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

From: Masahiro Yamada
Date: Mon Feb 18 2019 - 23:14:59 EST

On Fri, Feb 15, 2019 at 11:48 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
> On Mon, Feb 11, 2019 at 09:35:59AM -0500, Joel Fernandes (Google) wrote:
> > Introduce in-kernel headers and other artifacts which are made available
> > as an archive through proc (/proc/kheaders.txz file).

The extension '.txz' is not used in kernel code.

'.tar.xz' is used for 'tarxz-pkg', 'perf-tarxz-src-pkg' etc.

$ git grep '\.txz'
$ git grep '\.tar\.xz'
Documentation/admin-guide/README.rst: xz -cd linux-4.X.tar.xz | tar xvf -
arch/x86/crypto/camellia-aesni-avx-asm_64.S: *
crypto/testmgr.h: *
crypto/testmgr.h: *
crypto/testmgr.h: *
crypto/testmgr.h: *
crypto/testmgr.h: *
scripts/package/Makefile: @echo ' perf-tarxz-src-pkg - Build
$(perf-tar).tar.xz source tarball'

I prefer '.tar.xz' for consistency.

BTW, have you ever looked at scripts/extract-ikconfig?

You added IKHD_ST and IKHD_ED
just to mimic kernel/configs.c

It is currently pointless without the extracting tool,
but you might think it is useful to extract headers
from vmlinux or the module without mounting procfs.

> > 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/
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 was able to compile it in-tree
but it makes the incremental build extremely slow.

(Here, the incremental build means
"make" without changing any code after the full build.)

Before this patch, "make -j8" took 11 sec on my machine.

real 0m11.777s
user 0m16.608s
sys 0m5.164s

After this patch, x86_64_defconfig + CONFIG_IKHEADERS_PROC=y
takes 53 sec for me since kernel/kheaders_data.txz is regenerated
every time even when you did not touch any source file.

$ time make -j8
DESCEND objtool
CALL scripts/
CHK include/generated/compile.h
GEN kernel/kheaders_data.txz
UPD kernel/kheaders_data.h
CC kernel/kheaders.o
AR kernel/built-in.a
GEN .version
CHK include/generated/compile.h
UPD include/generated/compile.h
CC init/version.o
AR init/built-in.a
AR built-in.a
LD vmlinux.o
MODPOST vmlinux.o
KSYM .tmp_kallsyms1.o
KSYM .tmp_kallsyms2.o
LD vmlinux
SORTEX vmlinux
Building modules, stage 2.
CC arch/x86/boot/version.o
MODPOST 17 modules
VOFFSET arch/x86/boot/compressed/../voffset.h
OBJCOPY arch/x86/boot/compressed/vmlinux.bin
RELOCS arch/x86/boot/compressed/vmlinux.relocs
CC arch/x86/boot/compressed/kaslr.o
GZIP arch/x86/boot/compressed/vmlinux.bin.gz
CC arch/x86/boot/compressed/misc.o
MKPIGGY arch/x86/boot/compressed/piggy.S
AS arch/x86/boot/compressed/piggy.o
LD arch/x86/boot/compressed/vmlinux
ZOFFSET arch/x86/boot/zoffset.h
OBJCOPY arch/x86/boot/vmlinux.bin
AS arch/x86/boot/header.o
LD arch/x86/boot/setup.elf
OBJCOPY arch/x86/boot/setup.bin
BUILD arch/x86/boot/bzImage
Setup is 15612 bytes (padded to 15872 bytes).
System is 12673 kB
CRC 697aaf88
Kernel: arch/x86/boot/bzImage is ready (#6)

real 0m53.024s
user 0m32.076s
sys 0m9.296s

Also, I notice $(ARCH) must be fixed to $(SRCARCH),
but that is one of minor issues.

We should take time for careful review and test.

Please give me more time for thorough review.

Best Regards
Masahiro Yamada