linux-kernel - Re: [PATCH 2/2] s390/iucv: Fix vargs handling in iucv_alloc

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Message-ID: <2024081319-patriarch-brutishly-653f@gregkh>
Date: Tue, 13 Aug 2024 14:43:33 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Vasily Gorbik <gor@...ux.ibm.com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Alexandra Winter <wintera@...ux.ibm.com>,
	Heiko Carstens <hca@...ux.ibm.com>, linux-s390@...r.kernel.org
Subject: Re: [PATCH 2/2] s390/iucv: Fix vargs handling in iucv_alloc_device()

On Tue, Aug 13, 2024 at 01:50:27PM +0200, Vasily Gorbik wrote:
> On Tue, Aug 13, 2024 at 12:52:19PM +0200, Greg Kroah-Hartman wrote:
> > On Tue, Aug 13, 2024 at 12:42:37PM +0200, Vasily Gorbik wrote:
> > > From: Heiko Carstens <hca@...ux.ibm.com>
> > > 
> > > iucv_alloc_device() gets a format string and a varying number of
> > > arguments. This is incorrectly forwarded by calling dev_set_name() with
> > > the format string and a va_list, while dev_set_name() expects also a
> > > varying number of arguments.
> > > 
> > > Fix this and call kobject_set_name_vargs() instead which expects a
> > > va_list parameter.
> > 
> > I don't understand, why can't dev_set_name() be called here?
> > 
> > Calling "raw" kobject functions is almost never the correct thing to be
> > doing, ESPECIALLY as you have a struct device here.
> 
> struct device *iucv_alloc_device(const struct attribute_group **attrs,
>                                  void *priv, const char *fmt, ...);
> 
> va_start(vargs, fmt); initializes vargs to point to the first argument after fmt.
> 
> __printf(2, 0) int kobject_set_name_vargs(struct kobject *kobj, const char *fmt, va_list vargs);
> 
> __printf(2, 3) int dev_set_name(struct device *dev, const char *name, ...);
> 
> dev_set_name is expecting to receive individual variable arguments
> directly (...), not a va_list.
> 
> The (...) in dev_set_name is meant to be expanded into individual
> arguments, but when you pass a va_list to it, this expansion doesn't
> happen. Instead, the va_list is just treated as a pointer or a single
> argument, leading to undefined or incorrect behavior.
> 
> So, would it be okay to reuse kobject_set_name_vargs() here, or would you propose
> introducing another helper just for this case? e.g.
> 
> int dev_set_name_vargs(struct device *dev, const char *fmt, va_list vargs)
> {
> ჻·······return kobject_set_name_vargs(&dev->kobj, fmt, vargs);
> }
> EXPORT_SYMBOL_GPL(dev_set_name_vargs)

This function makes more sense if you really want to do this.

But step back, why is this needed at all anyway?  No other subsystem or
driver needs/wants this, what makes this api so special?  Why not figure
out your name beforehand?

thanks,

greg k-h