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

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Date:   Sat, 03 Oct 2020 07:59:46 -0700
From:   Joe Perches <joe@...ches.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.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 Sat, 2020-10-03 at 15:55 +0200, Greg Kroah-Hartman wrote:
> On Fri, Oct 02, 2020 at 03:30:30PM -0700, Kees Cook wrote:
> > On Wed, Sep 30, 2020 at 09:22:19PM -0700, Joe Perches wrote:
> > > 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...
> > 
> > dump_stack() is going to report symbols and register contents.
> > 
> > I was just pointing out that %p has no value here[1]. The interesting
> > states are: "was it NULL?" "how offset was it?". Its actual content
> > won't matter.
> 
> Ok, suggestions for a better error message are always welcome :)

For sysfs_emit, the offset_in_buf test also
effectively checks if buf not PAGE_SIZE so
it helps identify if it is being called from
a non _show function.

That's actually why %p can be somewhat valuable.