Bug#1123750: [5.10] regression: virtual consoles 2-12 unusable

["Barry K. Nathan" <barryn@pobox.com>]

On 1/8/26 5:23 AM, Greg Kroah-Hartman wrote:
> On Fri, Jan 02, 2026 at 05:26:22PM +0100, Ben Hutchings wrote:
> > Hello stable maintainers,
> >
> > Several Debian users reported a regression after updating to kernel
> > version 5.10.247.
> >
> > Commit f0982400648a ("fbdev: Add bounds checking in bit_putcs to fix
> > vmalloc-out-of-bounds"), a backport of upstream commit 3637d34b35b2,
> > depends on vc_data::vc_font.charcount being initialised correctly.
> >
> > However, before commit a1ac250a82a5 ("fbcon: Avoid using FNTCHARCNT()
> > and hard-coded built-in font charcount") in 5.11, this member was set
> > to 256 for VTs initially created with a built-in font and 0 for VTs
> > initially created with a user font.
> >
> > Since Debian normally sets a user font before creating VTs 2 and up,
> > those additional VTs became unusable.  VT 1 also doesn't work correctly
> > if the user font has > 256 characters, and the bounds check is
> > ineffective if it has < 256 characters.
> >
> > This can be fixed by backporting the following commits from 5.11:
> >
> > 7a089ec7d77f console: Delete unused con_font_copy() callback implementations
> > 259a252c1f4e console: Delete dummy con_font_set() and con_font_default() callback implementations
> > 4ee573086bd8 Fonts: Add charcount field to font_desc
> > 4497364e5f61 parisc/sticore: Avoid hard-coding built-in font charcount
> > a1ac250a82a5 fbcon: Avoid using FNTCHARCNT() and hard-coded built-in font charcount
> >
> > These all apply without fuzz and builds cleanly for x86_64 and parisc64.
> >
> > I tested on x86_64 that:
> >
> > - VT 2 works again
> > - bit_putcs_aligned() is setting charcnt = 256
> > - After loading a font with 512 characters, bit_putcs_aligned() sets
> >    charcnt = 512 and is able to display characters at positions >= 256
>
> All now queued up, thanks!
>
> greg k-h

For what it's worth, now that the above commits are queued up for 
5.10.y: There are two more commits, which were previously applied to 
5.15.y, that now apply to 5.10.y without merge conflicts (also without 
fuzz, if you apply the 5.15.y versions of the patches):


a5a923038d70
fbdev: fbcon: Properly revert changes when vc_resize() failed
(previously applied to 5.15.64 and 5.19.6)

3c3bfb8586f8
fbdev: fbcon: release buffer when fbcon_do_set_font() failed
(previously applied to 5.15.86, 6.0.16, and 6.1.2)


After looking at these two commits, it seems to me that they are now 
applicable to 5.10.y, and I think they probably should be applied 
(unless I'm overlooking or missing something).

By the way, for the past few days I have been running 5.10.247 + the 
latest 5.10.y queue + a5a923038d70 + 3c3bfb8586f8 (updating as new 
patches have been added to the queue). It seems to be running fine, and 
the virtual consoles seem to be working fine; it's also what I'm running 
now as I write and send this email.

(To be clear, 5.10.247 + the 5.10.y queue, without the two additional 
commits, also fixes the original 5.10.247 virtual console regression for 
me. However, applying commits a5a923038d70 and 3c3bfb8586f8 on top of 
5.10.y doesn't appear to create any new regressions in my testing so far.)

--
-Barry K. Nathan  <barryn@pobox.com>