linux-kernel - Re: [PATCH] serial: 8250: Prevent kernel crash with nr

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Message-ID: <5547C001.8040400@hurleysoftware.com>
Date:	Mon, 04 May 2015 14:52:49 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH] serial: 8250: Prevent kernel crash with nr_uarts=0

On 05/04/2015 01:45 PM, Jan Kiszka wrote:
> On 2015-05-04 19:02, Peter Hurley wrote:
>> Hi Jan,
>>
>> On 05/04/2015 12:01 PM, Jan Kiszka wrote:
>>> When nr_uarts was set to 0 (via config or 8250_core.nr_uarts), we crash
>>> early on x86 because serial8250_isa_init_ports dereferences base_ops
>>> which remains NULL. In fact, there is nothing to do for that function if
>>> there are no uarts.
>>
>> Thanks for finding this.
>>
>> So nr_uarts == 0 effectively disables the 8250 driver. Is there any 
>> reason not to simply abort the driver init instead?
> 
> I'm not very deep into this code, just stumbled over this while trying
> some, well, unusual configurations.

Ok; I wasn't sure if this was related to some weird setup that needed
the 8250 driver loaded but in-op.

> If you prefer to handle this differently, I can recode it, of course.

Yes, please. We should bail out of any initialization if nr_uarts == 0.

That would be:
1. univ8250_console_init()
	The return value is ignored but the console should not be registered.

2. early_serial_setup()

	if (nr_uarts == 0)
		return -ENODEV;

3. serial8250_init()

	if (nr_uarts == 0)
		return -ENODEV;


Regards,
Peter Hurley
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