From: Nicolas Pitre npitre@baylibre.com
[ Upstream commit ff78538e07fa284ce08cbbcb0730daa91ed16722 ]
Programs using poll() on /dev/vcsa to be notified when VT changes occur were missing one case: the switch from gfx to text mode.
Signed-off-by: Nicolas Pitre npitre@baylibre.com Link: https://lore.kernel.org/r/9o5ro928-0pp4-05rq-70p4-ro385n21n723@onlyvoer.pbz Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Sasha Levin sashal@kernel.org ---
**YES**
This commit should be backported to stable kernel trees for the following reasons:
1. **It fixes a real bug affecting userspace programs**: The commit addresses a missing notification that prevents programs using `poll()` on `/dev/vcsa` from being properly notified when the VT switches from graphics mode back to text mode. This is a functional regression that breaks userspace applications monitoring VT state changes.
2. **The fix is minimal and low-risk**: The change adds only a single line - `notify_update(vc);` - in the `do_unblank_screen()` function. This follows an established pattern where `notify_update()` is called after VT state changes to notify userspace watchers.
3. **Consistent with existing code patterns**: Looking at the codebase, `notify_update()` is already called in similar contexts throughout vt.c. For example, in `vc_do_resize()`, we see: ```c vt_event_post(VT_EVENT_RESIZE, vc->vc_num, vc->vc_num); notify_update(vc); ``` The same pattern should apply when unblanking the screen.
4. **Similar commits were backported**: The historical commits provided show that previous fixes adding or correcting `notify_update()` calls were marked for stable backporting (all three YES examples had `Cc: stable@vger.kernel.org`). These commits fixed similar issues where userspace poll() notifications were missing.
5. **Clear symptom and fix**: The bug has a clear symptom (missing notifications when switching from graphics to text mode) and a straightforward fix that directly addresses the root cause. The `leaving_gfx` parameter in `do_unblank_screen()` specifically indicates this transition scenario.
6. **No architectural changes**: This is purely a bug fix that restores expected behavior without introducing new features or changing the architecture of the VT subsystem.
The commit meets all the criteria for stable backporting: it fixes an important bug affecting userspace, the fix is minimal and contained, and it follows established patterns for similar fixes that were previously backported.
drivers/tty/vt/vt.c | 1 + 1 file changed, 1 insertion(+)
diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c index bd125ea5c51f4..4898299768121 100644 --- a/drivers/tty/vt/vt.c +++ b/drivers/tty/vt/vt.c @@ -4443,6 +4443,7 @@ void do_unblank_screen(int leaving_gfx) set_palette(vc); set_cursor(vc); vt_event_post(VT_EVENT_UNBLANK, vc->vc_num, vc->vc_num); + notify_update(vc); } EXPORT_SYMBOL(do_unblank_screen);