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

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Message-ID: <202010021527.DF20CE0@keescook>
Date:   Fri, 2 Oct 2020 15:30:30 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "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, 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.

-Kees

[1] "New uses of %p should not be added to the kernel"
    https://www.kernel.org/doc/html/latest/process/deprecated.html#p-format-specifier

-- 
Kees Cook