Re: [PATCH v5 14/16] printk: Do not set Braille console as preferred_console

[Petr Mladek]

On Thu 2026-10-29 15:19:59, Chris Down wrote:
> Petr Mladek writes:
> >-	if (brl_options)
> >+	if (is_new_entry)
> >+		INIT_LIST_HEAD(&pc->preferred_dev_node);
> 
> is_new_entry is derived from pc->name[0]/pc->devname[0] being empty, and
> console_setup() can store an empty name:
> 
>     console=,115200 console=tty1
> 
> The first one creates an entry with name "" (and links it). The second one 
> stops the lookup loop at the same slot, sees it as new again, and 
> re-initialises a node which is still linked, followed by list_add(). With 
> DEBUG_LIST + BUG_ON_DATA_CORRUPTION:
> 
>     list_add corruption. next->prev should be prev (ffffb010209fc0c0), but was ffffb01023cd1fa0. (next=ffffb01023cd1fa0).
>     kernel BUG at lib/list_debug.c:29!
>     Call trace:
>      __list_add_valid_or_report+0xbc/0x1c0 (P)
>      add_or_update_preferred_console+0x6fc/0x9c0
>      __add_preferred_console+0x11c/0x160
>      console_setup+0x2e4/0x348
>      obsolete_checksetup+0x148/0x190
>      unknown_bootoption+0x150/0x328
>      parse_one+0x108/0x480
>      parse_args+0x158/0x3d0
>      start_kernel+0x134/0x488
> 
> "console=,foo:bar" does the same on the first console= when SPCR added an 
> entry, since it matches the SPCR slot via the empty devname (see 7/16).
> 
> Rejecting empty name/devname in add_or_update_preferred_console() (see my 
> comment on 7/16) should fix that.

Yup, it will be fixed in v6 this way.

> >-		preferred_dev_console = i;
> >+		if (!list_empty(&pc->preferred_dev_node))
> >+			list_del_init(&pc->preferred_dev_node);
> >+	} else if (!is_braille_console_preferred(pc)) {
> >+		if (list_empty(&pc->preferred_dev_node))
> >+			list_add(&pc->preferred_dev_node, &preferred_dev_console);
> >+		else
> >+			list_move(&pc->preferred_dev_node, &preferred_dev_console);
> 
> list_del_init() and list_move() are both fine on an initialised self-linked 
> node (list_move() passes the DEBUG_LIST checks in __list_del_entry_valid()), so 
> the list_empty() checks can go.

I was lazy to check this and wanted to stay on the safe side ;-)
OK, I am going to do the following changes for v6:

--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2666,13 +2666,9 @@ static int add_or_update_preferred_console(unsigned int i,
 	 */
 	if (brl_options) {
 		want_braille_console = true;
-		if (!list_empty(&pc->preferred_dev_node))
-			list_del_init(&pc->preferred_dev_node);
+		list_del_init(&pc->preferred_dev_node);
 	} else if (!is_braille_console_preferred(pc)) {
-		if (list_empty(&pc->preferred_dev_node))
-			list_add(&pc->preferred_dev_node, &preferred_dev_console);
-		else
-			list_move(&pc->preferred_dev_node, &preferred_dev_console);
+		list_move(&pc->preferred_dev_node, &preferred_dev_console);
 	}
 
 	if (user_specified) {


> With these fixed:
> 
> Acked-by: Chris Down <chris@xxxxxxxxxxxxxx>

Thanks.

Best Regards,
Petr