arm64: stacktrace: avoid listing stacktrace functions in stacktrace

The stacktraces always begin as follows:

[<c00117b4>] save_stack_trace_tsk+0x0/0x98
[<c0011870>] save_stack_trace+0x24/0x28
...

This is because the stack trace code includes the stack frames for
itself. This is incorrect behaviour, and also leads to "skip" doing the
wrong thing (which is the number of stack frames to avoid recording.)

Perversely, it does the right thing when passed a non-current thread.
Fix this by ensuring that we have a known constant number of frames
above the main stack trace function, and always skip these.

This was fixed for arch arm by commit 3683f44c42e9 ("ARM: stacktrace:
avoid listing stacktrace functions in stacktrace")

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Prakash Gupta <[e-mail]>
Cc: Russell King <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Joonsoo Kim <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/arm64/kernel/stacktrace.c

···
 		trace->entries[trace->nr_entries++] = ULONG_MAX;
 }
 
-void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
+static noinline void __save_stack_trace(struct task_struct *tsk,
+	struct stack_trace *trace, unsigned int nosched)
 {
 	struct stack_trace_data data;
 	struct stackframe frame;
···
 
 	data.trace = trace;
 	data.skip = trace->skip;
+	data.no_sched_functions = nosched;
 
 	if (tsk != current) {
-		data.no_sched_functions = 1;
 		frame.fp = thread_saved_fp(tsk);
 		frame.pc = thread_saved_pc(tsk);
 	} else {
-		data.no_sched_functions = 0;
+		/* We don't want this function nor the caller */
+		data.skip += 2;
 		frame.fp = (unsigned long)__builtin_frame_address(0);
-		frame.pc = (unsigned long)save_stack_trace_tsk;
+		frame.pc = (unsigned long)__save_stack_trace;
 	}
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 	frame.graph = tsk->curr_ret_stack;
···
 }
 EXPORT_SYMBOL_GPL(save_stack_trace_tsk);
 
+void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
+{
+	__save_stack_trace(tsk, trace, 1);
+}
+
 void save_stack_trace(struct stack_trace *trace)
 {
-	save_stack_trace_tsk(current, trace);
+	__save_stack_trace(current, trace, 0);
 }
+
 EXPORT_SYMBOL_GPL(save_stack_trace);
 #endif