Currently, kheaders_data.tar.xz contains some build scripts as well as
headers. None of them is needed in the header archive.

For ARCH=x86, this commit excludes the following from the archive:

  arch/x86/include/asm/Kbuild
  arch/x86/include/uapi/asm/Kbuild
  include/asm-generic/Kbuild
  include/config/auto.conf
  include/config/kernel.release
  include/config/tristate.conf
  include/uapi/asm-generic/Kbuild
  include/uapi/Kbuild
  kernel/gen_kheaders.sh

This change is actually motivated for the planned header compile-testing
because it will generate more build artifacts, which should not be
included in the archive.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NJoel Fernandes (Google) <joel@joelfernandes.org>

The -R option of 'ls' is supposed to be used for directories.

   -R, --recursive
          list subdirectories recursively

Since 'find ... -type f' only matches to regular files, we do not
expect directories passed to the 'ls' command here.

Giving -R is harmless at least, but unneeded.
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NJoel Fernandes (Google) <joel@joelfernandes.org>

Linus reported an issue that doing an allmodconfig was causing the
kheaders archive to be regenerated even though the config is the same.
This patch fixes the issue by ignoring the config-related header files
for "knowing when to regenerate based on timestamps".  Instead, if the
CONFIG_X_Y option really changes, then we there are the
include/config/X/Y.h which will already tells us "if a config really
changed". So we don't really need these files for regeneration detection
anyway, and ignoring them fixes Linus's issue.
Reported-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NJoel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

The kheaders archive consisting of the kernel headers used for compiling
bpf programs is in /proc. However there is concern that moving it here
will make it permanent. Let us move it to /sys/kernel as discussed [1].

[1] https://lore.kernel.org/patchwork/patch/1067310/#1265969Suggested-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NJoel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

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: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NJoel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>