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

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Date:   Fri, 31 May 2019 15:29:42 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Petr Mladek <pmladek@...e.com>
cc:     Jiri Kosina <jikos@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        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, 31 May 2019, Petr Mladek wrote:

> On Fri 2019-05-31 14:32:34, 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.
> 
> IMHO, any extra check will not improve the situation much. Quiet
> return is as useless as the misleading pr_debug() that will
> not normally get printed anyway.

I disagree here. I think the silent return would be perfectly fine. The 
user was warned in klp_enable_patch() already.

Miroslav