On 10/2/25 10:52, Albin Babu Varghese wrote:
Hi Helge, I tested your suggestions and they seem to work well.
Do you know if this affects the selection?
If so, would modifying (reducing/shortening) the selection maybe fix it?
The syzkaller reproducer uses really weird values where xs > xe and ys > ye
(xs=0xa00, xe=0x101, ys=0xc7e, ye=0x100) and set_selection() already swaps them
if needed and clamps the values.
Ok.
I added debug prints to check what's happening and the clamping in
set_selection() is working and the values coming through are within bounds. But
the crash still happens when you remap the framebuffer because of a slight
overflow.
I also discovered that when image.width is clipped on the X-axis, the character
count (cnt) must also be updated to match, otherwise bit_putcs_aligned()
receives mismatched buffer size and character count parameters, causing
out-of-bounds writes.
So I changed it to something like this:
+ if (image.dx >= info->var.xres)
+ break;
+ if (image.dx + image.width > info->var.xres) {
+ image.width = info->var.xres - image.dx;
+ cnt = image.width / vc->vc_font.width;
+ if (cnt == 0)
+ break;
+ image.width = cnt * vc->vc_font.width;
+ }
Looks good.
I tested it in syzbot, with the syzkaller reproducer, and also manually in QEMU
and verified that the buffer switches from tty1 to tty2 work correctly.
I couldn’t find a dedicated fbdev/fbcon test suite. Beyond kselftests, do you
recommend anything specific before sending v2?
There is Geert's fbdev test tool
(https://git.kernel.org/pub/scm/linux/kernel/git/geert/fbtest.git/),
but it does not involve testing of fbcon.
Helge
