Message-ID: <CAMj1kXGMmpLF8Sfcm16E+QpKosPgUGQ7FpO9qknoKxjyXY1zUQ@mail.gmail.com>
Date:   Mon, 5 Sep 2022 11:50:48 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc:     linux-efi@...r.kernel.org, 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 Mon, 29 Aug 2022 at 15:04, Guilherme G. Piccoli <gpiccoli@...lia.com> wrote:
>
> 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.

Hey,

No worries about the ping - apologies for the late response, I was on vacation.

I still don't see the point of this series, but I will take the fix if
you could please rebase it so it doesn't depend on the first patch.

Thanks,
Ard.

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