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

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Date:   Thu, 29 Sep 2016 18:36:26 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sean Hudson <sean_hudson@...tor.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 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...

-Kees

>
> Sean Hudson (2):
>   printk: collect offsets into replaceable structure
>   printk: external log buffer (CONFIG_LOGBUFFER)
>
>  init/Kconfig           |  12 +
>  init/main.c            |   2 +
>  kernel/printk/printk.c | 598 +++++++++++++++++++++++++++++++++++--------------
>  3 files changed, 445 insertions(+), 167 deletions(-)
>
> --
> 1.9.1
>
>



-- 
Kees Cook
Nexus Security