[PATCH] x86_64: stack unwinder crash fix

the new dwarf2 unwinder crashes while trying to dump the stack:

  Leftover inexact backtrace:

  Unable to handle kernel paging request at ffffffff82800000 RIP:
   [<ffffffff8026cf26>] dump_trace+0x35b/0x3d2
  PGD 203027 PUD 205027 PMD 0
  Oops: 0000 [2] PREEMPT SMP
  CPU 0
  Modules linked in:
  Pid: 30, comm: khelper Not tainted 2.6.19-rc6-rt1 #11
  RIP: 0010:[<ffffffff8026cf26>]  [<ffffffff8026cf26>] dump_trace+0x35b/0x3d2
  RSP: 0000:ffff81003fb9d848  EFLAGS: 00010006
  RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
  RDX: 0000000000000000 RSI: ffffffff805b3520 RDI: 0000000000000000
  RBP: ffffffff827ffff9 R08: ffffffff80aad000 R09: 0000000000000005
  R10: ffffffff80aae000 R11: ffffffff8037961b R12: ffff81003fb9d858
  R13: 0000000000000000 R14: ffffffff80598460 R15: ffffffff80ab1fc0
  FS:  0000000000000000(0000) GS:ffffffff806c4200(0000) knlGS:0000000000000000
  CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
  CR2: ffffffff82800000 CR3: 0000000000201000 CR4: 00000000000006e0

this crash happened because it did not sanitize the dwarf2 data it
got, and got an unaligned stack pointer - which happily walked past
the process stack (and eventually reached the end of kernel memory
and pagefaulted there) due to this naive iteration condition:

        HANDLE_STACK (((long) stack & (THREAD_SIZE-1)) != 0);

note that i386 is alot more conservative when it comes to trusting
stack pointers:

  static inline int valid_stack_ptr(struct thread_info *tinfo, void *p)
  {
         return  p > (void *)tinfo &&
                 p < (void *)tinfo + THREAD_SIZE - 3;
  }

but the x86_64 code did not take this bit of i386 code.

The fix is to align the stack pointer.
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Cc: Andi Kleen <ak@suse.de>
Cc: Jan Beulich <jbeulich@novell.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

arch/x86_64/kernel/traps.c

@@ -290,6 +290,12 @@ void dump_trace(struct task_struct *tsk, struct pt_regs *regs, unsigned long * s
 		if (tsk && tsk != current)
 			stack = (unsigned long *)tsk->thread.rsp;
 	}
+	/*
+	 * Align the stack pointer on word boundary, later loops
+	 * rely on that (and corruption / debug info bugs can cause
+	 * unaligned values here):
+	 */
+	stack = (unsigned long *)((unsigned long)stack & ~(sizeof(long)-1));
 
 	/*
 	 * Print function call entries within a stack. 'cond' is the