csky: Add support for function error injection

Inspired by the commit 42d038c4 ("arm64: Add support for function
error injection"), this patch supports function error injection for
csky.

This patch mainly support two functions: one is regs_set_return_value()
which is used to overwrite the return value; the another function is
override_function_with_return() which is to override the probed
function returning and jump to its caller.

Test log:

 cd /sys/kernel/debug/fail_function/
 echo sys_clone > inject
 echo 100 > probability
 echo 1 > interval
 ls /
[  108.644163] FAULT_INJECTION: forcing a failure.
[  108.644163] name fail_function, interval 1, probability 100, space 0, times 1
[  108.647799] CPU: 0 PID: 104 Comm: sh Not tainted 5.8.0-rc5+ #46
[  108.648384] Call Trace:
[  108.649339] [<8005eed4>] walk_stackframe+0x0/0xf0
[  108.649679] [<8005f16a>] show_stack+0x32/0x5c
[  108.649927] [<8040f9d2>] dump_stack+0x6e/0x9c
[  108.650271] [<80406f7e>] should_fail+0x15e/0x1ac
[  108.650720] [<80118ba8>] fei_kprobe_handler+0x28/0x5c
[  108.651519] [<80754110>] kprobe_breakpoint_handler+0x144/0x1cc
[  108.652289] [<8005d6da>] trap_c+0x8e/0x110
[  108.652816] [<8005ce8c>] csky_trap+0x5c/0x70
-sh: can't fork: Invalid argument
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Arnd Bergmann <arnd@arndb.de>

arch/csky/Kconfig

@@ -45,6 +45,7 @@ config CSKY
 	select HAVE_DYNAMIC_FTRACE_WITH_REGS
 	select HAVE_FUNCTION_TRACER
 	select HAVE_FUNCTION_GRAPH_TRACER
+	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_FTRACE_MCOUNT_RECORD
 	select HAVE_KERNEL_GZIP
 	select HAVE_KERNEL_LZO

arch/csky/include/asm/ptrace.h

@@ -52,6 +52,12 @@ static inline unsigned long regs_return_value(struct pt_regs *regs)
 	return regs->a0;
 }
 
+static inline void regs_set_return_value(struct pt_regs *regs,
+					 unsigned long val)
+{
+	regs->a0 = val;
+}
+
 /* Valid only for Kernel mode traps. */
 static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
 {

arch/csky/lib/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
 lib-y  := usercopy.o delay.o
+obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o

arch/csky/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->lr);
+}
+NOKPROBE_SYMBOL(override_function_with_return);