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

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Date:   Sat, 27 Apr 2019 15:38:44 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.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
Subject: Re: [PATCH v7 resend 1/2] Provide in-kernel headers to make
 extending kernel easier

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>