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

[Masahiro Yamada]

On Fri, Feb 8, 2019 at 6:13 AM Joel Fernandes (Google)
<joel@xxxxxxxxxxxxxxxxx> wrote:
>
> Introduce in-kernel headers and other artifacts which are made available
> as an archive through proc (/proc/kheaders.txz 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.
>
> To build a module, the below steps have been tested on an x86 machine:
> modprobe kheaders
> rm -rf $HOME/headers
> mkdir -p $HOME/headers
> tar -xvf /proc/kheaders.txz -C $HOME/headers >/dev/null
> cd my-kernel-module
> make -C $HOME/headers M=$(pwd) modules
> rmmod kheaders
>
> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---
> Changes since RFC:
> Both changes bring size down to 3.8MB:
> - use xz for compression
> - strip comments except SPDX lines
> - Call out the module name in Kconfig
> - Also added selftests in second patch to ensure headers are always
> working.
>
>  Documentation/dontdiff    |  1 +
>  arch/x86/Makefile         |  2 ++
>  init/Kconfig              | 11 ++++++
>  kernel/.gitignore         |  2 ++
>  kernel/Makefile           | 29 +++++++++++++++
>  kernel/kheaders.c         | 74 +++++++++++++++++++++++++++++++++++++++
>  scripts/gen_ikh_data.sh   | 19 ++++++++++
>  scripts/strip-comments.pl |  8 +++++
>  8 files changed, 146 insertions(+)
>  create mode 100644 kernel/kheaders.c
>  create mode 100755 scripts/gen_ikh_data.sh
>  create mode 100755 scripts/strip-comments.pl
>
> diff --git a/Documentation/dontdiff b/Documentation/dontdiff
> index 2228fcc8e29f..05a2319ee2a2 100644
> --- a/Documentation/dontdiff
> +++ b/Documentation/dontdiff
> @@ -151,6 +151,7 @@ int8.c
>  kallsyms
>  kconfig
>  keywords.c
> +kheaders_data.h*
>  ksym.c*
>  ksym.h*
>  kxgettext
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 88398fdf8129..ad176d669da4 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -240,6 +240,8 @@ archmacros:
>  ASM_MACRO_FLAGS = -Wa,arch/x86/kernel/macros.s
>  export ASM_MACRO_FLAGS
>  KBUILD_CFLAGS += $(ASM_MACRO_FLAGS)
> +IKH_EXTRA += arch/x86/kernel/macros.s
> +export IKH_EXTRA


This does not exist in any of released kernels.

See commit 6ac389346e6



>
>  ###
>  # Kernel objects
> diff --git a/init/Kconfig b/init/Kconfig
> index a4112e95724a..b95d769b6098 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -549,6 +549,17 @@ config IKCONFIG_PROC
>           This option enables access to the kernel configuration file
>           through /proc/config.gz.
>
> +config IKHEADERS_PROC
> +       tristate "Enable kernel header artifacts through /proc/kheaders.txz"
> +       select BUILD_BIN2C
> +       depends on PROC_FS
> +       help
> +         This option enables access to the kernel header and other artifacts that
> +          are generated during the build process. These can be used to build kernel
> +          modules, and other in-kernel programs such as those generated by eBPF
> +          and systemtap tools. If you build the headers as a module, a module
> +          called kheaders.ko is built which can be loaded to get access to them.
> +
>  config LOG_BUF_SHIFT
>         int "Kernel log buffer size (16 => 64KB, 17 => 128KB)"
>         range 12 25
> diff --git a/kernel/.gitignore b/kernel/.gitignore
> index b3097bde4e9c..6acf71acbdcb 100644
> --- a/kernel/.gitignore
> +++ b/kernel/.gitignore
> @@ -3,5 +3,7 @@
>  #
>  config_data.h
>  config_data.gz
> +kheaders_data.h
> +kheaders_data.txz
>  timeconst.h
>  hz.bc
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 7343b3a9bff0..aa2d3f9b9f49 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -73,6 +73,7 @@ obj-$(CONFIG_UTS_NS) += utsname.o
>  obj-$(CONFIG_USER_NS) += user_namespace.o
>  obj-$(CONFIG_PID_NS) += pid_namespace.o
>  obj-$(CONFIG_IKCONFIG) += configs.o
> +obj-$(CONFIG_IKHEADERS_PROC) += kheaders.o
>  obj-$(CONFIG_SMP) += stop_machine.o
>  obj-$(CONFIG_KPROBES_SANITY_TEST) += test_kprobes.o
>  obj-$(CONFIG_AUDIT) += audit.o auditfilter.o
> @@ -131,3 +132,31 @@ $(obj)/config_data.gz: $(KCONFIG_CONFIG) FORCE
>  targets += config_data.h
>  $(obj)/config_data.h: $(obj)/config_data.gz FORCE
>         $(call filechk,ikconfiggz)
> +
> +# Build a list of in-kernel headers for building kernel modules
> +# Any other files will be stored in IKH_EXTRA variable.
> +ikh_file_list := include/
> +ikh_file_list += arch/$(ARCH)/Makefile
> +ikh_file_list += arch/$(ARCH)/include/
> +ikh_file_list += $(IKH_EXTRA)

IKH_EXTRA is unneeded.


> +ikh_file_list += scripts/
> +ikh_file_list += Makefile
> +ikh_file_list += Module.symvers
> +ifeq ($(CONFIG_STACK_VALIDATION), y)
> +ikh_file_list += $(objtree)/tools/objtool/objtool
> +endif
> +
> +$(obj)/kheaders.o: $(obj)/kheaders_data.h
> +
> +targets += kheaders_data.txz
> +
> +quiet_cmd_genikh = GEN     $(obj)/kheaders_data.txz
> +cmd_genikh = $(srctree)/scripts/gen_ikh_data.sh $@ $^ >/dev/null 2>&1
> +$(obj)/kheaders_data.txz: $(ikh_file_list) FORCE
> +       $(call cmd,genikh)
> +
> +filechk_ikheadersxz = (echo "static const char kernel_headers_data[] __used = KH_MAGIC_START"; cat $< | scripts/bin2c; echo "KH_MAGIC_END;")
> +
> +targets += kheaders_data.h
> +$(obj)/kheaders_data.h: $(obj)/kheaders_data.txz FORCE
> +       $(call filechk,ikheadersxz)
> diff --git a/kernel/kheaders.c b/kernel/kheaders.c
> new file mode 100644
> index 000000000000..c39930f51202
> --- /dev/null
> +++ b/kernel/kheaders.c
> @@ -0,0 +1,74 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * kernel/kheaders.c
> + * Provide headers and artifacts needed to build kernel modules.
> + * (Borrowed code from kernel/configs.c)
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/proc_fs.h>
> +#include <linux/seq_file.h>
> +#include <linux/init.h>
> +#include <linux/uaccess.h>
> +
> +/*
> + * Define kernel_headers_data and kernel_headers_data_size, which contains the
> + * compressed kernel headers.  The file is first compressed with xz and then
> + * bounded by two eight byte magic numbers to allow extraction from a binary
> + * kernel image:
> + *
> + *   IKHD_ST
> + *   <image>
> + *   IKHD_ED
> + */
> +#define KH_MAGIC_START "IKHD_ST"
> +#define KH_MAGIC_END   "IKHD_ED"
> +#include "kheaders_data.h"
> +
> +
> +#define KH_MAGIC_SIZE (sizeof(KH_MAGIC_START) - 1)
> +#define kernel_headers_data_size \
> +       (sizeof(kernel_headers_data) - 1 - KH_MAGIC_SIZE * 2)
> +
> +static ssize_t
> +ikheaders_read_current(struct file *file, char __user *buf,
> +                     size_t len, loff_t *offset)
> +{
> +       return simple_read_from_buffer(buf, len, offset,
> +                                      kernel_headers_data + KH_MAGIC_SIZE,
> +                                      kernel_headers_data_size);
> +}
> +
> +static const struct file_operations ikheaders_file_ops = {
> +       .owner = THIS_MODULE,
> +       .read = ikheaders_read_current,
> +       .llseek = default_llseek,
> +};
> +
> +static int __init ikheaders_init(void)
> +{
> +       struct proc_dir_entry *entry;
> +
> +       /* create the current headers file */
> +       entry = proc_create("kheaders.txz", S_IFREG | S_IRUGO, NULL,
> +                           &ikheaders_file_ops);
> +       if (!entry)
> +               return -ENOMEM;
> +
> +       proc_set_size(entry, kernel_headers_data_size);
> +
> +       return 0;
> +}
> +
> +static void __exit ikheaders_cleanup(void)
> +{
> +       remove_proc_entry("kheaders.txz", NULL);
> +}
> +
> +module_init(ikheaders_init);
> +module_exit(ikheaders_cleanup);
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Joel Fernandes");
> +MODULE_DESCRIPTION("Echo the kernel header artifacts used to build the kernel");
> diff --git a/scripts/gen_ikh_data.sh b/scripts/gen_ikh_data.sh
> new file mode 100755
> index 000000000000..609196b5cea2
> --- /dev/null
> +++ b/scripts/gen_ikh_data.sh
> @@ -0,0 +1,19 @@
> +#!/bin/bash
> +# SPDX-License-Identifier: GPL-2.0
> +
> +spath="$(dirname "$(readlink -f "$0")")"
> +
> +rm -rf $1.tmp
> +mkdir $1.tmp
> +
> +for f in "${@:2}";
> +       do find "$f" ! -name "*.c" ! -name "*.o" ! -name "*.cmd" ! -name ".*";
> +done | cpio -pd $1.tmp
> +
> +for f in $(find $1.tmp); do
> +       $spath/strip-comments.pl $f
> +done
> +
> +tar -Jcf $1 -C $1.tmp/ . > /dev/null
> +
> +rm -rf $1.tmp
> diff --git a/scripts/strip-comments.pl b/scripts/strip-comments.pl
> new file mode 100755
> index 000000000000..f8ada87c5802
> --- /dev/null
> +++ b/scripts/strip-comments.pl
> @@ -0,0 +1,8 @@
> +#!/usr/bin/perl -pi
> +# SPDX-License-Identifier: GPL-2.0
> +
> +# This script removes /**/ comments from a file, unless such comments
> +# contain "SPDX". It is used when building compressed in-kernel headers.
> +
> +BEGIN {undef $/;}
> +s/\/\*((?!SPDX).)*?\*\///smg;
> --
> 2.20.1.611.gfbb209baf1-goog



-- 
Best Regards
Masahiro Yamada