csky: Add support for function error injection

Inspired by the commit 42d038c4fb00 ("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 <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>

arch/csky/Kconfig

···
 	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

···
 	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);