[PATCH] x86: Make backtracer fallback logic more bullet-proof

The unwinder fallback logic still had potential for falling through to
the legacy stack trace code without printing an indication (at once
serving as a separator) of this.

Further, the stack pointer retrieval for the fallback should be as
restrictive as possible (in order to avoid having the legacy stack
tracer try to access invalid memory). The patch tightens that, but
this could certainly be further improved.

Also making the call_trace command line option now conditional upon
CONFIG_STACK_UNWIND (as it's meaningless otherwise).
Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/traps.c

@@ -92,7 +92,11 @@ asmlinkage void spurious_interrupt_bug(void);
 asmlinkage void machine_check(void);
 
 static int kstack_depth_to_print = 24;
+#ifdef CONFIG_STACK_UNWIND
 static int call_trace = 1;
+#else
+#define call_trace (-1)
+#endif
 ATOMIC_NOTIFIER_HEAD(i386die_chain);
 
 int register_die_notifier(struct notifier_block *nb)
@@ -187,22 +191,21 @@ static void show_trace_log_lvl(struct task_struct *task, struct pt_regs *regs,
 			if (unwind_init_blocked(&info, task) == 0)
 				unw_ret = show_trace_unwind(&info, log_lvl);
 		}
-		if (unw_ret > 0 && !arch_unw_user_mode(&info)) {
-#ifdef CONFIG_STACK_UNWIND
+		if (unw_ret > 0) {
+			if (call_trace == 1 && !arch_unw_user_mode(&info)) {
 				print_symbol("DWARF2 unwinder stuck at %s\n",
 					     UNW_PC(&info));
-			if (call_trace == 1) {
+				if (UNW_SP(&info) >= PAGE_OFFSET) {
 					printk("Leftover inexact backtrace:\n");
-				if (UNW_SP(&info))
 					stack = (void *)UNW_SP(&info);
-			} else if (call_trace > 1)
+				} else
+					printk("Full inexact backtrace again:\n");
+			} else if (call_trace >= 1)
 				return;
 			else
 				printk("Full inexact backtrace again:\n");
-#else
+		} else
 			printk("Inexact backtrace:\n");
-#endif
-		}
 	}
 
 	if (task == current) {
@@ -1241,6 +1244,7 @@ static int __init kstack_setup(char *s)
 }
 __setup("kstack=", kstack_setup);
 
+#ifdef CONFIG_STACK_UNWIND
 static int __init call_trace_setup(char *s)
 {
 	if (strcmp(s, "old") == 0)
@@ -1254,3 +1258,4 @@ static int __init call_trace_setup(char *s)
 	return 1;
 }
 __setup("call_trace=", call_trace_setup);
+#endif

arch/x86_64/kernel/traps.c

@@ -107,7 +107,11 @@ static inline void preempt_conditional_cli(struct pt_regs *regs)
 }
 
 static int kstack_depth_to_print = 12;
+#ifdef CONFIG_STACK_UNWIND
 static int call_trace = 1;
+#else
+#define call_trace (-1)
+#endif
 
 #ifdef CONFIG_KALLSYMS
 # include <linux/kallsyms.h>
@@ -274,21 +278,21 @@ void show_trace(struct task_struct *tsk, struct pt_regs *regs, unsigned long * s
 			if (unwind_init_blocked(&info, tsk) == 0)
 				unw_ret = show_trace_unwind(&info, NULL);
 		}
-		if (unw_ret > 0 && !arch_unw_user_mode(&info)) {
-#ifdef CONFIG_STACK_UNWIND
-			unsigned long rip = info.regs.rip;
-			print_symbol("DWARF2 unwinder stuck at %s\n", rip);
-			if (call_trace == 1) {
+		if (unw_ret > 0) {
+			if (call_trace == 1 && !arch_unw_user_mode(&info)) {
+				print_symbol("DWARF2 unwinder stuck at %s\n",
+					     UNW_PC(&info));
+				if ((long)UNW_SP(&info) < 0) {
 					printk("Leftover inexact backtrace:\n");
-				stack = (unsigned long *)info.regs.rsp;
-			} else if (call_trace > 1)
+					stack = (unsigned long *)UNW_SP(&info);
+				} else
+					printk("Full inexact backtrace again:\n");
+			} else if (call_trace >= 1)
 				return;
 			else
 				printk("Full inexact backtrace again:\n");
-#else
+		} else
 			printk("Inexact backtrace:\n");
-#endif
-		}
 	}
 
 	/*
@@ -1120,6 +1124,7 @@ static int __init kstack_setup(char *s)
 }
 __setup("kstack=", kstack_setup);
 
+#ifdef CONFIG_STACK_UNWIND
 static int __init call_trace_setup(char *s)
 {
 	if (strcmp(s, "old") == 0)
@@ -1133,3 +1138,4 @@ static int __init call_trace_setup(char *s)
 	return 1;
 }
 __setup("call_trace=", call_trace_setup);
+#endif

include/asm-i386/unwind.h

@@ -87,6 +87,7 @@ static inline int arch_unw_user_mode(const struct unwind_frame_info *info)
 #else
 
 #define UNW_PC(frame) ((void)(frame), 0)
+#define UNW_SP(frame) ((void)(frame), 0)
 
 static inline int arch_unw_user_mode(const void *info)
 {

include/asm-x86_64/unwind.h

@@ -95,6 +95,7 @@ static inline int arch_unw_user_mode(const struct unwind_frame_info *info)
 #else
 
 #define UNW_PC(frame) ((void)(frame), 0)
+#define UNW_SP(frame) ((void)(frame), 0)
 
 static inline int arch_unw_user_mode(const void *info)
 {