bpf: mark a NULL kfunc argument precisegrafted

check_kfunc_arg() allows bpf_register_is_null() for nullable arguments
w/o marking the underlying scalar register precise. Hence a checkpoint
created on such a path would prune against arbitrary scalar value.

Fixes: 3bda08b63670 ("bpf: Allow NULL buffers in bpf_dynptr_slice(_rw)")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260904-register-is-null-precise-fixes-v1-5-0f5a360ff15d@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 28 insertions, 0 deletions

diff --git a/rust/helpers/refcount.c b/rust/helpers/refcount.c
new file mode 100644
index 000000000..36334a674
--- /dev/null
+++ b/rust/helpers/refcount.c
@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/refcount.h>
+
+__rust_helper refcount_t rust_helper_REFCOUNT_INIT(int n)
+{
+	return (refcount_t)REFCOUNT_INIT(n);
+}
+
+__rust_helper void rust_helper_refcount_set(refcount_t *r, int n)
+{
+	refcount_set(r, n);
+}
+
+__rust_helper void rust_helper_refcount_inc(refcount_t *r)
+{
+	refcount_inc(r);
+}
+
+__rust_helper void rust_helper_refcount_dec(refcount_t *r)
+{
+	refcount_dec(r);
+}
+
+__rust_helper bool rust_helper_refcount_dec_and_test(refcount_t *r)
+{
+	return refcount_dec_and_test(r);
+}