Re: [PATCH v5] drm/fbdev-generic: prohibit potential out-of-bounds access

[Thomas Zimmermann]

Hi

Am 20.04.23 um 14:25 schrieb Geert Uytterhoeven:
> Hi Thomas,
>
> On Thu, Apr 20, 2023 at 1:10 PM Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:
> > Am 20.04.23 um 12:04 schrieb Sui Jingfeng:
> > > What will happen if the 'screen_size' is not page_size aligned and mmap
> > > will mapping in the granularity of pages ?
> >
> > You need to map at page granularity. If screen_size is not page-size
> > aligned, there's this trailing buffer that is accessible, but cannot be
> > displayed. But userspace has no direct way of knowing that, so let's
> > ignore that problem for now.
>
> Userspace can know, if fb_fix_screeninfo.smem_{start,len} match
> the actual offset and size.

Can you elaborate? How can userspace detect/compute the actually usable 
space?

From grep'ing fbdev drivers, smem_len appears to be a multiple of the 
pagesize. (?) screen_size is not exported and line_length in the fixed 
portion. Or can line_length change between modes? In that case it should 
be (yres_virtual * line_length), right?

Best regards
Thomas


> Gr{oetje,eeting}s,
>
>                          Geert

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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