linux-kernel - Re: [RFC PATCH v1 0/2] printk: Shared kernel logging

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Date:   Fri, 30 Sep 2016 19:42:23 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sean Hudson <darknighte@...knighte.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yang Shi <yang.shi@...aro.org>, Ingo Molnar <mingo@...nel.org>,
        Prarit Bhargava <prarit@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Yaowei Bai <baiyaowei@...s.chinamobile.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Petr Mladek <pmladek@...e.com>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Borislav Petkov <bp@...e.de>,
        Tim Bird <tim.bird@...ymobile.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [RFC PATCH v1 0/2] printk: Shared kernel logging

On Fri, Sep 30, 2016 at 6:56 PM, Sean Hudson <darknighte@...knighte.com> wrote:
> On 9/29/2016 8:36 PM, Kees Cook wrote:
>> On Thu, Sep 29, 2016 at 5:55 PM, Sean Hudson <sean_hudson@...tor.com>
>> wrote:
>>> This patch set is based on Linus' v4.8-rc8 tag.
>>>
>>> This debug feature allows the kernel to use an external buffer and
>>> control block for kernel log messages. The feature is controlled by
>>> an optional command line parameter. The existing buffer and control
>>> block can contain existing log messages from previous boot cycles
>>> and/or the bootloader. The command line parameter was chosen for
>>> flexibility, cross arch portability, and the ability to dynamically
>>> enable/disable this feature. The parameter specifies the address of
>>> a control block used to replace the default log buffer. Existing
>>> bootloader and kernel log messages are kept, in order, inside the
>>> new buffer. After a boot that preserves the buffer contents, a
>>> bootloader can display both kernel and bootloader log entries from
>>> multiple, previous boots. It also allows the kernel to display
>>> bootloader log entries along with its own messages.
>>>
>>> This feature is intended for debug purposes and has no effect
>>> unless the command line parameter is specified. Further, it
>>> validates the passed control block carefully and if any checks
>>> fail, it falls back to the default behaviour. As such, it can be
>>> left enabled by default.
>>>
>>> Memory Reservation
>>>
>>> This feature expects the bootloader to reserve/preserve the shared
>>> buffer memory. This reservation needs to prevent the kernel from
>>> overwriting the external log control block and log entries. In my
>>> testing, I've used the 'fdt' commands in uboot to dynamically
>>> inject reserved memory regions via the DT to the kernel.
>>
>> Interesting! I wonder if this can be adjusted to incorporate the
>> existing console logging feature in the pstore which does a similar
>> thing? Though pstore doesn't know about bootloader logs, really,
>> it's just storing kernel logs in a ring buffer. Maybe this can
>> provide a backend to pstore or something, especially since pstore
>> initialization happens "too late" for this to really be very
>> sensible. It just seems like it'd be nice to have a single persistent
>> console memory region...
>>
>
> I don't know that much about pstore.  From your description though, it
> sounds feasible to put the two together at some point.  How arch
> specific is pstore?

The only "weird" part is how to declare the reserved memory range.
That depends on either a platform driver or a Device Tree entry, but
isn't strictly "arch specific". Otherwise, console writes are
preserved in persistent memory as declared by the reservation info. It
doesnt, however, _load_ console from such a place.

-Kees

-- 
Kees Cook
Nexus Security