On Sat, May 30, 2020 at 10:21:19AM +0800, Wangshaobo (bobo) wrote:

1) when a user mode task just fork start excuting ret_from_fork() till
schedule_tail, unwind_next_frame found

orc->sp_reg is ORC_REG_UNDEFINED but orc->end not equals zero, this time
arch_stack_walk_reliable()

terminates it's backtracing loop for unwind_done() return true. then 'if
(!(task->flags & (PF_KTHREAD | PF_IDLE)))'

in arch_stack_walk_reliable() true and return -EINVAL after.

* The stack trace looks like that:

ret_from_fork

      -=> UNWIND_HINT_EMPTY

      -=> schedule_tail             /* schedule out */

      ...

      -=> UNWIND_HINT_REGS      /*  UNDO */

Yes, makes sense.

2) when using call_usermodehelper_exec_async() to create a user mode task,
ret_from_fork() still not exec whereas

the task has been scheduled in __schedule(), at this time, orc->sp_reg is
ORC_REG_UNDEFINED but orc->end equals zero,

unwind_error() return true and also terminates arch_stack_walk_reliable()'s
backtracing loop, end up return from

'if (unwind_error())' branch.

* The stack trace looks like that:

-=> call_usermodehelper_exec

                 -=> do_exec

                           -=> search_binary_handler

                                      -=> load_elf_binary

                                                -=> elf_map

                                                         -=> vm_mmap_pgoff

-=> down_write_killable

-=> _cond_resched

             -=> __schedule           /* scheduled to work */

-=> ret_from_fork       /* UNDO */

I don't quite follow the stacktrace, but it sounds like the issue is the
same as the first one you originally reported:

1) The task was not actually scheduled to excute, at this time
UNWIND_HINT_EMPTY in ret_from_fork() has not reset unwind_hint, it's
sp_reg and end field remain default value and end up throwing an error
in unwind_next_frame() when called by arch_stack_walk_reliable();

Or am I misunderstanding?

And to reiterate, these are not "livepatch failures", right?  Livepatch
doesn't fail when stack_trace_save_tsk_reliable() returns an error.  It
recovers gracefully and tries again later.

-- 
Josh