linux-kernel - Re: [PATCH v3 1/2] Provide in-kernel headers for making it easy to extend the kernel

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Date:   Thu, 28 Feb 2019 17:04:59 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Joel Fernandes <joel@...lfernandes.org>,
        Qais Yousef <qais.yousef@....com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, ast@...nel.org,
        atishp04@...il.com, dancol@...gle.com,
        Dan Williams <dan.j.williams@...el.com>,
        gregkh@...uxfoundation.org, 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>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        mhiramat@...nel.org, paulmck@...ux.vnet.ibm.com,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        rdunlap@...radead.org, rostedt@...dmis.org,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, yhs@...com
Subject: Re: [PATCH v3 1/2] Provide in-kernel headers for making it easy to
 extend the kernel

Hi Joel,

On 2/28/19 3:47 PM, Joel Fernandes wrote:
> On Thu, Feb 28, 2019 at 01:53:43PM +0000, Qais Yousef wrote:
>> Hi Joel
>>
>> On 02/27/19 14:37, Joel Fernandes (Google) wrote:

[...]

> Ah good catch, I made this change for "file_list=${@:2}" in my tree but
> forgot to push it. Below is the updated patch. Sorry and I'll refresh the
> series with the change after we finish the discussion in the other thread.
> Meanwhile the updated patch is as follows...
> 
> ---8<-----------------------
> 
> From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> Subject: [PATCH v3.1] Provide in-kernel headers for making it easy to extend the kernel
> 
> Introduce in-kernel headers and other artifacts which are made available
> as an archive through proc (/proc/kheaders.tar.xz file). This archive makes
> it possible to build kernel modules, run eBPF programs, and other
> tracing programs that need to extend the kernel for tracing purposes
> without any dependency on the file system having headers and build
> artifacts.
> 
> On Android and embedded systems, it is common to switch kernels but not
> have kernel headers available on the file system. Raw kernel headers
> also cannot be copied into the filesystem like they can be on other
> distros, due to licensing and other issues. There's no linux-headers
> package on Android. Further once a different kernel is booted, any
> headers stored on the file system will no longer be useful. 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 feature is also buildable as a module just in case the user desires
> it not being part of the kernel image. This makes it possible to load
> and unload the headers on demand. A tracing program, or a kernel module
> builder can load the module, do its operations, and then unload the
> module to save kernel memory. The total memory needed is 3.8MB.
> 
> The code to read the headers is based on /proc/config.gz code and uses
> the same technique to embed the headers.

This version gives me the header files on a v5.0-rc8 kernel on my arm64 
box but does not compile anymore on v4.20:

kernel/kheaders.c:25:22: error: expected identifier or ‘(’ before string 
constant
  #define KH_MAGIC_END "IKHD_ED"
                       ^
kernel/kheaders_data.h:1:1: note: in expansion of macro ‘KH_MAGIC_END’
  KH_MAGIC_END;
  ^~~~~~~~~~~~
kernel/kheaders.c: In function ‘ikheaders_read_current’:
kernel/kheaders.c:38:12: error: ‘kernel_headers_data’ undeclared (first 
use in this function); did you mean ‘kernel_headers_data_size’?
             kernel_headers_data + KH_MAGIC_SIZE,
             ^~~~~~~~~~~~~~~~~~~
             kernel_headers_data_size
kernel/kheaders.c:38:12: note: each undeclared identifier is reported 
only once for each function it appears in
kernel/kheaders.c: In function ‘ikheaders_init’:
kernel/kheaders.c:31:10: error: ‘kernel_headers_data’ undeclared (first 
use in this function); did you mean ‘kernel_headers_data_size’?
   (sizeof(kernel_headers_data) - 1 - KH_MAGIC_SIZE * 2)
           ^
kernel/kheaders.c:57:23: note: in expansion of macro 
‘kernel_headers_data_size’
   proc_set_size(entry, kernel_headers_data_size);
                        ^~~~~~~~~~~~~~~~~~~~~~~~
kernel/kheaders.c: In function ‘ikheaders_read_current’:
kernel/kheaders.c:40:1: warning: control reaches end of non-void 
function [-Wreturn-type]
  }


The reason for me to stay on v4.20 is that with v5.0-rc8 I don't have 
ebpf 'raw tracepoint' support any more on my arm64 board. But this issue 
is not related to your patch though.

Another point which supports the functionality your patch provides is 
the fact that maintainers don't want to see new TRACE_EVENTs in their 
code. So here your patch comes handy when using ebpf for tracing in 
embedded environments.