[PATCH bpf-next v4 6/8] bpf: Let bpf_iter_task_new accept null task ptr

[Chuyi Zhou <zhouchuyi@xxxxxxxxxxxxx>]

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_{PROCS, THREADS} 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 add "__nullable" annotation:
	__bpf_kfunc void FN(struct TYPE_A *obj__nullable);
Here __nullable 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 c2e1c3cbffea..773be9a221f5 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -873,7 +873,7 @@ enum {
 };
 
 __bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it,
-		struct task_struct *task, unsigned int flags)
+		struct task_struct *task__nullable, unsigned int flags)
 {
 	struct bpf_iter_task_kern *kit = (void *)it;
 
@@ -885,14 +885,17 @@ __bpf_kfunc int bpf_iter_task_new(struct bpf_iter_task *it,
 	switch (flags) {
 	case BPF_TASK_ITER_ALL_THREADS:
 	case BPF_TASK_ITER_ALL_PROCS:
+		break;
 	case BPF_TASK_ITER_PROC_THREADS:
+		if (!task__nullable)
+			return -EINVAL;
 		break;
 	default:
 		return -EINVAL;
 	}
 
 	if (flags == BPF_TASK_ITER_PROC_THREADS)
-		kit->task = task;
+		kit->task = task__nullable;
 	else
 		kit->task = &init_task;
 	kit->pos = kit->task;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3a60cc87520e..d09697dbfd9c 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10310,6 +10310,11 @@ static bool is_kfunc_arg_refcounted_kptr(const struct btf *btf, const struct btf
 	return __kfunc_param_match_suffix(btf, arg, "__refcounted_kptr");
 }
 
+static bool is_kfunc_arg_nullable(const struct btf *btf, const struct btf_param *arg)
+{
+	return __kfunc_param_match_suffix(btf, arg, "__nullable");
+}
+
 static bool is_kfunc_arg_scalar_with_name(const struct btf *btf,
 					  const struct btf_param *arg,
 					  const char *name)
@@ -10452,6 +10457,7 @@ 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 {
@@ -10608,6 +10614,8 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
 	if (is_kfunc_arg_callback(env, meta->btf, &args[argno]))
 		return KF_ARG_PTR_TO_CALLBACK;
 
+	if (is_kfunc_arg_nullable(meta->btf, &args[argno]) && register_is_null(reg))
+		return KF_ARG_PTR_TO_NULL;
 
 	if (argno + 1 < nargs &&
 	    (is_kfunc_arg_mem_size(meta->btf, &args[argno + 1], &regs[regno + 1]) ||
@@ -11158,7 +11166,8 @@ 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_nullable(meta->btf, &args[i])) {
 			verbose(env, "Possibly NULL pointer passed to trusted arg%d\n", i);
 			return -EACCES;
 		}
@@ -11183,6 +11192,8 @@ 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