Re: [PATCH 1/2] printk: Unconditionally unregister boot consoles if in init section

[Petr Mladek]

Hi all,

let's first make sure that we understand the code the same way.

On Fri 2017-07-07 08:58:01, Matt Redfearn wrote:
> On 07/07/17 05:45, Sergey Senozhatsky wrote:
> >On (07/06/17 11:38), Matt Redfearn wrote:
> >>Commit 4c30c6f566c0 ("kernel/printk: do not turn off bootconsole in
> >>printk_late_init() if keep_bootcon") added a check on keep_bootcon to
> >>ensure that boot consoles were kept around until the real console is
> >>registered.
> >>
> >>This can lead to problems if the boot console data and code are in the
> >>init section, since it can be freed before the boot console is
> >>deregistered.

Yes.

> >>This was fixed by commit 81cc26f2bd11 ("printk: only
> >>unregister boot consoles when necessary").

This does not make sense to me in this context. This commit has an
effect only when keep_bootcon is false. While the commit 4c30c6f566c0
("kernel/printk: do not turn off boot console in printk_late_init()
if keep_bootcon") _causes problems only_ when keep_bootcon is true.

What I want to say is that the two commits have effect when
keep_bootcon has different value. Therefore they could not fix
each other.

> >>The keep_bootcon flag prevents the unregistration of a boot console,
> >>even if it's data and code reside in the init section and are about to
> >>be freed. This can lead to crashes and weird panics when the bootconsole
> >>is accessed after free, especially if page poisoning is in use and the
> >>code / data have been overwritten with a poison value.
> >>To prevent this, always free the boot console if it is within the init
> >>section.

> >if someone asked to `keep_bootcon' but we actually don't keep it, then
> >what's the purpose of the flag and
> >	pr_info("debug: skip boot console de-registration.\n")?

Exactly. The important information is in the commit 7bf693951a8e5f7e
("console: allow to retain boot console via boot option keep_bootcon"):

    On some architectures, the boot process involves de-registering the boot
    console (early boot), initialize drivers and then re-register the console.

    This mechanism introduces a window in which no printk can happen on the
    console and messages are buffered and then printed once the new console is
    available.

    If a kernel crashes during this window, all it's left on the boot console
    is "console [foo] enabled, boot console disabled" making debug of the crash
    rather 'interesting'.


> >keeping `early_printk' sometimes can be helpful and people even want to
> >use `early_printk' as a panic() console fallback (because of those nasty
> >deadlocks that spoil printk->call_console_drivers()).
> >
> 
> Sure, but as a user, how are you supposed to know that?

Good point! I wonder if the authors of the keep_bootcon option
actually knew about it. I do not see this risk mentioned anywhere
and the early consoles might work long enough by chance.

One problem is that real consoles might be registered much later
when it is done using an async probe calls. It might open a big window
when there is no visible output and debugging is "impossible".

I am not comfortable with removing the only way to debug some type
of bugs. But the current state is broken as well.

IMHO, the reasonable solution is to move early console code and data
out of the init sections. We should do this for the early consoles
where the corresponding real console is registered using a deferred
probe. Others should be already replaced by the real console when
printk_late_init() is called. At least this is how I understand it.

Matt, is there any chance that you look into this possibility?

Best Regards,
Petr