[PATCHSET v1 RFC liburing 2/6] Add kernel error header `src/kernel_err.h`

[Ammar Faizi <ammar.faizi@students.amikom.ac.id>]

We take the `include/linux/err.h` file from Linux kernel with a bit
modification.

The purpose of this file is to use `PTR_ERR()`, `ERR_PTR()`, and
similar stuff to implement the kernel style return value which is
discussed at [1].

The small modification summary:
  1) Add `__must_check` attribute macro.
  2) `#include <liburing.h>` to take the `uring_likely` and
     `uring_unlikely` macros.

This file is licensed under the GPL-2.0.

Link: https://github.com/axboe/liburing/issues/443 [1]
Signed-off-by: Ammar Faizi <ammar.faizi@students.amikom.ac.id>
---
 src/kernel_err.h | 75 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)
 create mode 100644 src/kernel_err.h

diff --git a/src/kernel_err.h b/src/kernel_err.h
new file mode 100644
index 0000000..b9ea5fe
--- /dev/null
+++ b/src/kernel_err.h
@@ -0,0 +1,75 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_ERR_H
+#define _LINUX_ERR_H
+
+#include <linux/types.h>
+
+#include <asm/errno.h>
+
+#include <stdbool.h>
+#include <liburing.h>
+
+/*
+ * Kernel pointers have redundant information, so we can use a
+ * scheme where we can return either an error code or a normal
+ * pointer with the same return value.
+ *
+ * This should be a per-architecture thing, to allow different
+ * error and pointer decisions.
+ */
+#define MAX_ERRNO	4095
+
+#ifndef __ASSEMBLY__
+
+#define IS_ERR_VALUE(x) uring_unlikely((unsigned long)(void *)(x) >= (unsigned long)-MAX_ERRNO)
+
+/*
+ *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-warn_005funused_005fresult-function-attribute
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#nodiscard-warn-unused-result
+ */
+#define __must_check __attribute__((__warn_unused_result__))
+
+static inline void * __must_check ERR_PTR(long error)
+{
+	return (void *) error;
+}
+
+static inline long __must_check PTR_ERR(const void *ptr)
+{
+	return (long) ptr;
+}
+
+static inline bool __must_check IS_ERR(const void *ptr)
+{
+	return IS_ERR_VALUE((unsigned long)ptr);
+}
+
+static inline bool __must_check IS_ERR_OR_NULL(const void *ptr)
+{
+	return uring_unlikely(!ptr) || IS_ERR_VALUE((unsigned long)ptr);
+}
+
+/**
+ * ERR_CAST - Explicitly cast an error-valued pointer to another pointer type
+ * @ptr: The pointer to cast.
+ *
+ * Explicitly cast an error-valued pointer to another pointer type in such a
+ * way as to make it clear that's what's going on.
+ */
+static inline void * __must_check ERR_CAST(const void *ptr)
+{
+	/* cast away the const */
+	return (void *) ptr;
+}
+
+static inline int __must_check PTR_ERR_OR_ZERO(const void *ptr)
+{
+	if (IS_ERR(ptr))
+		return PTR_ERR(ptr);
+	else
+		return 0;
+}
+
+#endif /* #ifndef __ASSEMBLY__ */
+
+#endif /* #ifndef _LINUX_ERR_H */
-- 
2.30.2