linux-kernel - Re: [PATCH 3/3] livepatch: Cleanup message handling in klp_try_switch

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Date:   Wed, 24 Apr 2019 20:48:58 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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 3/3] livepatch: Cleanup message handling in
 klp_try_switch_task()

On Wed, 24 Apr 2019, Josh Poimboeuf wrote:

> On Wed, Apr 24, 2019 at 10:55:50AM +0200, Petr Mladek wrote:
> > WARN_ON_ONCE() could not be called safely under rq lock because
> > of console deadlock issues. Fortunately, simple printk_deferred()
> > is enough because the warning is printed from a well defined
> > location and context.
> > 
> > Also klp_try_switch_task() is called under klp_mutex.
> > Therefore, the buffer for debug messages could be static.
> > 
> > Signed-off-by: Petr Mladek <pmladek@...e.com>
> > ---
> >  kernel/livepatch/transition.c | 19 ++++++++++++++-----
> >  1 file changed, 14 insertions(+), 5 deletions(-)
> > 
> > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> > index 9c89ae8b337a..e8183d18227f 100644
> > --- a/kernel/livepatch/transition.c
> > +++ b/kernel/livepatch/transition.c
> > @@ -254,6 +254,7 @@ static int klp_check_stack_func(struct klp_func *func,
> >  static int klp_check_stack(struct task_struct *task, char *err_buf)
> >  {
> >  	static unsigned long entries[MAX_STACK_ENTRIES];
> > +	static int enosys_warned;
> >  	struct stack_trace trace;
> >  	struct klp_object *obj;
> >  	struct klp_func *func;
> > @@ -263,8 +264,16 @@ static int klp_check_stack(struct task_struct *task, char *err_buf)
> >  	trace.nr_entries = 0;
> >  	trace.max_entries = MAX_STACK_ENTRIES;
> >  	trace.entries = entries;
> > +
> >  	ret = save_stack_trace_tsk_reliable(task, &trace);
> > -	WARN_ON_ONCE(ret == -ENOSYS);
> > +	if (ret == -ENOSYS) {
> > +		if (!enosys_warned) {
> > +			printk_deferred(KERN_WARNING "%s: save_stack_trace_tsk_reliable() not supported on this architecture.\n",
> > +					__func__);
> > +			enosys_warned = 1;
> > +		}
> > +		return ret;
> > +	}
> 
> We already have a similar printk in patch 1, so is this warning really
> needed?

I don't think so. pr_warn() in klp_enable_patch() should be enough in my 
opinion.

However,

if (ret == -ENOSYS)
	return ret;

would be justified, wouldn't it?

Miroslav