linux-kernel - Re: [PATCH V3 1/8] sysfs: Add sysfs_emit and sysfs_emit

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Date:   Fri, 2 Oct 2020 15:27:35 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Joe Perches <joe@...ches.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Denis Efremov <efremov@...ux.com>,
        Julia Lawall <julia.lawall@...ia.fr>,
        Alex Dewar <alex.dewar90@...il.com>,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH V3 1/8] sysfs: Add sysfs_emit and sysfs_emit_at to format
 sysfs output

On Thu, Oct 01, 2020 at 10:50:29PM +0200, Greg Kroah-Hartman wrote:
> 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.

Sure, it's just a question if you want log spamming vs how reachable you
think something might be. I would expect this to be uncommon to
encounter, but very repeatable for whatever system DOES hit it, so doing
_ONCE means they see the report and don't get completely flooded with
it.

I'm fine either way.

-- 
Kees Cook