linux-kernel - Re: [PATCH 2/3] livepatch: Remove duplicate warning about missing reliable stacktrace support

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Message-ID: <20190531193740.fdyhlgvfz32yzxgj@treble>
Date:   Fri, 31 May 2019 14:37:40 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Petr Mladek <pmladek@...e.com>, Jiri Kosina <jikos@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] livepatch: Remove duplicate warning about missing
 reliable stacktrace support

On Fri, May 31, 2019 at 02:32:34PM +0200, Miroslav Benes wrote:
> On Fri, 31 May 2019, Petr Mladek wrote:
> 
> > WARN_ON_ONCE() could not be called safely under rq lock because
> > of console deadlock issues.
> > 
> > It can be simply removed. A better descriptive message is written
> > in klp_enable_patch() when klp_have_reliable_stack() fails.
> > The remaining debug message is good enough.
> > 
> > Signed-off-by: Petr Mladek <pmladek@...e.com>
> > ---
> >  kernel/livepatch/transition.c | 1 -
> >  1 file changed, 1 deletion(-)
> > 
> > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> > index abb2a4a2cbb2..1bf362df76e1 100644
> > --- a/kernel/livepatch/transition.c
> > +++ b/kernel/livepatch/transition.c
> > @@ -247,7 +247,6 @@ static int klp_check_stack(struct task_struct *task, char *err_buf)
> >  	int ret, nr_entries;
> >  
> >  	ret = stack_trace_save_tsk_reliable(task, entries, ARRAY_SIZE(entries));
> > -	WARN_ON_ONCE(ret == -ENOSYS);
> >  	if (ret < 0) {
> >  		snprintf(err_buf, STACK_ERR_BUF_SIZE,
> >  			 "%s: %s:%d has an unreliable stack\n",
> 
> The current situation is not the best, but I think the patch improves it 
> only slightly. I see two possible solutions.
> 
> 1. we either revert commit 1d98a69e5cef ("livepatch: Remove reliable 
> stacktrace check in klp_try_switch_task()"), so that klp_check_stack() 
> returns right away.
> 
> 2. or we test ret from stack_trace_save_tsk_reliable() for ENOSYS and 
> return.
> 
> In my opinion either of them is better than what we have now (and what we 
> would have with the patch), because klp_check_stack() returns, but it 
> prints out that a task has an unreliable stack. Yes, it is pr_debug() only 
> in the end, but still.
> 
> I don't have a preference and my understanding is that Petr does not want 
> to do v4. I can prepare a patch, but it would be nice to choose now. Josh? 
> Anyone else?

My vote would be #1 -- just revert 1d98a69e5cef.

-- 
Josh