Introduce in-kernel headers which are made available as an archive through proc (/proc/kheaders.tar.xz file). This archive makes it possible to run eBPF and other tracing programs that need to extend the kernel for tracing purposes without any dependency on the file system having headers.

A github PR is sent for the corresponding BCC patch at: https://github.com/iovisor/bcc/pull/2312

On Android and embedded systems, it is common to switch kernels but not have kernel headers available on the file system. Further once a different kernel is booted, any headers stored on the file system will no longer be useful. This is an issue even well known to distros. 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 best way to use this feature is by building it in. Several users have a need for this, when they switch debug kernels, they do not want to update the filesystem or worry about it where to store the headers on it. However, the feature is also buildable as a module in case the user desires it not being part of the kernel image. This makes it possible to load and unload the headers from memory on demand. A tracing program can load the module, do its operations, and then unload the module to save kernel memory. The total memory needed is 3.3MB.

By having the archive available at a fixed location independent of filesystem dependencies and conventions, all debugging tools can directly refer to the fixed location for the archive, without concerning with where the headers on a typical filesystem which significantly simplifies tooling that needs kernel headers.

The code to read the headers is based on /proc/config.gz code and uses the same technique to embed the headers.

Other approaches were discussed such as having an in-memory mountable filesystem, but that has drawbacks such as requiring an in-kernel xz decompressor which we don't have today, and requiring usage of 42 MB of kernel memory to host the decompressed headers at anytime. Also this approach is simpler than such approaches.

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

v6 -> v7: - Minor nits from Masahiro Yamada are addressed.

v5 -> v6: (Masahiro Yamada suggestions mostly) - Dropped support for module building. - Rebuild archive if script changes. - Move archive file list to script. - Move build script to kernel directory.

v4 -> v5: (v4 was Tested-by the following folks) Tested-by: qais.yousef@arm.com Tested-by: dietmar.eggemann@arm.com Tested-by: linux@manojrajarao.com (Thanks to Masahiro Yamada for several excellent suggestions) - used incbin instead of bin2c (Masahiro did similar idea) - added module.lds if ia64 otherwise ia64 may fail to build. - added clean-files rule to Makefile - removed strip-comments script and doing it inline - added set -e to header generated to die on errorsr - fixed a minor issue where find command was noisy. - removed unneeded tar.xz rule from kernel/.gitignore - added Tested-by tags from ARM folks.

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.

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

init/Kconfig | 10 +++++ kernel/.gitignore | 1 + kernel/Makefile | 10 +++++ kernel/gen_ikh_data.sh | 89 ++++++++++++++++++++++++++++++++++++++++++ kernel/kheaders.c | 74 +++++++++++++++++++++++++++++++++++ 5 files changed, 184 insertions(+) create mode 100755 kernel/gen_ikh_data.sh create mode 100644 kernel/kheaders.c

diff --git a/init/Kconfig b/init/Kconfig index 4592bf7997c0..47c0db6e63a5 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -580,6 +580,16 @@ 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" + 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 eBPF + tracing programs, or similar programs. If you build the headers as a + module, a module called kheaders.ko is built which can be loaded on-demand + to get access to the headers. + 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 6e699100872f..34d1e77ee9df 100644 --- a/kernel/.gitignore +++ b/kernel/.gitignore @@ -1,5 +1,6 @@ # # Generated files # +kheaders.md5 timeconst.h hz.bc diff --git a/kernel/Makefile b/kernel/Makefile index 6c57e78817da..12399614c350 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 @@ -121,3 +122,12 @@ $(obj)/configs.o: $(obj)/config_data.gz targets += config_data.gz $(obj)/config_data.gz: $(KCONFIG_CONFIG) FORCE $(call if_changed,gzip) + +$(obj)/kheaders.o: $(obj)/kheaders_data.tar.xz + +quiet_cmd_genikh = CHK $(obj)/kheaders_data.tar.xz +cmd_genikh = $(srctree)/kernel/gen_ikh_data.sh $@ +$(obj)/kheaders_data.tar.xz: FORCE + $(call cmd,genikh) + +clean-files := kheaders_data.tar.xz kheaders.md5 diff --git a/kernel/gen_ikh_data.sh b/kernel/gen_ikh_data.sh new file mode 100755 index 000000000000..591a94f7b387 --- /dev/null +++ b/kernel/gen_ikh_data.sh @@ -0,0 +1,89 @@ +#!/bin/bash +# SPDX-License-Identifier: GPL-2.0 + +# This script generates an archive consisting of kernel headers +# for CONFIG_IKHEADERS_PROC. +set -e +spath="$(dirname "$(readlink -f "$0")")" +kroot="$spath/.." +outdir="$(pwd)" +tarfile=$1 +cpio_dir=$outdir/$tarfile.tmp + +# Script filename relative to the kernel source root +# We add it to the archive because it is small and any changes +# to this script will also cause a rebuild of the archive. +sfile="$(realpath --relative-to $kroot "$(readlink -f "$0")")" + +src_file_list=" +include/ +arch/$SRCARCH/include/ +$sfile +" + +obj_file_list=" +include/ +arch/$SRCARCH/include/ +" + +# 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 + +# include/generated/compile.h is ignored because it is 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 $src_file_list -type f | + grep -v "include/generated/compile.h" | + xargs ls -lR | md5sum | cut -d ' ' -f1)" +popd > /dev/null +obj_files_md5="$(find $obj_file_list -type f | + 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)" == "$tarfile_md5" ]; then + exit +fi + +if [ "${quiet}" != "silent_" ]; then + echo " GEN $tarfile" +fi + +rm -rf $cpio_dir +mkdir $cpio_dir + +pushd $kroot > /dev/null +for f in $src_file_list; + do find "$f" ! -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 "*.cmd" ! -name ".*"; +done | cpio --quiet -pd $cpio_dir >/dev/null 2>&1 + +# Remove comments except SDPX lines +find $cpio_dir -type f -print0 | + xargs -0 -P8 -n1 perl -pi -e 'BEGIN {undef $/;}; s//*((?!SPDX).)*?*///smg;' + +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/kernel/kheaders.c b/kernel/kheaders.c new file mode 100644 index 000000000000..19526294a4e8 --- /dev/null +++ b/kernel/kheaders.c @@ -0,0 +1,74 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Provide kernel headers useful to build tracing programs + * such as for running eBPF tracing tools. + * + * (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_end, within which the the + * compressed kernel headers are stpred. The file is first compressed with xz. + */ + +asm ( +" .pushsection .rodata, "a" \n" +" .global kernel_headers_data \n" +"kernel_headers_data: \n" +" .incbin "kernel/kheaders_data.tar.xz" \n" +" .global kernel_headers_data_end \n" +"kernel_headers_data_end: \n" +" .popsection \n" +); + +extern char kernel_headers_data; +extern char kernel_headers_data_end; + +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, + &kernel_headers_data_end - + &kernel_headers_data); +} + +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_end - + &kernel_headers_data); + 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");