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    Subject[PATCH 1/2] Provide in-kernel headers for making it easy to extend the kernel
    Date
    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@joelfernandes.org>
    ---
    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

    ###
    # 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 += $(IKH_EXTRA)
    +ikh_file_list += scripts/
    +ikh_file_list += Makefile
    +ikh_file_list += Module.symvers
    +ifeq ($(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
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