LoongArch: Add support for function error injection

Inspired by the commit 42d038c4 ("arm64: Add support for function
error injection") and the commit ee55ff80 ("riscv: Add support for
function error injection"), this patch supports function error injection
for LoongArch.

Mainly implement two functions:
(1) regs_set_return_value() which is used to overwrite the return value,
(2) override_function_with_return() which is used to override the probed
function returning and jump to its caller.

Here is a simple test under CONFIG_FUNCTION_ERROR_INJECTION and
CONFIG_FAIL_FUNCTION:

  # echo sys_clone > /sys/kernel/debug/fail_function/inject
  # echo 100 > /sys/kernel/debug/fail_function/probability
  # dmesg
  bash: fork: Invalid argument
  # dmesg
  ...
  FAULT_INJECTION: forcing a failure.
  name fail_function, interval 1, probability 100, space 0, times 1
  ...
  Call Trace:
  [<90000000002238f4>] show_stack+0x5c/0x180
  [<90000000012e384c>] dump_stack_lvl+0x60/0x88
  [<9000000000b1879c>] should_fail_ex+0x1b0/0x1f4
  [<900000000032ead4>] fei_kprobe_handler+0x28/0x6c
  [<9000000000230970>] kprobe_breakpoint_handler+0xf0/0x118
  [<90000000012e3e60>] do_bp+0x2c4/0x358
  [<9000000002241924>] exception_handlers+0x1924/0x10000
  [<900000000023b7d0>] sys_clone+0x0/0x4
  [<90000000012e4744>] do_syscall+0x7c/0x94
  [<9000000000221e44>] handle_syscall+0xc4/0x160
Tested-by: Hengqi Chen <hengqi.chen@gmail.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Kconfig

@@ -100,6 +100,7 @@ config LOONGARCH
 	select HAVE_FAST_GUP
 	select HAVE_FTRACE_MCOUNT_RECORD
 	select HAVE_FUNCTION_ARG_ACCESS_API
+	select HAVE_FUNCTION_ERROR_INJECTION
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER
 	select HAVE_GENERIC_VDSO

arch/loongarch/include/asm/ptrace.h

@@ -154,6 +154,11 @@ static inline long regs_return_value(struct pt_regs *regs)
 	return regs->regs[4];
 }
 
+static inline void regs_set_return_value(struct pt_regs *regs, unsigned long val)
+{
+	regs->regs[4] = val;
+}
+
 #define instruction_pointer(regs) ((regs)->csr_era)
 #define profile_pc(regs) instruction_pointer(regs)
 

arch/loongarch/lib/Makefile

@@ -5,3 +5,5 @@
 
 lib-y	+= delay.o memset.o memcpy.o memmove.o \
 	   clear_user.o copy_user.o csum.o dump_tlb.o unaligned.o
+
+obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o

arch/loongarch/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->regs[1]);
+}
+NOKPROBE_SYMBOL(override_function_with_return);