Re: [PATCH v4 1/2] Provide in-kernel headers for making it easy to extend the kernel

On 03/01/19 11:08, Joel Fernandes (Google) wrote:

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.

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.tar.xz -C $HOME/headers >/dev/null cd my-kernel-module make -C $HOME/headers M=$(pwd) modules rmmod kheaders

Additional notes: (1) external modules must be built on the same arch as the host that built vmlinux. This can be done either in a qemu emulated chroot on the target, or natively. This is due to host arch dependency of kernel scripts.

(2) A limitation of module building with this is, since Module.symvers is not available in the archive due to a cyclic dependency with building of the archive into the kernel or module binaries, the modules built using the archive will not contain symbol versioning (modversion). This is usually not an issue since the idea of this patch is to build a kernel module on the fly and load it into the same kernel. An appropriate warning is already printed by the kernel to alert the user of modules not having modversions when built using the archive. For building with modversions, the user can use traditional header packages. For our tracing usecases, we build modules on the fly with this so it is not a concern.

(3) I have left IKHD_ST and IKHD_ED markers as is to facilitate future patches that would extract the headers from a kernel or module image.

Signed-off-by: Joel Fernandes (Google) joel@joelfernandes.org

I could get the headers using this patch in both built-in and modules options.

You can add my tested-and-reviewed-by: Qais Yousef qais.yousef@arm.com

I am not familiar with running kselftests so didn't get a chance to try the next patch.

Thanks

-- Qais Yousef

---

Changes since v3:
- Blank tar was being generated because of a one line I
  forgot to push. It is updated now.
- Added module.lds since arm64 needs it to build modules.

Changes since v2:
(Thanks to Masahiro Yamada for several excellent suggestions)
- Added support for out of tree builds.
- Added incremental build support bringing down build time of
  incremental builds from 50 seconds to 5 seconds.
- Fixed various small nits / cleanups.
- clean ups to kheaders.c pointed by Alexey Dobriyan.
- Fixed MODULE_LICENSE in test module and kheaders.c
- Dropped Module.symvers from archive due to circular dependency.

Changes since v1:
- removed IKH_EXTRA variable, not needed (Masahiro Yamada)
- small fix ups to selftest
   - added target to main Makefile etc
   - added MODULE_LICENSE to test module
   - made selftest more quiet

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.

Other notes: By the way I still see this error (without the patch) when doing a clean build: Makefile:594: include/config/auto.conf: No such file or directory

It appears to be because of commit 0a16d2e8cb7e ("kbuild: use 'include' directive to load auto.conf from top Makefile")

Documentation/dontdiff | 1 + init/Kconfig | 11 ++++++ kernel/.gitignore | 3 ++ kernel/Makefile | 37 +++++++++++++++++++ kernel/kheaders.c | 72 ++++++++++++++++++++++++++++++++++++ scripts/gen_ikh_data.sh | 78 +++++++++++++++++++++++++++++++++++++++ scripts/strip-comments.pl | 8 ++++ 7 files changed, 210 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/init/Kconfig b/init/Kconfig index c9386a365eea..63ff0990ae55 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -563,6 +563,17 @@ 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.tar.xz"
• 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..484018945e93 100644 --- a/kernel/.gitignore +++ b/kernel/.gitignore @@ -3,5 +3,8 @@ # config_data.h config_data.gz +kheaders.md5 +kheaders_data.h +kheaders_data.tar.xz timeconst.h hz.bc diff --git a/kernel/Makefile b/kernel/Makefile index 6aa7543bcdb2..240685a6b638 100644 --- a/kernel/Makefile +++ b/kernel/Makefile @@ -70,6 +70,7 @@ 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 @@ -130,3 +131,39 @@ filechk_ikconfiggz = \ 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 +ikh_file_list := include/ +ikh_file_list += arch/$(SRCARCH)/Makefile +ikh_file_list += arch/$(SRCARCH)/include/ +ikh_file_list += arch/$(SRCARCH)/kernel/module.lds +ikh_file_list += scripts/ +ikh_file_list += Makefile

+# Things we need from the $objtree. "OBJDIR" is for the gen_ikh_data.sh +# script to identify that this comes from the $objtree directory +ikh_file_list += OBJDIR/scripts/ +ikh_file_list += OBJDIR/include/ +ikh_file_list += OBJDIR/arch/$(SRCARCH)/include/ +ifeq ($(CONFIG_STACK_VALIDATION), y) +ikh_file_list += OBJDIR/tools/objtool/objtool +endif

+$(obj)/kheaders.o: $(obj)/kheaders_data.h

+targets += kheaders_data.tar.xz

+quiet_cmd_genikh = GEN $(obj)/kheaders_data.tar.xz +cmd_genikh = $(srctree)/scripts/gen_ikh_data.sh $@ $(ikh_file_list) +$(obj)/kheaders_data.tar.xz: 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 +targets += kheaders.md5 +$(obj)/kheaders_data.h: $(obj)/kheaders_data.tar.xz FORCE

• $(call filechk,ikheadersxz)

diff --git a/kernel/kheaders.c b/kernel/kheaders.c new file mode 100644 index 000000000000..46a6358301e5 --- /dev/null +++ b/kernel/kheaders.c @@ -0,0 +1,72 @@ +// 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/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 = {

• .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.tar.xz", 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.tar.xz", NULL);

+}

+module_init(ikheaders_init); +module_exit(ikheaders_cleanup);

+MODULE_LICENSE("GPL v2"); +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..1fa5628fcc30 --- /dev/null +++ b/scripts/gen_ikh_data.sh @@ -0,0 +1,78 @@ +#!/bin/bash +# SPDX-License-Identifier: GPL-2.0

+spath="$(dirname "$(readlink -f "$0")")" +kroot="$spath/.." +outdir="$(pwd)" +tarfile=$1 +cpio_dir=$outdir/$tarfile.tmp

+file_list=${@:2}

+src_file_list="" +for f in $file_list; do

• src_file_list="$src_file_list $(echo $f | grep -v OBJDIR)"

+done

+obj_file_list="" +for f in $file_list; do

• f=$(echo $f | grep OBJDIR | sed -e 's/OBJDIR///g')
• obj_file_list="$obj_file_list $f";

+done

+# Support incremental builds by skipping archive generation +# if timestamps of files being archived are not changed.

+# This block is useful for debugging the incremental builds. +# Uncomment it for debugging. +# iter=1 +# if [ ! -f /tmp/iter ]; then echo 1 > /tmp/iter; +# else; iter=$(($(cat /tmp/iter) + 1)); fi +# find $src_file_list -type f | xargs ls -lR > /tmp/src-ls-$iter +# find $obj_file_list -type f | xargs ls -lR > /tmp/obj-ls-$iter

+# modules.order and include/generated/compile.h are ignored because these are +# touched even when none of the source files changed. This causes pointless +# regeneration, so let us ignore them for md5 calculation. +pushd $kroot > /dev/null +src_files_md5="$(find $src_file_list -type f ! -name modules.order |

• grep -v "include/generated/compile.h"		   |
  

• xargs ls -lR | md5sum | cut -d ' ' -f1)"
  

+popd > /dev/null +obj_files_md5="$(find $obj_file_list -type f ! -name modules.order |

• grep -v "include/generated/compile.h"		   |
  

• xargs ls -lR | md5sum | cut -d ' ' -f1)"
  

+if [ -f $tarfile ]; then tarfile_md5="$(md5sum $tarfile | cut -d ' ' -f1)"; fi +if [ -f kernel/kheaders.md5 ] &&

• [ "$(cat kernel/kheaders.md5|head -1)" == "$src_files_md5" ] &&
• [ "$(cat kernel/kheaders.md5|head -2|tail -1)" == "$obj_files_md5" ] &&
• [ "$(cat kernel/kheaders.md5|tail -1)" == "$tarfile_md5" ]; then

• exit
  

+fi

+rm -rf $cpio_dir +mkdir $cpio_dir

+pushd $kroot > /dev/null +for f in $src_file_list;

• do find "$f" ! -name "*.c" ! -name "*.o" ! -name "*.cmd" ! -name ".*";

+done | cpio --quiet -pd $cpio_dir +popd > /dev/null

+# The second CPIO can complain if files already exist which can +# happen with out of tree builds. Just silence CPIO for now. +for f in $obj_file_list;

• do find "$f" ! -name "*.c" ! -name "*.o" ! -name "*.cmd" ! -name ".*";

+done | cpio --quiet -pd $cpio_dir >/dev/null 2>&1

+find $cpio_dir -type f -print0 |

• xargs -0 -P8 -n1 -I {} sh -c "$spath/strip-comments.pl {}"

+tar -Jcf $tarfile -C $cpio_dir/ . > /dev/null

+echo "$src_files_md5" > kernel/kheaders.md5 +echo "$obj_files_md5" >> kernel/kheaders.md5 +echo "$(md5sum $tarfile | cut -d ' ' -f1)" >> kernel/kheaders.md5

+rm -rf $cpio_dir 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;

2.21.0.352.gf09ad66450-goog