LoongArch: ptrace: Add function argument access API

Add regs_get_argument() which returns N th argument of the function
call, This enables ftrace kprobe events to access kernel function
arguments via $argN syntax for later use.

E.g.:
echo 'p bio_add_page arg1=$arg1' > kprobe_events
bash: echo: write error: Invalid argument
Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

arch/loongarch/Kconfig

@@ -98,6 +98,7 @@ config LOONGARCH
 	select HAVE_EXIT_THREAD
 	select HAVE_FAST_GUP
 	select HAVE_FTRACE_MCOUNT_RECORD
+	select HAVE_FUNCTION_ARG_ACCESS_API
 	select HAVE_FUNCTION_GRAPH_TRACER
 	select HAVE_FUNCTION_TRACER
 	select HAVE_GENERIC_VDSO

arch/loongarch/include/asm/ptrace.h

@@ -109,6 +109,40 @@ static inline unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs, unsi
 
 struct task_struct;
 
+/**
+ * regs_get_kernel_argument() - get Nth function argument in kernel
+ * @regs:       pt_regs of that context
+ * @n:          function argument number (start from 0)
+ *
+ * regs_get_argument() returns @n th argument of the function call.
+ * Note that this chooses most probably assignment, in some case
+ * it can be incorrect.
+ * This is expected to be called from kprobes or ftrace with regs
+ * where the top of stack is the return address.
+ */
+static inline unsigned long regs_get_kernel_argument(struct pt_regs *regs,
+						     unsigned int n)
+{
+#define NR_REG_ARGUMENTS 8
+	static const unsigned int args[] = {
+		offsetof(struct pt_regs, regs[4]),
+		offsetof(struct pt_regs, regs[5]),
+		offsetof(struct pt_regs, regs[6]),
+		offsetof(struct pt_regs, regs[7]),
+		offsetof(struct pt_regs, regs[8]),
+		offsetof(struct pt_regs, regs[9]),
+		offsetof(struct pt_regs, regs[10]),
+		offsetof(struct pt_regs, regs[11]),
+	};
+
+	if (n < NR_REG_ARGUMENTS)
+		return regs_get_register(regs, args[n]);
+	else {
+		n -= NR_REG_ARGUMENTS;
+		return regs_get_kernel_stack_nth(regs, n);
+	}
+}
+
 /*
  * Does the process account for user or for system time?
  */