On Friday 13 July 2007 2:56:00 pm Bodo Eggert wrote: > I toyed with setting up a diskless system in initramfs. In the process, I > came across some things: > > 1) There is no way to have the kernel not mount a filesystem, > unless you use /init or rdinit=.
Er, yes. By design. The kernel has to run an init program in order to hand off control to userspace. In initramfs, this is defined as /init. It looks in exactly one documented place. The older root= mechanism fell back to a half-dozen places (eventually trying things like /bin/sh if it couldn't find anything else). This is because there wasn't a documented standard for what should be executed, like there is with initramfs. > 1a) In the process of writing these patches, I found prepare_namespace not > to be called if /init is present. prepare_namespace will call > security_sb_post_mountroot after mounting the root fs. I did not yet > see a way to call this from /init, and grepping kinit for "security" did > not help, too. I don't use selinux. I'm neither a Fortune 500 company nor the NSA. However, if you don't trust your own initramfs, where everything starts out running as root, you have bigger problems. > This is probably a bug, but using the features of this patchset, you'll > avoid hitting it. Therefore this patchset does nothing about that. > > 2) If you want to use tmpfs, you need a script which essentially > duplicates the work the kernel just did: Mount the root fs, unpack or move > the files. mkdir sub mount -t tmpfs sub sub cp -ax / sub switch_root sub /init-stage-2 I haven't tried it but it really doesn't sound like brain surgery. If your switch_root is statically linked, you can use mv instead of cp. > Using tmpfs instead for the first root mount is as cheap as > using ramfs, as long as tmpfs is used anyway (and most likely it is). *shrug* I don't object to doing so, but I've heard nebulous technical objections from people who know more about the implementation details fo tmpfs than I do. > 2a) I figured if you prepared the root fs to contain a running system, you > woud probably also set up a runnable system on it. *scratches head* That's a tautology, right? > Therefore I changed > the default to boot from tmpfs if there was no /init nor a root= > option. (If there is a /init, it will be executed as usural.) I have no idea what you just said. If there's an /init, we boot from it. If there's no /init, they just violated the spec and we don't know what to boot from. They made an initramfs. They constructed it, explicitly, for the Linux kernel, and /init is what the kernel will try to boot. I documented this years ago. If they chose not to put an /init, then they didn't want us to boot from initramfs. (Maybe they're supplying firmware and an /sbin/hotplug for a statically linked device. I dunno.) > Unfortunately the way I do it, this will override the rdev setting, but > that should be OK, since rdev is dead. Isn't it? > > 3) While I was at it, I figured I would not need most of the init/mount* > code anymore. Therefore I made patch 3, which ifdefs it out as far as > possible while still aiming for a small change. > > Patch 1 adds the capability to use root=rootfs. I've been using initramfs since back around 2.6.12. It works fine. Why are you patching the kernel for something that already works? > Patch 2 adds the capability to use tmpfs for root, default root=rootfs. > ramfs becomes optional if rootfs=tmpfs. > Patch 3 allows to remove the capability of automounting filesystems. > > All patches appyl to 2.6.22.1 Rob -- "One of my most productive days was throwing away 1000 lines of code." - Ken Thompson. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
