Re: [PATCH v2 00/38] fbdev: Make userspace interfaces optional

[Sam Ravnborg]

Hi Thomas,

Nice series, quite some preparations.

On Mon, Jun 12, 2023 at 04:07:38PM +0200, Thomas Zimmermann wrote:
> Add the new config option FB_DEVICE. If enabled, fbdev provides
> traditional userspace interfaces in devfs, sysfs and procfs, such
> as /dev/fb0 or /proc/fb.
> 
> Modern Linux distrobutions have adopted DRM drivers for graphics
> output and use fbdev only for the kernel's framebuffer console.
> Userspace has also moved on, with no new fbdev code being written
> and existing support being removed.
> 
> OTOH, fbdev provides userspace a way of accessing kernel or I/O
> memory, which might compromise the system's security. See the recent
> commit c8687694bb1f ("drm/fbdev-generic: prohibit potential
> out-of-bounds access") for an example. Disabling fbdev userspace
> interfaces is therefore a useful feature to limit unnecessary
> exposure of fbdev code to processes of low privilegues.
> 
> Patches 1 to 31 fix various bugs and issues in fbdev-related code.
> In most cases the code uses the fbdev device where it should use
> the Linux hardware device or something else. Most of these patches
> fix existing problems and should therefore be considered in any case.
> 
> Patches 32 to 37 refactor the fbdev core code. The patches move
> support for backlights, sysfs, procfs and devfs into separate files
> and hide it behind simple interfaces. These changes will allow to
> easily build the userspace support conditionally.
> 
> Patch 38 introduces the config option FB_DEVICE and adapts the fbdev
> core to support it. The field struct fb_info.dev is now optional,
> hence the name of the config option.
> 
> Tested on simpledrm and i915, including the device handover.
> 
> Future directions: With the support for disabling fbdev userspace
> interfaces in place, it will be possible to make most fbdev drivers'
> file-I/O code in struct fb_ops optional as well. 
> 
> v2:
> 	* fix fsl-diu-fb and sh7760fb
> 	* split backlight patches
> 	* set 'default y' for FB_CONFIG
> 	* minor fixes and corrections
> 
> Thomas Zimmermann (38):
>   backlight/bd6107: Compare against struct fb_info.device
>   backlight/bd6107: Rename struct bd6107_platform_data.fbdev to 'dev'
>   backlight/gpio_backlight: Compare against struct fb_info.device
>   backlight/gpio_backlight: Rename field 'fbdev' to 'dev'
>   backlight/lv5207lp: Compare against struct fb_info.device
>   backlight/lv5207lp: Rename struct lv5207lp_platform_data.fbdev to
>     'dev'
>   fbdev/atyfb: Reorder backlight and framebuffer init/cleanup
>   fbdev/atyfb: Use hardware device as backlight parent
>   fbdev/aty128fb: Reorder backlight and framebuffer init/cleanup
>   fbdev/aty128fb: Use hardware device as backlight parent
>   fbdev/broadsheetfb: Call device_remove_file() with hardware device
>   fbdev/ep93xx-fb: Output messages with fb_info() and fb_err()
>   fbdev/ep93xx-fb: Do not assign to struct fb_info.dev
>   fbdev/fsl-diu-fb: Output messages with fb_*() helpers
>   fbdev/mb862xxfb: Output messages with fb_dbg()
>   fbdev/metronomefb: Use hardware device for dev_err()
>   fbdev/nvidiafb: Reorder backlight and framebuffer init/cleanup
>   fbdev/nvidiafb: Use hardware device as backlight parent
>   fbdev/pxa168fb: Do not assign to struct fb_info.dev
>   fbdev/radeonfb: Reorder backlight and framebuffer cleanup
>   fbdev/radeonfb: Use hardware device as backlight parent
>   fbdev/rivafb: Reorder backlight and framebuffer init/cleanup
>   fbdev/rivafb: Use hardware device as backlight parent
>   fbdev/sh7760fb: Use fb_dbg() in sh7760fb_get_color_info()
>   fbdev/sh7760fb: Output messages with fb_dbg()
>   fbdev/sh7760fb: Use hardware device with dev_() output during probe
>   fbdev/sm501fb: Output message with fb_err()
>   fbdev/tdfxfb: Set i2c adapter parent to hardware device
The above are all:
Reviewed-by: Sam Ravnborg <sam@xxxxxxxxxxxx>

>   fbdev/smscufx: Detect registered fb_info from refcount
I did not try to understand the code, so others must review.

>   fbdev/ep93xx-fb: Alloc DMA memory from hardware device
>   fbdev/sh7760fb: Alloc DMA memory from hardware device
This smells like bug-fixes, and I do not see what impact the change has.
So again, someone else needs to provide review here.

The rest are already reviewed or got a dedicated reply.

	Sam