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

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