linux-kernel - Re: Question: livepatch failed for new fork() task stack unreliable

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Date:   Fri, 29 May 2020 12:44:33 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Wang ShaoBo <bobo.shaobowang@...wei.com>
Cc:     huawei.libin@...wei.com, xiexiuqi@...wei.com,
        cj.chengjian@...wei.com, mingo@...hat.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
        mbenes@...e.cz, devel@...ukata.com, viro@...iv.linux.org.uk,
        esyr@...hat.com
Subject: Re: Question: livepatch failed for new fork() task stack unreliable

On Fri, May 29, 2020 at 06:10:59PM +0800, Wang ShaoBo wrote:
> Stack unreliable error is reported by stack_trace_save_tsk_reliable() when trying
> to insmod a hot patch for module modification, this results in frequent failures
> sometimes. We found this 'unreliable' stack is from task just fork.

For livepatch, this shouldn't actually be a failure.  The patch will
just stay in the transition state until after the fork has completed.
Which should happen in a reasonable amount of time, right?

> 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();

Yes, this seems to be true for forked-but-not-yet-scheduled tasks.

I can look at fixing that.  I have some ORC cleanups in progress which
are related to UNWIND_HINT_EMPTY and the end of the stack.  I can add
this issue to the list of improvements.

> 2) The task has been scheduled but UNWIND_HINT_REGS not finished, at this time
> arch_stack_walk_reliable() terminates it's backtracing loop for pt_regs unknown
> and return -EINVAL because it's a user task.

Hm, do you see this problem with upstream?  It seems like it should
work.  arch_stack_walk_reliable() has this:

			/* Success path for user tasks */
			if (user_mode(regs))
				return 0;

Where exactly is the error coming from?

-- 
Josh