s390/ptrace: add function argument access API

Add regs_get_kernel_argument() which returns Nth argument of a
function call.

This enables ftrace kprobe events to access kernel function arguments
via $argN syntax.

This is the s390 variant of commit a823c35f ("arm64: ptrace: Add
function argument access API").
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/Kconfig

@@ -160,6 +160,7 @@ config S390
 	select HAVE_FAST_GUP
 	select HAVE_FENTRY
 	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

arch/s390/include/asm/ptrace.h

@@ -196,6 +196,25 @@ const char *regs_query_register_name(unsigned int offset);
 unsigned long regs_get_register(struct pt_regs *regs, unsigned int offset);
 unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs, unsigned int n);
 
+/**
+ * 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_kernel_argument() returns @n th argument of the function call.
+ */
+static inline unsigned long regs_get_kernel_argument(struct pt_regs *regs,
+						     unsigned int n)
+{
+	unsigned int argoffset = STACK_FRAME_OVERHEAD / sizeof(long);
+
+#define NR_REG_ARGUMENTS 5
+	if (n < NR_REG_ARGUMENTS)
+		return regs_get_register(regs, 2 + n);
+	n -= NR_REG_ARGUMENTS;
+	return regs_get_kernel_stack_nth(regs, argoffset + n);
+}
+
 static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
 {
 	return regs->gprs[15];