Re: [PATCH v5 07/16] printk: Ensure consistent name and devname when updating preferred console

[Petr Mladek <pmladek@suse.com>]

On Thu 2026-10-29 15:09:52, Chris Down wrote:
> Petr Mladek writes:
> >-		strscpy(pc->name, name);
> >-		pc->index = idx;
> >+		if (!pc->name[0]) {
> >+			is_new_entry = true;
> >+			strscpy(pc->name, name);
> >+			pc->index = idx;
> 
> console_setup() can hand us an empty name or devname:
> 
>     console=,115200   -> name "", idx 0, options "115200"
>     console=,foo:bar  -> devname "", idx -1, options "foo:bar"
> 
> For the devname case, __add_preferred_console() matches the first name-only 
> entry (`strcmp(pc->devname, "") == 0`), e.g. the SPCR "pl011"/0 entry, and this 
> hunk now treats it as new.
> 
> Then index becomes -1 and options become "foo:bar", so the SPCR console never 
> matches again. Before the series that command line only moved 
> preferred_console.
> 
> For the `!pc->name[0]` branch below the slot keeps looking unused, which turns 
> into list corruption with 14/16 (I'll put some details over there instead of 
> duplicating it here).

Great catch! I think that even the original code did not expect that
anyone could pass "" as either @name or @devname.

> Rejecting empty strings up front, after the `WARN_ON(!name && !devname)`, fixes 
> both:
> 
>     	if ((name && !name[0]) || (devname && !devname[0])) {
>     		pr_err("Adding a preferred console with an empty name\n");
>     		return -EINVAL;
>     	}

I like this. I am going to add this as a separate patch.
As mentioned above, I think that it was not handled before...

I have the following as the 5th patch:

From d7990601069c2facc8ca47efe7ed36ac21038be7 Mon Sep 17 00:00:00 2001
From: Petr Mladek <pmladek@suse.com>
Date: Tue, 3 Nov 2026 18:02:20 +0100
Subject: [PATCH] printk: Do not accept an empty name or devname as a preferred
 console

The current code for storing preferred consoles does not expect
the case when an empty string is passed via @name or @devname.

Empty console driver name does not make any sense. Avoid handling
such entries completely.

Suggested-by: Chris Down <chris@chrisdown.name>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/printk.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b7543fea865b..6bba67e142e1 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2558,7 +2558,9 @@ asmlinkage __visible void early_printk(const char *fmt, ...)
  *
  * The function adds or updates an entry in preferred_consoles[] array. Also
  * it updates some global variables which are used to make the right decisions
- * in register_console().
+ * in register_console(). Finally, it enforces some rules:
+ *
+ *   1. Do not accept empty @name or @devname.
  */
 static int add_or_update_preferred_console(unsigned int i,
 					   const char *name, const short idx,
@@ -2576,6 +2578,11 @@ static int add_or_update_preferred_console(unsigned int i,
 	if (!name && !devname)
 		return -EINVAL;
 
+	if ((name && !name[0]) || (devname && !devname[0])) {
+		pr_err("Adding a preferred console with an empty name\n");
+		return -EINVAL;
+	}
+
 	/*
 	 * We use a signed short index for struct console for device drivers to
 	 * indicate a not yet assigned index or port. However, a negative index
-- 
2.56.0

Best Regards,
Petr