Re: [PATCH v3 0/6] allow ramoops to collect all kmesg_dump events

From: Kees Cook <keescook@chromium.org>
To: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Petr Mladek <pmladek@suse.com>,
	Anton Vorontsov <anton@enomsg.org>,
	Colin Cross <ccross@android.com>, Tony Luck <tony.luck@intel.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Rob Herring <robh+dt@kernel.org>,
	Benson Leung <bleung@chromium.org>,
	Enric Balletbo i Serra <enric.balletbo@collabora.com>,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	James Morris <jmorris@namei.org>, Sasha Levin <sashal@kernel.org>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	devicetree@vger.kernel.org
Subject: Re: [PATCH v3 0/6] allow ramoops to collect all kmesg_dump events
Date: Tue, 12 May 2020 11:53:42 -0700	[thread overview]
Message-ID: <202005121146.3B3C1FE0D@keescook> (raw)
In-Reply-To: <CA+CK2bC0argMNHzynedpwN6ekOg8yypN03JvmAKGWQ5Aegxh+Q@mail.gmail.com>

On Tue, May 12, 2020 at 12:49:10PM -0400, Pavel Tatashin wrote:
> On Tue, May 12, 2020 at 11:52 AM Petr Mladek <pmladek@suse.com> wrote:
> > I wonder if anyone is actually using the ramoops.dump_oops parameter
> > in reality. I would personally make it deprecated and change the
> > default behavior to work according to printk.always_kmsg_dump parameter.
> 
> This sounds alright to me with one slight problem. I am doing this
> work for an embedded arm64 SoC, so controlling everything via device
> tree is preferable compared to having some settings via device tree
> and others via kernel parameters, especially because the kernel
> parameters are hardcoded by firmware that we try not to update too
> often for uptime reasons.

I'm entirely convinced that this area of pstore needs to be cleaned up
and I want to have the pstore backends be able to declare their kmsg
dump reason filters in a configurable fashion. So at least on the pstore
end, I intend to have some way to do this.

> > IMHO, ramoops.dump_oops just increases complexity and should not have
> > been introduced at all. I would try hard to avoid introducing even bigger
> > complecity and mess.
> 
> I agree, amoops.dump_oops should be depricated with or without
> max_reason change.

Yup. dump_oops will be deprecated in favor of whatever we settle on here.

> > I know that there is the "do not break existing userspace" rule. The
> > question is if there is any user and if it is worth it.
> >
> > > I agree, the reasons in kmsg_dump_reason do not order well  (I
> > > actually want to add another reason for kexec type reboots, and where
> > > do I put it?), so how about if we change the ordering list to
> > > bitfield/flags, and instead of max_reason provide: "reasons" bitset?
> >
> > It looks too complicated. I would really try hard to avoid the
> > parameter at all.
> 
> OK. Should we remove max_reason from struct kmsg_dumper and also
> remove the misleading comment about kmsg_dump_reason ordering?

I'm also fine with this. I can have pstore infrastructure doing the
filtering if kmsg dump doesn't want to. Given the existence of
printk.always_kmsg_dump, though, it seemed like it was better to have
kmsg dump do this filtering instead.

At this point my preference is to switch to a bit field -- I don't see a
reason for ordering. The only cases that remain "special" appear to be
PANIC and EMERG (which, again, aren't ordered adjacent).

-Kees

-- 
Kees Cook