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SubjectRe: [PATCH v3 1/2] Provide in-kernel headers for making it easy to extend the kernel
On 02/28/19 17:04, Dietmar Eggemann wrote:
> 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@joelfernandes.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’

I believe this is caused by this change ad774086356d (kbuild: change filechk
to surround the given command with { }) which changes how filechk defined - and
since filechk_ikheadersxz is used to generate kheaders_data.h it fails when
backported because the syntax has changed.

HTH

--
Qais Yousef

> 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.

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