On Mon, Apr 08, 2019 at 09:29:30AM -0700, Olof Johansson wrote:
Hi,
On Wed, Mar 20, 2019 at 9:31 AM Joel Fernandes (Google) joel@joelfernandes.org 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. 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 best way to use this feature is by building it in. Several users have a need for this, when they switch debug kernels, they donot 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, 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.
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.
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) If module building is used, 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.
(v4 was Tested-by the following folks, v5 only has minor changes and has passed my testing). Tested-by: qais.yousef@arm.com Tested-by: dietmar.eggemann@arm.com Tested-by: linux@manojrajarao.com Signed-off-by: Joel Fernandes (Google) joel@joelfernandes.org
Sorry to be late at the party with this kind of feedback, but I find the whole ".tar.gz in procfs" to be an awkward solution, especially if there's expected to be userspace tooling that depends on this long-term.
No problem, your feedback is welcome.
Wouldn't it be more convenient to provide it in a standardized format such that you won't have to take an additional step, and always have This is that form IMO.
The location of the archive is fixed/known. If you are talking of the location where the user decompresses it to, then they a;ready know where they are decompressing to.
Something like:
- Pseudo-filesystem, that can just be mounted under
/sys/kernel/headers or something (similar to debugfs or /proc/device-tree).
The headers are huge if uncompressed (~30MB). Currently we use xz compression in the archive. It would be a huge waste to decompress everything into memory such as through an in-memory filesystem. And compressing on a per-file basis would be too slow for build time. Currently the build of the archive is extrememly fast.
- Exporting something like a squashfs image instead, allowing
loopback mounting of it (or by providing a pseudo-/dev entry for it), again allowing direct export of the contents and avoiding the extracted directory from being out of sync with currently running kernel.
One drawback of squashfs (other than possibly the compression ratio) is that this would be kernel build unfriendly in comparison to tar+xz. On my machine, squashfs-tools needed to be installed. For users who don't have this package, that would break their kernel build.
Having to copy and extract the tarball is the most awkward step, IMHO. I also find the waste of kernel memory for it to be an issue, but given that it can be built as a module I guess that's the obvious solution for those who care about memory consumption.
Yes. We discussed in previous threads that for users who really want the archive to be completely uncompressed and in-memory, can just load the module, decompress into tmpfs, and unload the module. That is an extra step, yes.
We had close to 2-3 months of discussions now with various folks up until v5. I am about to post v6 which is in line with Masahiro Yamada's expecations. In that I will be dropping module building artifacts due to his module building concerns and only include the headers.
thanks,
- Joel