Re: [PATCH V3 1/8] sysfs: Add sysfs_emit and sysfs_emit_at to format sysfs output - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Kees Cook <keescook@chromium.org>
Cc: Joe Perches <joe@perches.com>,
	"Rafael J . Wysocki" <rafael@kernel.org>,
	Denis Efremov <efremov@linux.com>,
	Julia Lawall <julia.lawall@inria.fr>,
	Alex Dewar <alex.dewar90@gmail.com>,
	linux-kernel@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
	linux-doc@vger.kernel.org
Subject: Re: [PATCH V3 1/8] sysfs: Add sysfs_emit and sysfs_emit_at to format sysfs output
Date: Thu, 1 Oct 2020 22:50:29 +0200	[thread overview]
Message-ID: <20201001205029.GC915579@kroah.com> (raw)
In-Reply-To: <202009302108.18B05CA38@keescook>

On Wed, Sep 30, 2020 at 09:17:03PM -0700, Kees Cook wrote:
> On Wed, Sep 30, 2020 at 01:57:40PM +0200, Greg Kroah-Hartman wrote:
> > Kees, and Rafael, I don't know if you saw this proposal from Joe for
> > sysfs files, questions below:
> 
> I'm a fan. I think the use of sprintf() in sysfs might have been one of
> my earliest complaints about unsafe code patterns in the kernel. ;)

Ok, great.

> > > +/**
> > > + *	sysfs_emit - scnprintf equivalent, aware of PAGE_SIZE buffer.
> > > + *	@buf:	start of PAGE_SIZE buffer.
> > > + *	@fmt:	format
> > > + *	@...:	optional arguments to @format
> > > + *
> > > + *
> > > + * Returns number of characters written to @buf.
> > > + */
> > > +int sysfs_emit(char *buf, const char *fmt, ...)
> > > +{
> > > +	va_list args;
> > > +	int len;
> > > +
> > > +	if (WARN(!buf || offset_in_page(buf),
> > > +		 "invalid sysfs_emit: buf:%p\n", buf))
> 
> I don't want the %p here, but otherwise, sure. I'd also make it a _ONCE
> variant:
> 
> 	if (WARN_ONCE(!buf || offset_in_page(buf),
> 		 "invalid sysfs_emit: offset_in_page(buf):%zd\n",
> 		  buf ? offset_in_page(buf) : 0))

As Joe points out, _ONCE doesn't work because this happens from all
sysfs files, not just one.

thanks,

greg k-h