linux-kernel - Re: [PATCH v7 resend 1/2] Provide in-kernel headers to make extending kernel easier

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Date:   Mon, 29 Apr 2019 09:26:02 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Andrew Morton <akpm@...ux-foundation.org>, ast@...nel.org,
        atishp04@...il.com, dancol@...gle.com,
        Dan Williams <dan.j.williams@...el.com>,
        dietmar.eggemann@....com, Guenter Roeck <groeck@...omium.org>,
        Jonathan Corbet <corbet@....net>, karim.yaghmour@...rsys.com,
        Kees Cook <keescook@...omium.org>, kernel-team@...roid.com,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-trace-devel@...r.kernel.org,
        Manoj Rao <linux@...ojrajarao.com>, mhiramat@...nel.org,
        qais.yousef@....com, rdunlap@...radead.org, rostedt@...dmis.org,
        Shuah Khan <shuah@...nel.org>, yhs@...com,
        Olof Johansson <olof@...om.net>
Subject: Re: [PATCH v7 resend 1/2] Provide in-kernel headers to make
 extending kernel easier

On Sat, Apr 27, 2019 at 03:38:44PM +0200, Greg KH wrote:
> On Fri, Apr 26, 2019 at 03:04:29PM -0400, Joel Fernandes (Google) wrote:
> > Introduce in-kernel headers which are made available as an archive
> > through proc (/proc/kheaders.tar.xz file). This archive makes it
> > possible to run eBPF and other tracing programs that need to extend the
> > kernel for tracing purposes without any dependency on the file system
> > having headers.
> > 
> > A github PR is sent for the corresponding BCC patch at:
> > https://github.com/iovisor/bcc/pull/2312
> > 
> > On Android and embedded systems, it is common to switch kernels but not
> > have kernel headers available on the file system. Further once a
> > different kernel is booted, any headers stored on the file system will
> > no longer be useful. This is an issue even well known to distros.
> > By storing the headers as a compressed archive within the kernel, we can
> > avoid these issues that have been a hindrance for a long time.
> > 
> > The best way to use this feature is by building it in. Several users
> > have a need for this, when they switch debug kernels, they do not want to
> > update the filesystem or worry about it where to store the headers on
> > it. However, the feature is also buildable as a module in case the user
> > desires it not being part of the kernel image. This makes it possible to
> > load and unload the headers from memory on demand. A tracing program can
> > load the module, do its operations, and then unload the module to save
> > kernel memory. The total memory needed is 3.3MB.
> > 
> > By having the archive available at a fixed location independent of
> > filesystem dependencies and conventions, all debugging tools can
> > directly refer to the fixed location for the archive, without concerning
> > with where the headers on a typical filesystem which significantly
> > simplifies tooling that needs kernel headers.
> > 
> > The code to read the headers is based on /proc/config.gz code and uses
> > the same technique to embed the headers.
> > 
> > Other approaches were discussed such as having an in-memory mountable
> > filesystem, but that has drawbacks such as requiring an in-kernel xz
> > decompressor which we don't have today, and requiring usage of 42 MB of
> > kernel memory to host the decompressed headers at anytime. Also this
> > approach is simpler than such approaches.
> > 
> > Reviewed-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

Thanks for the Reviewed-by tag. I believe there are still 2 logistical things
to merge this.
1. Location of the header archive:
Olof and Steve did not like it to be in /proc and instead /sys seemed a better
choice they are Ok with. Me and Greg were Ok with it being in /sys/kernel/.
Alexei, Greg and me are Ok with either proc or Sys.

2. Who is going to pull this patch: This seems a matter of where the header
archive resides. If it is in /sys/kernel/ then I am assuming Greg will pull
it.  Masahiro has given his Reviewed-by tag, is he the one to pull it?

Let us agree on these open questions so I can respin the patch to be based on
that and move this forward.

thanks!

 - Joel