linux-kernel - Re: [PATCH] printk: handle blank console arguments passed in.

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Date:   Tue, 6 Oct 2020 11:52:26 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Shreyas Joshi <shreyas.joshi@...mp.com>, rostedt@...dmis.org,
        shreyasjoshi15@...il.com, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] printk: handle blank console arguments passed in.

On Mon 2020-10-05 20:35:59, Guenter Roeck wrote:
> On 10/5/20 7:59 PM, Sergey Senozhatsky wrote:
> > On (20/05/22 12:00), Petr Mladek wrote:
> >> On Fri 2020-05-22 16:53:06, Shreyas Joshi wrote:
> >>> If uboot passes a blank string to console_setup then it results in a trashed memory.
> >>> Ultimately, the kernel crashes during freeing up the memory. This fix checks if there
> >>> is a blank parameter being passed to console_setup from uboot.
> >>> In case it detects that the console parameter is blank then
> >>> it doesn't setup the serial device and it gracefully exits.
> >>>
> > Petr, this patch's causing regressions for us. We use blank console= boot
> > param to bypass dts. It appears that it'd be better to revert the change.
> > 
> Not just to bypass dts, it was also possible to use console= to disable consoles
> passed as config option, as well as other default console options. A quick test
> confirms that this affects all platforms/architectures, not just Chromebooks.
> Prior to this patch, it was possible to disable a default console with an
> empty "console=" parameter. This is no longer possible. This means that
> this patch results in a substantial (and, as far as I can see, completely
> undiscussed) functionality change.

Where is this behavior documented, please?

I do not see it anywhere (documentation, git log, google) and it is far from
obvious from the code. It seems that any random string would do the
same job, e.g. console=none.

Of course, we need to restore the original behavior when it breaks
existing systems. But I want to be sure that there is no better
solution.

And it makes perfect sense to disable all consoles or drop all defined
by dts. But I would prefer to make it more obvious way, for
example by parameters like:

   + console=none
   + no-console
   + no-dtd-console
   + no-default-console


JFYI, the console= parameter handling is a real historical mess. We are
always surprised what undefined behavior people depend on. For
example, see:

  + commit 33225d7b0ac9903c5701b ("printk: Correctly set CON_CONSDEV
    even when preferred console was not registered")

  + commit e369d8227fd211be3624 ("printk: Fix preferred console
    selection with multiple matches")

> I don't understand why (yet), but the patch also causes regressions with
> seemingly unrelated functionality, specifically with dm-verity on at least
> one Chromebook platform. I filed crbug.com/1135157 to track the problem,
> and reverted the patch from all our stable releases immediately after
> the last round of stable release merges.
> 
> On a side note, I don't see the problem presumably fixed with this
> patch in any of my tests.

Console drivers might provide a custom match() callback to handle
various aliases. I guess that some driver wrongly matches the empty
string stored in the array of preferred consoles.

There are likely other ways to fix the original problem.

Best Regards,
Petr