- valid_stack_ptr() erroneously assumes that stack always lives in
  task_struct->thread_info.

- the main loop in show_trace() does not recalc ebp after stack
  switching.  With CONFIG_FRAME_POINTER every call to print_context_stack()
  will produce the same output.

With this patch, show_trace() does not use task argument in the main loop. 
Instead, it converts stack to thread_info* context, and passes it to
print_context_stack() and (implicitly) to valid_stack_ptr().

valid_stack_ptr() now does bounds checking against proper context.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>