s390: add USER_STACKTRACE support

Use the perf_callchain_user() code as blueprint to also add support for
USER_STACKTRACE. To describe how to use this cite the commit message of the
LoongArch implementation which came with commit 4d7bf939 ("LoongArch:
Add USER_STACKTRACE support"), but replace -fno-omit-frame-pointer option
with the s390 specific -mbackchain option:
====================================================================== To
get the best stacktrace output, you can compile your userspace programs
with frame pointers (at least glibc + the app you are tracing).

1, export "CC = gcc -mbackchain";
2, compile your programs with "CC";
3, use uprobe to get stacktrace output.

...
     echo 'p:malloc /usr/lib64/libc.so.6:0x0a4704 size=%r2:u64' > uprobe_events
     echo 'p:free /usr/lib64/libc.so.6:0x0a4d50 ptr=%r2:u64' >> uprobe_events
     echo 'comm == "demo"' > ./events/uprobes/malloc/filter
     echo 'comm == "demo"' > ./events/uprobes/free/filter
     echo 1 > ./options/userstacktrace
     echo 1 > ./options/sym-userobj
...
======================================================================
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/Kconfig

@@ -236,6 +236,7 @@ config S390
 	select THREAD_INFO_IN_TASK
 	select TRACE_IRQFLAGS_SUPPORT
 	select TTY
+	select USER_STACKTRACE_SUPPORT
 	select VIRT_CPU_ACCOUNTING
 	select ZONE_DMA
 	# Note: keep the above list sorted alphabetically

arch/s390/kernel/stacktrace.c

@@ -6,9 +6,12 @@
  */
 
 #include <linux/stacktrace.h>
+#include <linux/uaccess.h>
+#include <linux/compat.h>
 #include <asm/stacktrace.h>
 #include <asm/unwind.h>
 #include <asm/kprobes.h>
+#include <asm/ptrace.h>
 
 void arch_stack_walk(stack_trace_consume_fn consume_entry, void *cookie,
 		     struct task_struct *task, struct pt_regs *regs)
@@ -58,3 +61,43 @@ int arch_stack_walk_reliable(stack_trace_consume_fn consume_entry,
 		return -EINVAL;
 	return 0;
 }
+
+void arch_stack_walk_user(stack_trace_consume_fn consume_entry, void *cookie,
+			  const struct pt_regs *regs)
+{
+	struct stack_frame_user __user *sf;
+	unsigned long ip, sp;
+	bool first = true;
+
+	if (is_compat_task())
+		return;
+	if (!consume_entry(cookie, instruction_pointer(regs)))
+		return;
+	sf = (void __user *)user_stack_pointer(regs);
+	pagefault_disable();
+	while (1) {
+		if (__get_user(sp, &sf->back_chain))
+			break;
+		if (__get_user(ip, &sf->gprs[8]))
+			break;
+		if (ip & 0x1) {
+			/*
+			 * If the instruction address is invalid, and this
+			 * is the first stack frame, assume r14 has not
+			 * been written to the stack yet. Otherwise exit.
+			 */
+			if (first && !(regs->gprs[14] & 0x1))
+				ip = regs->gprs[14];
+			else
+				break;
+		}
+		if (!consume_entry(cookie, ip))
+			break;
+		/* Sanity check: ABI requires SP to be aligned 8 bytes. */
+		if (!sp || sp & 0x7)
+			break;
+		sf = (void __user *)sp;
+		first = false;
+	}
+	pagefault_enable();
+}