Subject: [PATCH 2/2] ppc64le save_stack_trace_tsk_reliable (Was: HAVE_RELIABLE_STACKTRACE)

From: Torsten Duwe <[email-masked]>
Date: Tue, 27 Feb 2018 17:09:24 +0100

On Tue, Dec 12, 2017 at 01:12:37PM +0100, Miroslav Benes wrote:
> 
> I think that this is not enough. You need to also implement 
> save_stack_trace_tsk_reliable() for powerpc defined as __weak in 
> kernel/stacktrace.c.

So here is my initial proposal. I'd really like to get the successful
exit stricter, i.e. hit the initial stack value exactly instead of just
a window. Also, the check for kernel code looks clumsy IMHO. IOW:
Comments more than welcome!

Most of it is Copy&Waste, nonetheless:

Signed-off-by: Torsten Duwe <[email-masked]>

diff --git a/arch/powerpc/kernel/stacktrace.c b/arch/powerpc/kernel/stacktrace.c
index d534ed901538..e08af49e71d0 100644
--- a/arch/powerpc/kernel/stacktrace.c
+++ b/arch/powerpc/kernel/stacktrace.c
@@ -13,6 +13,7 @@
 #include <linux/export.h>
 #include <linux/sched.h>
 #include <linux/sched/debug.h>
+#include <linux/sched/task_stack.h>
 #include <linux/stacktrace.h>
 #include <asm/ptrace.h>
 #include <asm/processor.h>
@@ -76,3 +77,58 @@ save_stack_trace_regs(struct pt_regs *regs, struct stack_trace *trace)
 	save_context_stack(trace, regs->gpr[1], current, 0);
 }
 EXPORT_SYMBOL_GPL(save_stack_trace_regs);
+
+#ifdef CONFIG_HAVE_RELIABLE_STACKTRACE
+int
+save_stack_trace_tsk_reliable(struct task_struct *tsk,
+                              struct stack_trace *trace)
+{
+	unsigned long sp;
+	unsigned long stack_page = (unsigned long)task_stack_page(tsk);
+	/* the last frame (unwinding first) may not yet have saved its LR onto the stack. */
+	int firstframe = 1;
+
+	if (tsk == current)
+		sp = current_stack_pointer();
+	else
+		sp = tsk->thread.ksp;
+
+	if (sp < stack_page + sizeof(struct thread_struct)
+	    || sp > stack_page + THREAD_SIZE - STACK_FRAME_OVERHEAD)
+		return 1;
+	
+	for (;;) {
+		unsigned long *stack = (unsigned long *) sp;
+		unsigned long newsp, ip;
+
+		newsp = stack[0];
+		/* Stack grows downwards; unwinder may only go up */
+		if (newsp <= sp)
+			return 1;
+
+		if (newsp >= stack_page + THREAD_SIZE)
+			return 1; /* invalid backlink, too far up! */
+
+		/* Examine the saved LR: it must point into kernel code. */
+		ip = stack[STACK_FRAME_LR_SAVE];
+		if ( (ip & 0xEFFF000000000000) != CONFIG_KERNEL_START
+		     && !firstframe)
+			return 1;
+		firstframe = 0;
+
+		if (!trace->skip)
+			trace->entries[trace->nr_entries++] = ip;
+		else
+			trace->skip--;
+
+		if (newsp > stack_page + THREAD_SIZE - STACK_FRAME_OVERHEAD)
+			break; /* hit the window for last frame */
+
+		if (trace->nr_entries >= trace->max_entries)
+			return -E2BIG;
+
+		sp = newsp;
+	}
+	return 0;
+}
+#endif /* CONFIG_HAVE_RELIABLE_STACKTRACE */