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Date: Sat, 9 Mar 2024 10:27:47 -0800
From: Kees Cook <keescook@chromium.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
	Joel Fernandes <joel@joelfernandes.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vincent Donnefort <vdonnefort@google.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>, suleiman@google.com,
	Thomas Gleixner <tglx@linutronix.de>,
	Vineeth Pillai <vineeth@bitbyteword.org>,
	Youssef Esmat <youssefesmat@google.com>,
	Beau Belgrave <beaub@linux.microsoft.com>,
	Alexander Graf <graf@amazon.com>, Baoquan He <bhe@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	David Howells <dhowells@redhat.com>
Subject: Re: [PATCH 0/8] tracing: Persistent traces across a reboot or crash
Message-ID: <202403091016.5CDF0E2EE@keescook>
References: <20240306015910.766510873@goodmis.org>
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In-Reply-To: <20240306015910.766510873@goodmis.org>

On Tue, Mar 05, 2024 at 08:59:10PM -0500, Steven Rostedt wrote:
> This is a way to map a ring buffer instance across reboots.

As mentioned on Fedi, check out the persistent storage subsystem
(pstore)[1]. It already does what you're starting to construct for RAM
backends (but also supports reed-solomon ECC), and supports several
other backends including EFI storage (which is default enabled on at
least Fedora[2]), block devices, etc. It has an existing mechanism for
handling reservations (including via device tree), and supports multiple
"frontends" including the Oops handler, console output, and even ftrace
which does per-cpu recording and event reconstruction (Joel wrote this
frontend).

It should be pretty straight forward to implement a new frontend if the
ftrace one isn't flexible enough. It's a bit clunky still to add one,
but search for "ftrace" in fs/pstore/ram.c to see how to plumb a new
frontend into the RAM backend.

I continue to want to lift the frontend configuration options up into
the pstore core, since it would avoid a bunch of redundancy, but this is
where we are currently. :)

-Kees

[1] CONFIG_PSTORE et. al. in fs/pstore/ https://docs.kernel.org/admin-guide/ramoops.html
[2] https://www.freedesktop.org/software/systemd/man/latest/systemd-pstore.service.html

-- 
Kees Cook