[MIPS] Improve unwind_stack()

This patch allows unwind_stack() to return ra for leaf function.
But it tries to detects cases where get_frame_info() wrongly
consider nested function as a leaf one.

It also pass 'unsinged long *sp' instead of 'unsigned long **sp'
as second parameter. The code looks cleaner.
Signed-off-by: Franck Bui-Huu <vagabon.xyz@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/process.c

@@ -448,15 +448,16 @@ unsigned long get_wchan(struct task_struct *p)
 }
 
 #ifdef CONFIG_KALLSYMS
-/* used by show_frametrace() */
-unsigned long unwind_stack(struct task_struct *task,
-			   unsigned long **sp, unsigned long pc)
+/* used by show_backtrace() */
+unsigned long unwind_stack(struct task_struct *task, unsigned long *sp,
+			   unsigned long pc, unsigned long ra)
 {
 	unsigned long stack_page;
 	struct mips_frame_info info;
 	char *modname;
 	char namebuf[KSYM_NAME_LEN + 1];
 	unsigned long size, ofs;
+	int leaf;
 
 	stack_page = (unsigned long)task_stack_page(task);
 	if (!stack_page)
@@ -469,18 +470,26 @@ unsigned long unwind_stack(struct task_struct *task,
 
 	info.func = (void *)(pc - ofs);
 	info.func_size = ofs;	/* analyze from start to ofs */
-	if (get_frame_info(&info)) {
-		/* leaf or unknown */
-		*sp += info.frame_size / sizeof(long);
+	leaf = get_frame_info(&info);
+	if (leaf < 0)
 		return 0;
-	}
-	if ((unsigned long)*sp < stack_page ||
-	    (unsigned long)*sp + info.frame_size / sizeof(long) >
-	    stack_page + THREAD_SIZE - 32)
+
+	if (*sp < stack_page ||
+	    *sp + info.frame_size > stack_page + THREAD_SIZE - 32)
 		return 0;
 
-	pc = (*sp)[info.pc_offset];
-	*sp += info.frame_size / sizeof(long);
-	return pc;
+	if (leaf)
+		/*
+		 * For some extreme cases, get_frame_info() can
+		 * consider wrongly a nested function as a leaf
+		 * one. In that cases avoid to return always the
+		 * same value.
+		 */
+		pc = pc != ra ? ra : 0;
+	else
+		pc = ((unsigned long *)(*sp))[info.pc_offset];
+
+	*sp += info.frame_size;
+	return __kernel_text_address(pc) ? pc : 0;
 }
 #endif

arch/mips/kernel/traps.c

@@ -74,8 +74,9 @@ void (*board_ejtag_handler_setup)(void);
 void (*board_bind_eic_interrupt)(int irq, int regset);
 
 
-static void show_raw_backtrace(unsigned long *sp)
+static void show_raw_backtrace(unsigned long reg29)
 {
+	unsigned long *sp = (unsigned long *)reg29;
 	unsigned long addr;
 
 	printk("Call Trace:");
@@ -99,30 +100,29 @@ static int __init set_raw_show_trace(char *str)
 }
 __setup("raw_show_trace", set_raw_show_trace);
 
-extern unsigned long unwind_stack(struct task_struct *task,
-				  unsigned long **sp, unsigned long pc);
+extern unsigned long unwind_stack(struct task_struct *task, unsigned long *sp,
+				  unsigned long pc, unsigned long ra);
+
 static void show_backtrace(struct task_struct *task, struct pt_regs *regs)
 {
-	unsigned long *sp = (long *)regs->regs[29];
+	unsigned long sp = regs->regs[29];
+	unsigned long ra = regs->regs[31];
 	unsigned long pc = regs->cp0_epc;
-	int top = 1;
 
 	if (raw_show_trace || !__kernel_text_address(pc)) {
 		show_raw_backtrace(sp);
 		return;
 	}
 	printk("Call Trace:\n");
-	while (__kernel_text_address(pc)) {
+	do {
 		print_ip_sym(pc);
-		pc = unwind_stack(task, &sp, pc);
-		if (top && pc == 0)
-			pc = regs->regs[31];	/* leaf? */
-		top = 0;
-	}
+		pc = unwind_stack(task, &sp, pc, ra);
+		ra = 0;
+	} while (pc);
 	printk("\n");
 }
 #else
-#define show_backtrace(task, r) show_raw_backtrace((long *)(r)->regs[29]);
+#define show_backtrace(task, r) show_raw_backtrace((r)->regs[29]);
 #endif
 
 /*