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, 63 insertions, 0 deletions

diff --git a/io_uring/refs.h b/io_uring/refs.h
new file mode 100644
index 000000000..0fe16b67c
--- /dev/null
+++ b/io_uring/refs.h
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef IOU_REQ_REF_H
+#define IOU_REQ_REF_H
+
+#include <linux/atomic.h>
+#include <linux/io_uring_types.h>
+
+/*
+ * Shamelessly stolen from the mm implementation of page reference checking,
+ * see commit f958d7b528b1 for details.
+ */
+#define req_ref_zero_or_close_to_overflow(req)	\
+	((unsigned int) atomic_read(&(req->refs)) + 127u <= 127u)
+
+static inline bool req_ref_inc_not_zero(struct io_kiocb *req)
+{
+	WARN_ON_ONCE(!(req->flags & REQ_F_REFCOUNT));
+	return atomic_inc_not_zero(&req->refs);
+}
+
+static inline bool req_ref_put_and_test_atomic(struct io_kiocb *req)
+{
+	WARN_ON_ONCE(!(data_race(req->flags) & REQ_F_REFCOUNT));
+	WARN_ON_ONCE(req_ref_zero_or_close_to_overflow(req));
+	return atomic_dec_and_test(&req->refs);
+}
+
+static inline bool req_ref_put_and_test(struct io_kiocb *req)
+{
+	if (likely(!(req->flags & REQ_F_REFCOUNT)))
+		return true;
+
+	WARN_ON_ONCE(req_ref_zero_or_close_to_overflow(req));
+	return atomic_dec_and_test(&req->refs);
+}
+
+static inline void req_ref_get(struct io_kiocb *req)
+{
+	WARN_ON_ONCE(!(req->flags & REQ_F_REFCOUNT));
+	WARN_ON_ONCE(req_ref_zero_or_close_to_overflow(req));
+	atomic_inc(&req->refs);
+}
+
+static inline void req_ref_put(struct io_kiocb *req)
+{
+	WARN_ON_ONCE(!(req->flags & REQ_F_REFCOUNT));
+	WARN_ON_ONCE(req_ref_zero_or_close_to_overflow(req));
+	atomic_dec(&req->refs);
+}
+
+static inline void __io_req_set_refcount(struct io_kiocb *req, int nr)
+{
+	if (!(req->flags & REQ_F_REFCOUNT)) {
+		req->flags |= REQ_F_REFCOUNT;
+		atomic_set(&req->refs, nr);
+	}
+}
+
+static inline void io_req_set_refcount(struct io_kiocb *req)
+{
+	__io_req_set_refcount(req, 1);
+}
+#endif