Re: [PATCH v11] firmware: google: Implement cbmem in sysfs driver

Jack Rosenthal
2022-09-30T22:14:50Z

On 2022-09-30 at 08:32 +0200, Greg KH wrote:
> symlink? Ick, no, do not do that at all please.
>
> As these are device attributes, just stick with them. Don't do a crazy
> symlink into a non-device-attribute portion of the sysfs tree, by doing
> that you break all userspace tools and stuff like libudev will never
> even see these attributes.

I guess I can fill in some info here about the use case needed:
userspace tools (in this case, a tool called "crossystem") needs to look
up a CBMEM entry by ID and read it. So, being able to find a fixed path
like /sys/firmware/cbmem/<id>/mem is significantly easier than scanning
all /sys/bus/coreboot/devices/coreboot*/id to find the right device
first.

What exactly do we break here by adding symlinks? udev won't look into
/sys/firmware, right?

Or, is there another good alternative that we could use to find a CBMEM
entry by its id without needing to scan thru all coreboot bus type
devices? Setting the device name to something more predictable (e.g.,
"cbmem-<id>") would require the coreboot bus type to "look ahead" and
notice it's a CBMEM entry before registering the device, which wouldn't
exactly be all that clean.

> > +What: /sys/firmware/cbmem/
> > +Date: August 2022
> > +Contact: Jack Rosenthal <jrosenth@chromium.org>
> > +Description:
> > + Coreboot provides a variety of data structures in CBMEM. This
> > + directory contains each CBMEM entry, which can be found via
> > + Coreboot tables.
>
> What happened to the coreboot name?

I removed it as it seemed like from your last message you didn't want
it?

> Why cbmem? What is CBMEM?

I can add this to the next patch once I get clarifications on the above.

> Also, I asked before, but some note about "exposing all of these bios
> values to userspace is not a security issue at all" would be nice, if
> only to point at in a few years and say "wow we were naive"...

Right, I'll add this too.

Thanks,

Jack