ARM: 9279/1: support function error injection

This enables HAVE_FUNCTION_ERROR_INJECTION by adding necessary
regs_set_return_value() and override_function_with_return().

Simply tested according to Documentation/fault-injection/fault-injection.rst.

Signed-off-by: Kefeng Wang <[e-mail]>
Signed-off-by: Russell King (Oracle) <[e-mail]>

arch/arm/Kconfig

···
 	select HAVE_EXIT_THREAD
 	select HAVE_FAST_GUP if ARM_LPAE
 	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
+	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER if !XIP_KERNEL
 	select HAVE_GCC_PLUGINS

arch/arm/include/asm/ptrace.h

···
 		((current_stack_pointer | (THREAD_SIZE - 1)) - 7) - 1;	\
 })
 
+static inline void regs_set_return_value(struct pt_regs *regs, unsigned long rc)
+{
+	regs->ARM_r0 = rc;
+}
 
 /*
  * Update ITSTATE after normal execution of an IT block instruction.

arch/arm/lib/Makefile

···
   CFLAGS_xor-neon.o		+= $(NEON_FLAGS)
   obj-$(CONFIG_XOR_BLOCKS)	+= xor-neon.o
 endif
+
+obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o

arch/arm/lib/error-inject.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/error-injection.h>
+#include <linux/kprobes.h>
+
+void override_function_with_return(struct pt_regs *regs)
+{
+	instruction_pointer_set(regs, regs->ARM_lr);
+}
+NOKPROBE_SYMBOL(override_function_with_return);