arch: introduce exc_handle.h

Defines some common macros and data structures for declaring
text section ranges with fixup handler addresses if an
exception occurs.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

include/exc_handle.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (c) 2018 Intel Corporation.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef _ZEPHYR_EXC_HANDLE_H_
+#define _ZEPHYR_EXC_HANDLE_H_
+
+/*
+ * This is used by some architectures to define code ranges which may
+ * perform operations that could generate a CPU exception that should not
+ * be fatal. Instead, the exception should return but set the program
+ * counter to a 'fixup' memory address which will gracefully error out.
+ *
+ * For example, in the case where user mode passes in a C string via
+ * system call, the length of that string needs to be measured. A specially
+ * written assembly language version of strlen (z_arch_user_string_len)
+ * defines start and end symbols where the memory in the string is examined;
+ * if this generates a fault, jumping to the fixup symbol within the same
+ * function will return an error result to the caller.
+ *
+ * To ensure precise control of the state of registers and the stack pointer,
+ * these functions need to be written in assembly.
+ *
+ * The arch-specific fault handling code will define an array of these
+ * z_exc_handle structures and return from the exception with the PC updated
+ * to the fixup address if a match is found.
+ */
+
+struct z_exc_handle {
+	void *start;
+	void *end;
+	void *fixup;
+};
+
+#define Z_EXC_HANDLE(name) \
+	{ &name ## _fault_start, &name ## _fault_end, &name ## _fixup }
+
+#define Z_EXC_DECLARE(name) \
+	extern void (*name ## _fault_start)(void); \
+	extern void (*name ## _fault_end)(void); \
+	extern void (*name ## _fixup)(void)
+
+#endif /* _ZEPHYR_EXC_HANDLE_H_ */