Re: [PATCH v2 2/3] firmware: google: memconsole: Escape unprintable characters

From: Greg Kroah-Hartman
Date: Mon May 01 2017 - 20:43:06 EST

On Mon, May 01, 2017 at 04:44:15PM -0700, Julius Werner wrote:
> On Fri, Apr 28, 2017 at 10:37 PM, Greg Kroah-Hartman
> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Fri, Apr 28, 2017 at 01:42:24PM -0700, Julius Werner wrote:
> >> Recent improvements in coreboot's memory console allow it to contain
> >> logs from more than one boot as long as the information persists in
> >> memory. Since trying to persist a memory buffer across reboots often
> >> doesn't quite work perfectly it is now more likely for random bit flips
> >> to occur in the console. With the current implementation this can lead
> >> to stray control characters that cause weird effects for the most common
> >> use cases (such as just dumping the console with 'cat').
> >>
> >> This patch changes the memconsole driver to replace unprintable
> >> characters with '?' by default. It also adds a new /sys/firmware/rawlog
> >> node next to the existing /sys/firmware/log for use cases where it's
> >> desired to read the raw characters.
> >
> > Again, you are doing multiple things here. Break it up.
> Sorry, I'm not sure what else you want me to break up here? If I add
> the escaping in one patch and then add the raw node in a different
> patch, there's a gap between the two patches where we're losing
> functionality. Isn't that undesirable?

Raw node first and then escape the old one?

> > And, can userspace handle this change? You are now changing the format
> > of the existing file.
> I'm escaping characters that previously hadn't been escaped, but
> really, I'm just trying to make sure that people can continue to use
> this node as before. The real change has already happened in coreboot,
> outside of Linux. This node could previously be relied upon to only
> contain printable characters, and with newer versions of coreboot
> that's not always the case anymore. I chose to do it this way because
> I thought it would be the least disruptive for most users.
> It can be handled in userspace, yes. It depends on what you use to
> read the file, mostly... people using less will be fine, but people
> using cat may get weird behavior. You're right that it doesn't really
> *need* to be escaped in the kernel, and honestly I don't care too
> much... I can leave it out if you want. But I think this is the most
> convenient way to do it for existing users.

Binary sysfs files are supposed to be "pass through" only, the kernel
should not be touching the data at all, it's up to userspace to do what
it wants to do with things. So don't escape anything at all, that's not
the kernel's job here.


greg k-h