Message-ID: <468b8369-84c2-da12-1eb0-6ec16d160b34@igalia.com>
Date:   Mon, 29 Aug 2022 10:04:14 -0300
From:   "Guilherme G. Piccoli" <gpiccoli@...lia.com>
To:     ardb@...nel.org, linux-efi@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-dev@...lia.com,
        kernel@...ccoli.net, anton@...msg.org, ccross@...roid.com,
        keescook@...omium.org, matt@...eblueprint.co.uk,
        mjg59@...f.ucam.org, tony.luck@...el.com
Subject: Re: [PATCH v2 0/3] The UEFI panic notification mechanism, 2nd round

On 29/07/2022 16:45, Guilherme G. Piccoli wrote:
> Hey folks, this is the 2nd iteration of the patchset adding a simple
> mechanism to notify the UEFI firmware about a panic event in the kernel.
> V1 here:
> https://lore.kernel.org/lkml/20220520195028.1347426-1-gpiccoli@igalia.com/
> 
> 
> First thing, the differences in this V2:
> 
> - Ardb response in V1 mentioned a refactor aimed for v5.20 that removes an
> obsolete/confusing way of setting EFI variables - this led to a weird
> condition, deleted variables stayed in sysfs after deletion. Well, I've
> refactored the code based on efi/next, so I'm using the recommended API
> now - thanks a bunch for the advice Ardb!
> 
> - I've changed NULL-terminating char in patch 1 to the format I've seen
> in Ardb's code, L'\0'.
> 
> - Patch 2 is new, it's somewhat a fix for a patch only in efi/next, part
> of the efivar refactor.
> 
> 
> In the V1 review, it was mentioned we could maybe use efi-pstore as a way
> to signal the firmware about a panic event - in the end, the efi-pstore
> mechanism can collect a dmesg, so it's even richer in the information level.
> But I disagree that it is the way to go, for 3 main reasons:
> 
> a) efi-pstore could be impossible to use, if the users are already using
> another pstore backend (like ramoops), which is _exactly_ our case!
> Of course, we could rework pstore and allow 2 backends, quite a bit of work,
> but...see next points!
> 
> b) Even if (a) is a not an issue, we have another one, even more important:
> signaling the firmware about a panic is *different* than collecting a bunch
> of data, a full dmesg even. This could be considered a security issue for
> some users; also, the dmesg collected consumes a bunch more memory in the
> (potentially scarce) UEFI available memory.
> Although related, the goal of pstore is orthogonal to our mechanism here:
> users rely on pstore to collect data, our proposal is a simple infrastructure
> to just let the firmware know about a panic. Our kernel module also shows a
> message and automatically clears the UEFI variable, so it tracks a single
> panic, whereas efi-pstore logs are kept by default, in order to provide
> data to users.
> 
> c) Finally, it's faster and less "invasive"/risky to just write a byte in a
> variable on a panic event than having a ksmg dumper collecting the full dmesg
> and writing it to the UEFI memory; again, some users wish to have the logs,
> but not all of them.
> 
> 
> With all of that said, I think this module makes sense, it's a very simple
> solution that opens doors to firmware panic handling approaches, like in our
> proposed case (a different splash screen on panic).
> 
> Finally, the variable name (PanicWarn) and value (0xFF by default, can be
> changed by a module parameter) are just my personal choices but I'm open to
> suggestions, not strongly attached to them heh
> 
> Thanks again for the reviews/suggestions!
> Cheers,
> 
> 
> Guilherme
> 
> 

Hi Ard, sorry for the ping =]

Any opinions in this one? Patch 2 is a simple fix, BTW.
Cheers,


Guilherme

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