x86, 32-bit: fix kernel_trap_sp()

Use &regs->sp instead of regs for getting the top of stack in kernel mode.
(on x86-64, regs->sp always points the top of stack)

[ Impact: Oprofile decodes only stack for backtracing on i386 ]
Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
[ v2: rename the API to kernel_stack_pointer(), move variable inside ]
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: systemtap@sources.redhat.com
Cc: Harvey Harrison <harvey.harrison@gmail.com>
Cc: Jan Blunck <jblunck@suse.de>
Cc: Christoph Hellwig <hch@infradead.org>
LKML-Reference: <20090511210300.17332.67549.stgit@localhost.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/include/asm/ptrace.h

@@ -187,14 +187,15 @@ static inline int v8086_mode(struct pt_regs *regs)
 
 /*
  * X86_32 CPUs don't save ss and esp if the CPU is already in kernel mode
- * when it traps.  So regs will be the current sp.
+ * when it traps.  The previous stack will be directly underneath the saved
+ * registers, and 'sp/ss' won't even have been saved. Thus the '&regs->sp'.
  *
  * This is valid only for kernel mode traps.
  */
-static inline unsigned long kernel_trap_sp(struct pt_regs *regs)
+static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
 {
 #ifdef CONFIG_X86_32
-	return (unsigned long)regs;
+	return (unsigned long)(&regs->sp);
 #else
 	return regs->sp;
 #endif

arch/x86/oprofile/backtrace.c

@@ -76,9 +76,9 @@ void
 x86_backtrace(struct pt_regs * const regs, unsigned int depth)
 {
 	struct frame_head *head = (struct frame_head *)frame_pointer(regs);
-	unsigned long stack = kernel_trap_sp(regs);
 
 	if (!user_mode_vm(regs)) {
+		unsigned long stack = kernel_stack_pointer(regs);
 		if (depth)
 			dump_trace(NULL, regs, (unsigned long *)stack, 0,
 				   &backtrace_ops, &depth);