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

[Greg KH <gregkh@linuxfoundation.org>]

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@socionext.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>