Re: [PATCH 8/8] printk: Try enable preferred consoles only when there are any

[Chris Down]

Petr Mladek writes:
> try_enable_preferred_console() used to be always called because it
> had several hidden effects, namely:
>
> - enabled Braille consoles which were ignored by "preferred_dev_console"
>  because they were not associated with /dev/console.
>
> - returned success when a console was pre-enabled using CON_ENABLED
>  flag.
>
> - returned success when a console was enabled by default because
>  try_enable_default_console() did not return success.
>
> The first two hidden effects were removed in previous patches. Remove
> the last one so that try_enable_preferred_console() can be called only
> when any non-Braille console is preferred.

There is a fourth hidden effect that was also removed, unconditionally 
initialising err :-) Unfortunately removing that causes an uninitialised read.

> @@ -4156,17 +4161,15 @@ void register_console(struct console *newcon)
> 	if (preferred_dev_console < 0) {
> 		if (hlist_empty(&console_list) || !console_first()->device ||
> 		    console_first()->flags & CON_BOOT) {
> -			try_enable_default_console(newcon);
> +			err = try_enable_default_console(newcon);
> 		}
> +	} else {
> +		err = try_enable_preferred_console(newcon, true);
> +
> +		if (err == -ENOENT)
> +			err = try_enable_preferred_console(newcon, false);
> 	}

When preferred_dev_console < 0 and the inner condition is false, nothing will 
initialise err.

> -	/* See if this console matches one we selected on the command line */
> -	err = try_enable_preferred_console(newcon, true);
> -
> -	/* If not, try to match against the platform default(s) */
> -	if (err == -ENOENT)
> -		err = try_enable_preferred_console(newcon, false);

You can see previously it was initialised without conditions here.

Later we read err with `if (err == -ENOENT)`, so it's possible to do an 
uninitialised read.

Doing `int err = -ENOENT` at declaration should fix it by the look of it.