpf_rcu_read_unlock];
 }
 
+static inline bool is_kfunc_arg_optional_null(struct bpf_reg_state *reg,
+				const struct btf *btf, const struct btf_param *arg)
+{
+	return register_is_null(reg) && is_kfunc_arg_optional(btf, arg);
+}
+
 static enum kfunc_ptr_arg_type
 get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
 		       struct bpf_kfunc_call_arg_meta *meta,
@@ -10497,6 +10504,8 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
 	 */
 	if (!btf_type_is_scalar(ref_t) && !__btf_type_is_scalar_struct(env, meta->btf, ref_t, 0) &&
 	    (arg_mem_size ? !btf_type_is_void(ref_t) : 1)) {
+			if (is_kfunc_arg_optional_null(reg, meta->btf, &args[argno]))
+				return KF_ARG_PTR_TO_NULL;
 		verbose(env, "arg#%d pointer type %s %s must point to %sscalar, or struct with scalar\n",
 			argno, btf_type_str(ref_t), ref_tname, arg_mem_size ? "void, " : "");
 		return -EINVAL;
@@ -11028,7 +11037,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 		}
 
 		if ((is_kfunc_trusted_args(meta) || is_kfunc_rcu(meta)) &&
-		    (register_is_null(reg) || type_may_be_null(reg->type))) {
+		    (register_is_null(reg) || type_may_be_null(reg->type)) && !is_kfunc_arg_optional(meta->btf, &args[i])) {
 			verbose(env, "Possibly NULL pointer passed to trusted arg%d\n", i);
 			return -EACCES;
 		}
@@ -11053,6 +11062,8 @@ 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_NULL:
+			continue;
 		case KF_ARG_PTR_TO_ALLOC_BTF_ID:
 		case KF_ARG_PTR_TO_BTF_ID:
 			if (!is_kfunc_trusted_args(meta) && !is_kfunc_rcu(meta))
-- 
2.20.1