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

From: Kees Cook
Date: Thu Sep 29 2016 - 21:36:37 EST

On Thu, Sep 29, 2016 at 5:55 PM, Sean Hudson <sean_hudson@xxxxxxxxxx> 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...


> 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