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

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Sep 29, 2022 at 05:44:32PM -0600, Jack Rosenthal wrote:
> The CBMEM area is a downward-growing memory region used by coreboot to
> dynamically allocate tagged data structures ("CBMEM entries") that
> remain resident during boot.
> 
> This implements a driver which exports access to the CBMEM entries
> via sysfs under /sys/firmware/cbmem/<id>.
> 
> This implementation is quite versatile.  Examples of how it could be
> used are given below:
> 
> * Tools like util/cbmem from the coreboot tree could use this driver
>   instead of finding CBMEM in /dev/mem directly.  Alternatively,
>   firmware developers debugging an issue may find the sysfs interface
>   more ergonomic than the cbmem tool and choose to use it directly.
> 
> * The crossystem tool, which exposes verified boot variables, can use
>   this driver to read the vboot work buffer.
> 
> * Tools which read the BIOS SPI flash (e.g., flashrom) can find the
>   flash layout in CBMEM directly, which is significantly faster than
>   searching the flash directly.
> 
> Write access is provided to all CBMEM regions via
> /sys/firmware/cbmem/<id>/mem, as the existing cbmem tooling updates
> this memory region, and envisioned use cases with crossystem
> can benefit from updating memory regions.
> 
> Link: https://issuetracker.google.com/239604743
> Cc: Stephen Boyd <swboyd@chromium.org>
> Cc: Tzung-Bi Shih <tzungbi@kernel.org>
> Reviewed-by: Guenter Roeck <groeck@chromium.org>
> Reviewed-by: Julius Werner <jwerner@chromium.org>
> Tested-by: Jack Rosenthal <jrosenth@chromium.org>
> Signed-off-by: Jack Rosenthal <jrosenth@chromium.org>
> ---
> Changes in v11:
> * Changed /sys/firmware/coreboot/cbmem -> /sys/firmware/cbmem
> * cbmem.c uses attribute groups to initialize files, which is much
>   cleaner.  The attributes are added under the device kobject, which
>   is now symlinked into /sys/firmware/cbmem.

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.


> * Changed documentation text as suggested by greg k-h
> 
>  .../ABI/testing/sysfs-firmware-cbmem          |  43 +++++
>  drivers/firmware/google/Kconfig               |   8 +
>  drivers/firmware/google/Makefile              |   3 +
>  drivers/firmware/google/cbmem.c               | 180 ++++++++++++++++++
>  drivers/firmware/google/coreboot_table.h      |  16 ++
>  5 files changed, 250 insertions(+)
>  create mode 100644 Documentation/ABI/testing/sysfs-firmware-cbmem
>  create mode 100644 drivers/firmware/google/cbmem.c
> 
> diff --git a/Documentation/ABI/testing/sysfs-firmware-cbmem b/Documentation/ABI/testing/sysfs-firmware-cbmem
> new file mode 100644
> index 000000000000..f769104ac4cd
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-firmware-cbmem
> @@ -0,0 +1,43 @@
> +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?

Why cbmem?  What is CBMEM?

And just stick with the attributes under the cbmem coreboot device in
the device tree, don't use /sys/firmware/.

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"...

thanks,

greg k-h