Re: [PATCH v2 1/2] Provide in-kernel headers for making it easy to extend the kernel

[Joel Fernandes]

On Tue, Feb 19, 2019 at 01:42:13PM +0900, Masahiro Yamada wrote:
[..]
> > > > 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 set looks good to me and since the main use case is building bpf progs
> > > I can route it via bpf-next tree if there are no objections.
> > > Masahiro, could you please ack it?
> >
> >
> > Honestly, I was not tracking this thread
> > since I did not know I was responsible for this.
> >
> >
> > I just started to take a closer look, then immediately got scared.
> >
> > This version is not mature enough for the merge.
> >
> >
> >
> > First of all, this patch cannot be compiled out-of-tree (O= option).
> >
> >
> > I do not know why 0-day bot did not catch this apparent breakage.
> >
> >
> > $ make -j8 O=hoge
> > make[1]: Entering directory '/home/masahiro/workspace/bsp/linux/hoge'
> >   GEN     Makefile
> >   Using .. as source for kernel
> >   DESCEND  objtool
> >   CALL    ../scripts/checksyscalls.sh
> >   CHK     include/generated/compile.h
> > make[2]: *** No rule to make target 'Module.symvers', needed by
> > 'kernel/kheaders_data.txz'.  Stop.
> > make[2]: *** Waiting for unfinished jobs....
> > /home/masahiro/workspace/bsp/linux/Makefile:1043: recipe for target
> > 'kernel' failed
> > make[1]: *** [kernel] Error 2
> > make[1]: *** Waiting for unfinished jobs....
> > make[1]: Leaving directory '/home/masahiro/workspace/bsp/linux/hoge'
> > Makefile:152: recipe for target 'sub-make' failed
> > make: *** [sub-make] Error 2
> 
> 
> 
> 
> I saw this build error for in-tree building as well.
> 
> We cannot build this from a pristine source tree.
> 
> For example, I observed the build error in the following procedure.
> 
> $ make mrproper
> $ make defconfig
>  Set CONFIG_IKHEADERS_PROC=y
> $ make
> 
> 
> 
> 
> Module.symvers is generated in the modpost stage
> (the very last stage of build).
> 
> But, vmlinux depends on kernel/kheaders_data.txz,
> which includes Module.symvers.
> 
> So, this is not so simple since it is a circular dependency...

I guess I was not building a pristine tree either and missed this circular
dependency :-/ . Any ideas on how we can fix the Module.symvers issue? One
idea is to reserve the space in the binaries, but only populate the space
reserved in the binary *after* the modpost stage, once the archive is ready..

thanks,

 - Joel