linux-kernel - Re: [PATCH 8/8] efi: pstore: Add module parameter for setting the record size

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Date:   Fri, 7 Oct 2022 10:00:02 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...lia.com>
To:     Ard Biesheuvel <ardb@...nel.org>, Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        kernel-dev@...lia.com, kernel@...ccoli.net, anton@...msg.org,
        ccross@...roid.com, tony.luck@...el.com, linux-efi@...r.kernel.org
Subject: Re: [PATCH 8/8] efi: pstore: Add module parameter for setting the
 record size

First of all, thanks Ard for the historical explanation!


On 07/10/2022 06:11, Ard Biesheuvel wrote:
> [...]
>> I think it'd be great to make it configurable! Ard, do you have any
>> sense of what the max/min, etc, should be here?
>>
> 
> Given that dbx on an arbitrary EFI system with secure boot enabled is
> already almost 4k, that seems like a reasonable default. As for the
> upper bound, there is no way to know what weird firmware bugs you
> might tickle by choosing highly unusual values here.
> 
> If you need to store lots of data, you might want to look at [0] for
> some work done in the past on using capsule update for preserving data
> across a reboot. In the general case, this is not as useful, as the
> capsule is only delivered to the firmware after invoking the
> ResetSystem() EFI runtime service (as opposed to SetVariable() calls
> taking effect immediately). However, if you need to capture large
> amounts of data, and can tolerate the uncertainty involved in the
> capsule approach, it might be a reasonable option.
> 
> [0] https://lore.kernel.org/all/20200312011335.70750-1-qiuxu.zhuo@intel.com/

So, you mean 4K as the default? I can change, but I would try to not
mess with the current users, is there a case you can imagine something
like 4k would fail? Maybe 2K is safer?

As for the maximum, I've tested with many values, and when it's larger
than ~30000 for edk2/ovmf, it fails with EFI_OUT_OF_RESOURCES and
doesn't collect the log; other than that, no issues observed.

When set to ~24000, the interesting is that we have fewer big logs in
/sys/fs/pstore, so it's nice to see compared to the bunch of 1K files heheh

Anyway, let's agree on the default and then I can resubmit that, I'm
glad you both consider that it's a good idea =)

Thanks,


Guilherme