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

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Date:   Wed, 30 Sep 2020 21:22:19 -0700
From:   Joe Perches <joe@...ches.com>
To:     Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "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 Wed, 2020-09-30 at 21:17 -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. ;)
[]
> > > +	if (WARN(!buf || offset_in_page(buf),
> > > +		 "invalid sysfs_emit: buf:%p\n", buf))

The dump_stack() is also going to emit pointers
so I don't see how it does anything but help
show where the buffer was.  It is hashed...

> 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))

I don't think that helps as multiple defects can easily
exist.  Log spam in this case isn't horrible either.