I am also thinking, what is the difference between "make the bootloader load the erofs into contiguous memory" part and doing something like storage-init. They are similar approaches, introduce something in the middle to handle the erofs. Is mise le meas/Regards, Eric Curtin On Mon, 11 Dec 2023 at 11:28, Eric Curtin <ecurtin@xxxxxxxxxx> wrote: > > On Mon, 11 Dec 2023 at 11:20, Eric Curtin <ecurtin@xxxxxxxxxx> wrote: > > > > On Mon, 11 Dec 2023 at 10:06, Lennart Poettering <mzerqung@xxxxxxxxxxx> wrote: > > > > > > On Fr, 08.12.23 17:59, Eric Curtin (ecurtin@xxxxxxxxxx) wrote: > > > > > > > Here is the boot sequence with initoverlayfs integrated, the > > > > mini-initramfs contains just enough to get storage drivers loaded and > > > > storage devices initialized. storage-init is a process that is not > > > > designed to replace init, it does just enough to initialize storage > > > > (performs a targeted udev trigger on storage), switches to > > > > initoverlayfs as root and then executes init. > > > > > > > > ``` > > > > fw -> bootloader -> kernel -> mini-initramfs -> initoverlayfs -> rootfs > > > > > > > > fw -> bootloader -> kernel -> storage-init -> init -----------------> > > > > ``` > > > > > > I am not sure I follow what these chains are supposed to mean? Why are > > > there two lines? > > > > The top line is the filesystem transition, the bottom is more like a > > process perspective. Will make this clearer in future. > > > > > > > > So, I generally would agree that the current initrd scheme is not > > > ideal, and we have been discussing better approaches. But I am not > > > sure your approach really is useful on generic systems for two > > > reasons: > > > > > > 1. no security model? you need to authenticate your initrd in > > > 2023. There's no execuse to not doing that anymore these days. Not > > > in automotive, and not anywhere else really. > > > > Yes you are right, there is no excuse, the plan was to mount using > > dm-verity most likely with the details from the initramfs, but > > admittedly we had not looked into that into great detail. > > > > > > > > 2. no way to deal with complex storage? i.e. people use FDE, want to > > > unlock their root disks with TPM2 and similar things. People use > > > RAID, LVM, and all that mess. > > > > We had 3 thoughts on this: > > > > 1. Just worry about the common use-cases and leave everyone else > > fallback to the approaches we use today. > > 2. Try and split up systemd to make it even smaller. We do use > > systemd-udev in the small initramfs storage-init process so far. > > 3. Reimplement some things? But as little as possible, on a case by > > case basis, we certainly don't want to fall into the trap of rewriting > > systemd that's for sure, systemd does these things very well. > > > > Tbh, if we try and implement this in kernelspace a lot of these > > questions go away. You just teach the kernel to deal with the > > filesystem image early (say erofs or whatever other filesystem) and > > have that data where initramfs data currently is. You still pay for > > the initial read, but you still save a bunch of kernel time. > > > > > > > > Actually the above are kinda the same problem in a way: you need > > > complex storage, but if you need that you kinda need udev, and > > > services, and then also systemd and all that other stuff, and that's > > > why the system works like the system works right now. > > > > True, but there is also a bunch of stuff in current initrd's today > > that aren't required to mount basic storage, but are designed around > > the whole idea of having an early throwaway filesystem. > > > > > > > > Whenever you devise a system like yours by cutting corners, and > > > declaring that you don't want TPM, you don't want signed initrds, you > > > don't want to support weird storage, you just solve your problem in a > > > very specific way, ignoring the big picture. Which is OK, *if* you can > > > actually really work without all that and are willing to maintain the > > > solution for your specific problem only. > > > > > > As I understand you are trying to solve multiple problems at once > > > here, and I think one should start with figuring out clearly what > > > those are before trying to address them, maybe without compromising on > > > security. So my guess is you want to address the following: > > > > > > 1. You don't want the whole big initrd to be read off disk on every > > > boot, but only the parts of it that are actually needed. > > > > > > 2. You don't want the whole big initrd to be fully decompressed on every > > > boot, but only the parts of it that are actually needed. > > > > > > 3. You want to share data between root fs and initrd > > > > > > 4. You want to save some boot time by not bringing up an init system > > > in the initrd once, then tearing it down again, and starting it > > > again from the root fs. > > > > It's mainly the top 3 that were the goals. And that people have the > > freedom to consider using heavier weight generic libraries, tools, > > etc. if they want. You want to use Rust (or languages X, Y, Z) to > > write something early boot, go ahead! You'll only pay the cost for the > > larger binary if you actually use it. The week I started tinkering at > > this, there was a mini-debate on whether we should include glib or not > > in the initrd. And we are regularly under pressure to reduce boot time > > at the moment. > > > > Number 4 was a convenient way to do an early version of this, stick a > > process in between systemd and the kernel. But it turns out, it works > > very well, the only problem is the reimplementation problem really. > > > > Theoretically this could be systemd-storage-init -> systemd also. Or > > systemd and dlopen more libraries as they become available later down > > the line. > > > > > > > > For the items listed above I think you can find different solutions > > > which do not necessarily compromise security as much. > > > > > > So, in the list above you could address the latter three like this: > > > > > > 2. Use an erofs rather than a packed cpio as initrd. Make the boot > > > loader load the erofs into contigous memory, then use memmap=X!Y on > > > the kernel cmdline to synthesize a block device from that, which > > > you then mount directly (without any initrd) via > > > root=/dev/pmem0. This means yout boot loader will still load the > > > whole image into memory, but only decompress the bits actually > > > neeed. (It also has some other nice benefits I like, such as an > > > immutable rootfs, which tmpfs-based initrds don't have.) > > What I am unsure about here, is the "make the bootloader load the > erofs into contiguous memory" part. I wonder could we try and use the > existing initramfs data as is. I dunno if > bootloaders make much assumptions about the format of that data, worst > case scenario we could encapsulate erofs in the initramfs, cpio looking > data. Teach the kernel not to decompress and process the whole > thing and mount it like an erofs alternatively. Does this sound crazy > or reasonable? > Sometimes you cannot change the code in a bootloader and it would be > nice if we could avoid introducing another layer of bootloader. > > > > > > Yes, lets explore this approach with the kernel community to gather > > their thoughts. I'm still happy I did the userspace version first, > > even if we end up doing it in kernelspace because it allowed me to > > test on various pieces of hardware to see if the benefits are genuine > > and they are.... > > > > > > > > 3. Simply never transition to the root fs, don't marke the initrds in > > > systemd's eyes as an initrd (specifically: don't add an > > > /etc/initrd-release file to it). Instead, just merge resources of > > > the root fs into your initrd fs via overlayfs. systemd has > > > infrastructure for this: "systemd-sysext". It takes immutable, > > > authenticated erofs images (with verity, we call them "DDIs", > > > i.e. "discoverable disk images") that it overlays into /usr/. [You > > > could also very nicely combine this approach with systemd's > > > portable services, and npsawn containers, which operate on the same > > > authenticated images]. At MSFT we have a major product that works > > > exactly like this: the OS runs off a rootfs that is loaded as an > > > initrd, and everything that runs on top of this are just these > > > verity disk images, using overlayfs and portable services. > > > > > > 4. The proposal in 3 also addresses goal 4. > > > > > > > I'm hoping we can benefit both use cases, the case where you want to > > transition to a rootfs and the case where you never want to transition > > to a rootfs. > > > > > Which leaves item 1, which is a bit harder to address. We have been > > > discussing this off an on internally too. A generic solution to this > > > is hard. My current thinking for this could be something like this, > > > covering the UEFI world: support sticking a DDI for the main initrd in > > > the ESP. The ESP is per definition unencrypted and unauthenticated, > > > but otherwise relatively well defined, i.e. known to be vfat and > > > discoverable via UUID on a GPT disk. So: build a minimal > > > single-process initrd into the kernel (i.e. UKI) that has exactly the > > > storage to find a DDI on the ESP, and set it up. i.e. vfat+erofs fs > > > drivers, and dm-verity. Then have a PID 1 that does exactly enough to > > > jump into the rootfs stored in the ESP. That latter then has proper > > > file system drivers, storage drivers, crypto stack, and can unlock the > > > real root. This would still be a pretty specific solution to one set > > > of devices though, as it could not cover network boots (i.e. where > > > there is just no ESP to boot from), but I think this could be kept > > > relatively close, as the logic in that case could just fall back into > > > loading the DDI that normally would still in the ESP fully into > > > memory. > > > > > > > I'm certainly a little biased here because I work with ARM, I would > > like it to be UEFI world, but it's not and convincing every SoC vendor > > you must use UEFI is hard. I know a UEFI covering solution only would > > not have much value for my team at least. > > > > > (If you are focussing on systems lacking UEFI, then replace the word > > > "ESP" in the above with a similar concept, i.e. a well discoverable, > > > unauthenticated relatively simple file system, such as vfat). > > > > Yeah, agree, this baseline, I think, is common enough to assume. Like > > Android Boot Images as an example are basically a UKI binary stuff in > > a boot partition. > > > > > > > > Anyway, I can't tell you how to solve your specific problems, but if > > > there's one thing I'd suggest you to keep in mind then it's the > > > security angle, i.e. keep in mind from the beginning how > > > authentication of every component of your process shall work, how > > > unatteneded disk encryption shall operate and how measurement shall > > > work. Security must be built into things from the beginning, not be > > > added as an afterthought. > > > > Yes and we certainly want something that fits with the UKI models and > > the other commonplace models around. > > > > > > > > Lennart > > > > > > -- > > > Lennart Poettering, Berlin > > >
