Re: [PATCH v2] Documentation/tracing: Mention that RESET_ATTACK_MITIGATION can clear memory

[Steven Rostedt <rostedt@goodmis.org>]

Jon,

This version should be good to go.

Thanks,

-- Steve


On Tue, 1 Oct 2024 09:57:34 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Steven Rostedt <rostedt@goodmis.org>
> 
> At the 2024 Linux Plumbers Conference, I was talking with Hans de Goede
> about the persistent buffer to display traces from previous boots. He
> mentioned that UEFI can clear memory. In my own tests I have not seen
> this. He later informed me that it requires the config option:
> 
>  CONFIG_RESET_ATTACK_MITIGATION
> 
> It appears that setting this will allow the memory to be cleared on boot
> up, which will definitely clear out the trace of the previous boot.
> 
> Add this information under the trace_instance in kernel-parameters.txt
> to let people know that this can cause issues.
> 
> Link: https://lore.kernel.org/all/20170825155019.6740-2-ard.biesheuvel@linaro.org/
> 
> Reported-by: Hans de Goede <hdegoede@redhat.com>
> Reviewed-by: Hans de Goede <hdegoede@redhat.com>
> Acked-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
> Changes since v1: https://lore.kernel.org/20240926130159.19e6d0e2@rorschach.local.home
> 
>  - Added more detail explanation that the system may not be able to use
>    memory to preserve the tracing ring buffer across reboots and use
>    the CONFIG_RESET_ATTACK_MITIGATION as one example.
> 
>  Documentation/admin-guide/kernel-parameters.txt | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 1518343bbe22..9881e3b857d0 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6867,6 +6867,12 @@
>  
>  				reserve_mem=12M:4096:trace trace_instance=boot_map^traceoff^traceprintk@trace,sched,irq
>  
> +			Note, saving the trace buffer across reboots does require that the system
> +			is set up to not wipe memory. For instance, CONFIG_RESET_ATTACK_MITIGATION
> +			can force a memory reset on boot which will clear any trace that was stored.
> +			This is just one of many ways that can clear memory. Make sure you system
> +			keeps the content of memory across reboots before relying on this option.
> +
>  			See also Documentation/trace/debugging.rst
>  
>