x86/unwind: Include __schedule() in stack traces

In the following commit:

  0100301bfd ("sched/x86: Rewrite the switch_to() code")

... the layout of the 'inactive_task_frame' struct was designed to have
a frame pointer header embedded in it, so that the unwinder could use
the 'bp' and 'ret_addr' fields to report __schedule() on the stack (or
ret_from_fork() for newly forked tasks which haven't actually run yet).

Finish the job by changing get_frame_pointer() to return a pointer to
inactive_task_frame's 'bp' field rather than 'bp' itself.  This allows
the unwinder to start one frame higher on the stack, so that it properly
reports __schedule().

Reported-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/598e9f7505ed0aba86e8b9590aa528c6c7ae8dcd.1483978430.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/stacktrace.h

@@ -52,16 +52,13 @@ static inline bool on_stack(struct stack_info *info, void *addr, size_t len)
 static inline unsigned long *
 get_frame_pointer(struct task_struct *task, struct pt_regs *regs)
 {
-	struct inactive_task_frame *frame;
-
 	if (regs)
 		return (unsigned long *)regs->bp;
 
 	if (task == current)
 		return __builtin_frame_address(0);
 
-	frame = (struct inactive_task_frame *)task->thread.sp;
-	return (unsigned long *)READ_ONCE_NOCHECK(frame->bp);
+	return &((struct inactive_task_frame *)task->thread.sp)->bp;
 }
 #else
 static inline unsigned long *

arch/x86/include/asm/switch_to.h

@@ -36,7 +36,10 @@ static inline void prepare_switch_to(struct task_struct *prev,
 
 asmlinkage void ret_from_fork(void);
 
-/* data that is pointed to by thread.sp */
+/*
+ * This is the structure pointed to by thread.sp for an inactive task.  The
+ * order of the fields must match the code in __switch_to_asm().
+ */
 struct inactive_task_frame {
 #ifdef CONFIG_X86_64
 	unsigned long r15;
@@ -48,6 +51,11 @@ struct inactive_task_frame {
 	unsigned long di;
 #endif
 	unsigned long bx;
+
+	/*
+	 * These two fields must be together.  They form a stack frame header,
+	 * needed by get_frame_pointer().
+	 */
 	unsigned long bp;
 	unsigned long ret_addr;
 };