linux-kernel - Re: [PATCH v2] workqueue.c: Increase workqueue name length

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Date: Wed, 10 Jan 2024 16:52:53 -0500
From: Rafael Aquini <aquini@...hat.com>
To: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc: Audra Mitchell <audra@...hat.com>, linux-kernel@...r.kernel.org,
	tj@...nel.org, jiangshanlai@...il.com,
	hirokazu.yamauchi.hk@...achi.com, ddouwsma@...hat.com,
	loberman@...hat.com, raquini@...hat.com
Subject: Re: [PATCH v2] workqueue.c: Increase workqueue name length

On Wed, Jan 10, 2024 at 09:47:56PM +0100, Rasmus Villemoes wrote:
> On 10/01/2024 21.29, Audra Mitchell wrote:
> 
> > @@ -4663,9 +4663,10 @@ struct workqueue_struct *alloc_workqueue(const char *fmt,
> >  					 unsigned int flags,
> >  					 int max_active, ...)
> >  {
> > -	va_list args;
> > +	va_list args, args_copy;
> >  	struct workqueue_struct *wq;
> >  	struct pool_workqueue *pwq;
> > +	int len;
> >  
> >  	/*
> >  	 * Unbound && max_active == 1 used to imply ordered, which is no longer
> > @@ -4692,6 +4693,13 @@ struct workqueue_struct *alloc_workqueue(const char *fmt,
> >  	}
> >  
> >  	va_start(args, max_active);
> > +	va_copy(args_copy, args);
> > +	len = vsnprintf(NULL, 0, fmt, args_copy);
> > +	WARN(len > WQ_NAME_LEN,
> > +		"workqueue: wq->name too long (%d). Truncated to WQ_NAME_LEN (%d)\n",
> > +		len, WQ_NAME_LEN);
> > +
> > +	va_end(args_copy);
> >  	vsnprintf(wq->name, sizeof(wq->name), fmt, args);
> 
> Eh, why not just _not_ throw away the return value from the existing
> vsnprintf() and do "len >= sizeof(wq->name)" to know if truncation
> happened? There's really no need need to do vsnprintf() twice. (And yes,
> you want >=, not >).
>

The extra vsnprintf call is required because the return of the existing 
vsnprintf() is going to be already capped by sizeof(wq->name).
 
> Oh, and definitely not WARN,  pr_warn() or pr_warn_once() please.
> 

Then you lose the ability to figure out what was trying to create the
wq with the inflated name. Also, the _once variants don't seem to do
good here, because alloc_workqueue() can be called by different 
drivers.

-- Rafael