A sub-Hurd is like a neighbor Hurd, however, makes use of some resources provided by another Hurd. For instance, backing store and the console.
Sub-hurds are extremely useful for debugging core servers as it is possible to attach to them with gdb from the parent (debugging via subhurds). This avoids deadlock, e.g., when the instance of gdb stops the server but requires its use. (Note: it is possible to use noninvasive debugging, but this is less flexible.) Vice versa, it is also possible to use a subhurd to debug the main Hurd system, for example, the latter's root file system, but that requires a privileged subhurd.
To run a subhurd, you need an additional partition with an installed Hurd system. In principle, you can also use your main partition in read-only mode; but this obviously will create severe limitations. Usually, you will want a complete independent system.
The system for the subhurd is a normal Hurd installation, which could just as well run standalone. You can use any of the various possible installation methods, or reuse an existing installation if you already have several. If using Debian GNU/Hurd, the easiest is probably to use debootstrap as root:
mke2fs /dev/hd0s6 settrans -ca mnt /hurd/ext2fs /dev/hd0s6 debootstrap sid mnt/ http://httpredir.debian.org/debian settrans -fga mnt
<gnu_srs> Never dared to try a subhurd, any link to the howto? <teythoon> gnu_srs: I followed http://www.gnu.org/software/hurd/hurd/subhurd.html though using crosshurd didn't work for me, I just used debootstrap <teythoon> gnu_srs: and you need a separate filesystem translator (i.e. not /) for that <teythoon> the easiest way is to add another virtual disk to you qemu setup <braunr> use the qemu image directly <braunr> simplest way to set up a subhurd <braunr> just change fstab from the host before the first boot to avoid making the subhurd use the same hd0 drive as the host <teythoon> braunr: nice idea :)
If you are using a recent version of the Hurd (>= 0.9), then you can simply boot the subhurd as an unprivileged user by issuing
hd0s6 by the name of your partition or backing file for the subhurd.)
The partition must be unmounted (or mounted read-only) before you boot from it!
You can provide the subhurd with a network card by passing a
-f option to
boot. For instance, if you have a second network card
/dev/eth1 in your
host hurd, pass
-f eth0=/dev/eth1 to make it appear as device eth0 in the
If you don't have a second network card, you can setup the eth-multiplexer to share one network card. To do so, install the multiplexer
settrans -c /dev/eth0m /hurd/eth-multiplexer --interface=/dev/eth0
Then configure your main Hurd system to use a virtual network
/dev/eth0m/0) instead. On Debian/Hurd, this can be
ifdown /dev/eth0 sed -i -e s#/dev/eth0#/dev/eth0m/0# /etc/network/interfaces ifup /dev/eth0m/0
You can now pass
-f eth0=/dev/eth0m/1 to
Now the subhurd should boot just like a normal Hurd started directly from GRUB,
finally presenting a login prompt. The
boot program serves as proxy for the
subhurd, so you can control it from the terminal where you issued the boot
To exit the subhurd, issue
reboot. This should exit it cleanly,
but for some reason it doesn't always work; sometimes it will output various
errors and then hang. If that happens, you need to kill the
manually from a different terminal. If the
boot process dies, the
proc server will kill all tasks belonging to the Subhurd.
In the subhurd, you can do basically all the same things as in the main Hurd.
You can even set up networking: Just invoke
settrans on the
/servers/socket/2 as usual inside the subhurd, using
/dev/eth0, only using a different local
IP than in the main Hurd. This way, the subhurd will be able to communicate to
the outside world with its own IP -- allowing for example to do
inside the subhurd, or to
ssh directly into the subhurd.
If you want to access the subhurd processes from the outside, e.g. for
debugging purposes (or to get rid of a subhurd that
didn't exit cleanly...), you need to find out how main Hurd s correspond to
subhurd processes: the subhurd processes appear in the main Hurd (e.g. if doing
ps -e) as unknown processes, but the s are different! To
find out which process is which, you can simply compare the order -- while the
numbers are different, the order should usually match. Often it also helps to
look at the number of threads (e.g. using
ps -l), as many servers have very
characteristic thread counts.
<teythoon> btw, is there a way to get dde-based networking into a subhurd? <teythoon> the wiki instructions look like they're for the mach driver <teythoon> and starting the dde translator inside the subhurd does not work for me <teythoon> that's probably a good thing though <youpi> the netdde process will need privileged access to mach <youpi> for hardware access <braunr> you can't easily use netdde from a subhurd, unless with a different nic <braunr> i usually rebuild mach with in kernel devices so both the main and subhurd can share on nic <braunr> one* <youpi> could a port to netdde perhaps forwarded to the subhurd? <braunr> zengh da wrote the eth-multiplexer for that iirc <youpi> it's a matter of making it appear as an eth0 device on the master port aiui <braunr> zheng* <teythoon> yes, I looked at that <teythoon> what is the master port? <youpi> on a plain hurd system it's the port that privileged processes can use to access mach devices <youpi> in a subhurd, it's the same for the subhurd, to access some devices that you choose to give access to <braunr> its real name is the "device master port" <teythoon> ah yes
<antrik> teythoon: use eth-multiplexer to use the NIC within a subhurd. that's exactly what it was created for. <antrik> I don't remember whether it's even possible to share a "raw" netdde device... I don't think I ever tried that; and I don't remember enough of the theory to tell whether it should be possible <antrik> but I really don't see the reason to, when eth-multiplexer is available <antrik> (IMHO running an eth-multiplexer on top of netdde should be the default setup in fact) <antrik> as for actually passing on the device, that should be perfectly possible with zhengda's modified subhurd... but I don't remember whether that was ever merged upstream <antrik> (you will definitely need that for using netdde in a subhurd, regardless whether through eth-multiplexer or directly)
<teythoon> I wonder if we can modify the boot program so that it passes ports from the mother hurd to the subhurd <teythoon> so that we could pass in a port to the eth-multiplexer <teythoon> or use like /hurd/remap as the root translator for the subhurd
<braunr> eth-multiplexer was created exactly for that iirc, <braunr> so it's probably already done somewhere
<gnu_srs> braunr: regarding subhurd did you mean to install sthibault/hurd-i386/debian-hurd.img.tar.gz <gnu_srs> on a separate partition and booting using the instructions for subhurds on the web. <braunr> gnu_srs: yes <braunr> be careful that the subhurd doesn't use the same partition as the main hurd, that's all <gnu_srs> what about changing fstab? <braunr> 12:17 < braunr> be careful that the subhurd doesn't use the same partition as the main hurd, that's all <teythoon> gnu_srs: yes, you need to change the fstab <teythoon> currently it is used for fscking stuff, so if it points to your main partition it will cause severe corruption <teythoon> gnu_srs: you also have to specify the right partition in the servers.boot file <gnu_srs> fstab of the subhurd image? <teythoon> yes <gnu_srs> how to unpack the .img file (just to be sure)? <teythoon> gnu_srs: you don't need to, just use the img file as secondary hard disk image <gnu_srs> Then how should I be able to change fstab of the image? <teythoon> boot your hurd box, mount the partition and change it <gnu_srs> I missed something here: on my partition /my_chroot I have have the file debian-hurd-20130504.img <teythoon> gnu_srs: ah, you copied it to the partition, braunr meant to use it as the secondary disk, e.g. qemu ... -hdb debian-hurd-20130504.img ... <gnu_srs> That is the same as installing another cd image, where does the subhurd come into play? <teythoon> mount the partition on the secondary hd, fix the fstab there, mount it r/o, get the servers.boot file from the wiki, modify it so that it points to the right partition, execute boot servers.boot /dev/<your partition>, probably /dev/hd1s1 <gnu_srs> BTW: unpacking was problematic: tar: debian-hurd-20130504.img: Cannot seek to 2147696640 (2G limitations) <teythoon> I wonder why you did this on your hurd system in the first place... <gnu_srs> I thought I could use that partition, /my_chroot as a chroot place. So it won't work for subhurds? <teythoon> well, there are several ways to setup a subhurd. one is to already have a spare partition for that and use crosshurd or as I did debootstrap to install a debian system there <teythoon> braunr suggested an even easier way, download the .img file and use it as secondary hard disk <teythoon> you ended up doing kind of both <gnu_srs> I tried once with debootstrap and that created a disaster... <teythoon> how so? <gnu_srs> The install errored out, and the whole filesystem (including /) was left in a broken state. Maybe I tried <gnu_srs> that without using a separate partition. Don't remember any longer. So you say it's safe now? <teythoon> I used it successfully to setup my subhurd <gnu_srs> and you have your subhurd in a separate partition, installed from there too, as root? <gnu_srs> the web page only mentions crosshurd, and that failed for you? <teythoon> yes, having a separate partition is (currently) necessary to run a subhurd <teythoon> yes, I used debootstrap as root, afaics that is necessary <teythoon> and yes, as I said the other day, I tried crosshurd first and it failed <teythoon> then again, I fail to see any reason to use crosshurd these days <teythoon> it's only a wrapper around debootstrap anyway, using it with --foreign and fixing up stuff later <teythoon> one has more control over the process if one uses debootstrap directly <gnu_srs> I still don't dare to do it yet. I'll create another image using netinst with a separate partition and try out first. <gnu_srs> When installing a new image using netinst.iso (2013-06-30) and rebooting /proc does not get mounted? <teythoon> gnu_srs: is that a statement or a question? <gnu_srs> A statement. <teythoon> it's not customary to end statements with question marks ;) <gnu_srs> s/mounted?/mounted, why?/ <teythoon> well, you seem to be the last person to perform such an installation, so you are in the perfect position to answer this question. <gnu_srs> cat /var/log/dmesg? <gnu_srs> On other images I have: fsysopts /proc; /hurd/procfs --clk-tck=100 --stat-mode=444 --fake-self=1 <youpi> gnu_srs: no, check the installation log <youpi> gnu_srs: and what does showtrans say? <gnu_srs> showtrans /proc; <empty> <gnu_srs> which log file to look for? <youpi> the installation log, somewhere in /var/log probably <gnu_srs> I only find /proc in /var/log/installer/syslog, mainly printing out errors not finding /proc/mounts <youpi> iirc the /proc translator should be set during the hurd package configuration <youpi> you should probably look for that part in the log <youpi> Setting up translators: /hurd/exec /hurd/proxy-defpager /hurd/pflocal (+link) /hurd/pfinet (+link) (+link) /hurd/procfs -c /hurd/password crash-kill crash-suspend crash-dump-core crash. <youpi> that part <gnu_srs> debootstrap: /hurd/procfs -c and in-target: /hurd/procfs -c No errors <youpi> I don't understand what that means <youpi> please explain in more details <gnu_srs> see: http://paste.debian.net/41195/ <youpi> makes much more sense :) <gnu_srs> Where is the 'Setting up translators' done? I cannot find anything in /var/lib/dpkg/info/hurd* or /etc/init.d/... <pinotree> /usr/lib/hurd/setup-translators, called in hurd.postinst <gnu_srs> tks:) <gnu_srs> Hi, when installing a new image with debootstrap to /chroot the script boot/servers.boot is already there (as well as in /boot/ + grub) <gnu_srs> Is it OK to use that file to boot the subhurd? <gnu_srs> using /boot/servers.boot or /chroot/boot/servers.boot (if the /chroot partition is unmounted it cannot be used?) <gnu_srs> and how to unmount /chroot: umount does not work? <gnu_srs> braunr: I'm also trying to find out what's wrong with glibc, when my subhurd is up and running 2.13-39 (if possible) <gnu_srs> I know I should issue settrans command, but I'm not yet fluent in translators. <gnu_srs> sorry:-/ <gnu_srs> Now this, after a reboot: unknown code P 30 while trying to open /dev/hd0s3 (/chroot) <gnu_srs> Disk write protected: use the -n option to do a read-only check of the device. <gnu_srs> fsysopts /dev/&hd0s1 --writable: Operation not supported?? <gnu_srs> OK, I'm giving up for now, no subhurd:-( and a broken install. <gnu_srs> Which terminal to use in rescue mode, TERM is not set, dumb,mach,hurd does not work with nano? <gnu_srs> e2fsck /dev/ho0s3; e2fsck: Unknown code P 2 while trying to open /dev/ho0s3; Possibly non-existent device? <gnu_srs> mke2fs /dev/hd0s3; /dev/hd0s3 is not a block special device.; Proceed anyway? (y,n) n: What's going on (hd0s3 not mounted)?? <gnu_srs> anybody, help? <gnu_srs> after removing and creating the partition again:mke2fs /dev/hd0s3, <same>, mke2fs: Unknown code P 13 while trying to determine filesystem size: What's going on? <gnu_srs> Where to find the glibc-2.13 versions which used to be at debian-ports?. <gnu_srs> seems they can be found on snapshot.debian.org
<gnu_srs> teythoon: Installing subhurd via debootstrap on partition /chroot fails miserably. Install hangs, and after reboot \rm -r /chroot/* fails for dev and proc <gnu_srs> Are there translators running there already? I have not booted the subhurd. <gnu_srs> translators for hd0s3 (/chroot) are storeio and ex2fs.static. Do I have to stop them to be able to clean out /chroot? <gnu_srs> mount -v /chroot; settrans -a /chroot /hurd/ext2fs /dev/hd0s3; <gnu_srs> ext2fs: /dev/hd0s3: panic: main: device too small for superblock (0 bytes); <gnu_srs> mount: cannot start translator /hurd/ext2fs: Translator died <gnu_srs> Please, somebody! <gnu_srs> don't ask to ask, just ask, right? <braunr> we've already told you everything you need <braunr> just get it right <braunr> for example, i told you to be careful about fstab so that the subhurd wouldn't use the main hurd partition <braunr> but you managed to screw that <braunr> good job <gnu_srs> I installed the subhurd in a partition /chroot /dev/hd0s3 using debootstrap <braunr> i don't know deboostrap, it may be broken, use the disk image youpi maintains <gnu_srs> ant the install screwed up with debootstrap <gnu_srs> ok; then I cannot use a partition, but another disk in kvm, e.g. hdb? <braunr> gnu_srs: hd1 <gnu_srs> something is fishy with glibc, definitely, that's why I'm trying to set up a subhurd to revert to 2.13-39 <gnu_srs> hi, when trying to boot a subhurd: /hurd/ext2fs.static: hd0s3: Gratuitous error; bye <braunr> gnu_srs: why hd0s3 ? <braunr> it should be hd1s1 <gnu_srs> I'm still using a separate partition /my_chroot /hd0s3. Will switch to hd1 next. teythoon? <gnu_srs> the servers.boot script use absolute paths:/hurd/ext2fs.static and /lib/ld.so.1 /hurd/exec, <gnu_srs> shouldn't they be relative to /my_chroot? <braunr> no <braunr> they're actually from your host <gnu_srs> teythoon: please, how did you succeed to boot a subhurd in a partition? <gnu_srs> using debootstrap <teythoon> gnu_srs: from my shell history: <teythoon> : 1374672426:0;debootstrap sid /mnt http://http.debian.net/debian/ <teythoon> : 1374673020:0;cp /etc/hosts /etc/resolv.conf /mnt/etc <teythoon> : 1374673048:0;cp /etc/passwd /etc/shadow /mnt/etc <braunr> teythoon: so it does work fine ? <braunr> great <teythoon> yes, why wouldn't it? <teythoon> gnu_srs: I then remounted that partition r/o and used the servers.boot file from the wiki to boot it <teythoon> braunr: why wouldn't it? (you do mean the debootstrap part, don't you?) <braunr> teythoon: i don't know <braunr> i've heard it wasn't maintained any more <braunr> not being maintained is a good reason for something to become unusable/untrustable with time <teythoon> o_O it is at the heart of d-i, isn't it? <teythoon> I actually do most Debian installations using debootstrap directly <braunr> ah <braunr> ok :) <braunr> teythoon: even hurd ones ? <teythoon> braunr: well, just the subhurd installation, but that went as expected <braunr> good <gnu_srs> Finally: I found the reason for Gratuitous error, I used the /boot/servers.boot script, <gnu_srs> that being different to the one on the wiki:-/ <gnu_srs> Is it possible to copy files between a host hurd and subhurd, what about access to eth0? <gnu_srs> Hi, when starting the subhurd I see some warnings/error: http://paste.debian.net/41963/ <gnu_srs> 1) A spelling error execunable-> executable <gnu_srs> 2) libports: invalid destination port <gnu_srs> 3) mach-defpager: another already running <pinotree> "execunable" is not a typo, but just "exec" and "unable ..." without a space-type character <gnu_srs> OK, sounds more plausible <gnu_srs> Ah, the printouts are mixed, no bug <gnu_srs> When setting up nework in the subhurd: /hurd/pfinet: file_name_lookup /dev/eth0: Translator died <gnu_srs> /hurd/pfinet: device_open(/dev/eth0): (os/device) no such device <gnu_srs> settrans: /hurd/pfinet: Translator died
<youpi> priority does not matter much <youpi> memory manager is not really surprising, there's indeed already one <youpi> what is actually the problem? <gnu_srs> So these are merely warnings? <youpi> gnu_srs: yes <gnu_srs> Real problems are I cannot set up networking, e.g. wget ...: Connecting to ... failed: Address family not supported by protocol. <youpi> gnu_srs: did you give the subhurd a network card? <gnu_srs> How? <gnu_srs> and do I need to set up fstab, for now it's empty. <gnu_srs> I just installed the base with dbootstrap <youpi> gnu_srs: -f option of boot <youpi> e2fsck will need fstab for sure <youpi> otherwise it can't divine what should be checked <gnu_srs> Why is the /boot/servers.boot different from the subhurd one on the wiki? Is it used at all, I thought grub was in charge. <youpi> it's not used at all <gnu_srs> maybe better to put in the subhurd one there then, with a comment? <youpi> no, since /boot/servers.boot is supposed to be used for machine boot <youpi> not subhurd boot <gnu_srs> what about putting a copy of the suhurd one there, with a different name? <youpi> probably a good idea, yes <youpi> matter of making it happen <gnu_srs> the wiki page on subhurd does not say how to set up networking, only that you can do it. <youpi> matter of adding the information <youpi> I remember it's the -f option of boot <youpi> make it work, and add the information for others <gnu_srs> I could try, but don't know how to add a network card to the subhurd, and e.g. how to set up swap <youpi> see -f option <gnu_srs> of boot? <youpi> "gnu_srs: -f option of boot" <youpi> if you could read what we write, that'd make things happen way faster <gnu_srs> yes I saw your comment above, it was just to be 100% sure:-D <gnu_srs> device_file=/dev/eth0 or something else? <gnu_srs> eth0 is used by the host already <youpi> did you read boot --help ? <youpi> iirc it's not a problem, both will receive all frames <gnu_srs> yes I did <youpi> then I don't see where you took device_file from <youpi> at least in that form <youpi> --device=device_name=device_file <youpi> that means rather something like --device=foo=bar <gnu_srs> so -f /dev/eth0 is correct usage then? <youpi> didn't you see that in what I wrote, there was a "=" in there? <gnu_srs> -f is the short option, --device is the long, I don't see the need for = in the short option? <youpi> in the long option there are *two* = <gnu_srs> yes, but in the short no? <youpi> why not? <youpi> long -> short usually drops one = <gnu_srs> to summarize: -f=/dev/eth0 or --device=eth_sub=/dev/eth0? <youpi> why shouldn't there be a eth_sub in the short version? <gnu_srs> 10:15:49) youpi: long -> short usually drops one = <youpi> yes, it drops the = <youpi> but nothing else <youpi> if the long option needs some information, the short needs it too? <youpi> -? <gnu_srs> correct now? -f eth_sub=/dev/eth0 or --device=eth_sub=/dev/eth0? <youpi> yes <gnu_srs> k!
Read about using a subhurd for debugging purposes.
Roland's tutorial about running a subhurd.
Note: This only works with privileged subhurds.
A subhurd can be used for debugging the main Hurd system. This works as long as the subhurd doesn't use any services provided by the main Hurd. For example, if you already have a subhurd running at the time it happens, you can use that one to debug a deadlocked ext2fs root file system in the main Hurd.
For this, you need to get a handle to the main Hurd's ext2fs translator's , but this is no problem, as currently s are visible across subhurd boundaries. (It is a open issue hurd whether this is the right thing to do in virtualization contexts, but that's how it currently is.)
freenode, #hurd channel, 2011-03-06:
From unit testing.
<youpi> it could be interesting to have scripts that automatically start a sub-hurd to do the tests <youpi> though that'd catch sub-hurd issues :) <foocraft> so a sub-hurd is a hurd that I can run on something that I know works(like linux)? <foocraft> Virtual machine I would think <foocraft> and over a network connection it would submit results back to the host :p * foocraft brain damage <youpi> sub-hurd is a bit like chroot <youpi> except that it's more complete <foocraft> oh okay <youpi> i.e. almost everything gets replaced with what you want, except the micro-kernel <youpi> that way you can even test the exec server for instance, without risks of damaging the host OS <foocraft> and we know the micro-kernel works correctly, right youpi? <youpi> well, at least it's small enough that most bugs are not there <foocraft> 1) all tests run in subhurd 2) output results in a place in the subhurd 3) tester in the host checks the result and pretty-prints it 4) rinse & repeat <youpi> the output can actually be redirected iirc <youpi> since you give the sub-hurd a "console" <foocraft> youpi, yup yeah, so now it's more like chroot if that's the case <youpi> it really looks like chroot, yes <foocraft> but again, there's this subset of tests that we need to have that ensures that even the tester running on the subhurd is valid, and it didn't break because of a bug in the subhurd <tschwinge> As long as you do in-system testing, you'll always (have to) rely on some functionality provided by the host system. <foocraft> the worst thing that could happen with unit testing is false results that lead someone to try to fix something that isn't broken :p <tschwinge> Yes. <youpi> usually one tries to repeat the test by hand in a normal environment