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mike155
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Joined: 17 Sep 2010
Posts: 4438
Location: Frankfurt, Germany
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 Posted: Mon Oct 26, 2020 1:02 pm Post subject: Why does 'df' print my root filesystem as '/dev/root'? |
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df on my Gentoo machine shows the root filesystem as '/dev/root':
| Code: | # df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/root 82045336 22397696 55436952 29% / |
df on a Debian machine shows the root filesystem with its partition name ('/dev/sda6'):
| Code: | # df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda6 38464044 3254560 33225896 9% /
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Why does my Gentoo machine print '/dev/root'? What can I do to make my Gentoo machine show the partition name instead of '/dev/root'? |
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Zucca
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Joined: 14 Jun 2007
Posts: 3345
Location: Rasi, Finland
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| Posted: Mon Oct 26, 2020 2:43 pm Post subject: |
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Does lsblk show the same?
What doestell?
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mike155
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Joined: 17 Sep 2010
Posts: 4438
Location: Frankfurt, Germany
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| Posted: Mon Oct 26, 2020 3:01 pm Post subject: |
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| Code: | # file /dev/root
/dev/root: cannot open '/dev/root' (No such file or directory) |
| Code: | # lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 74.5G 0 disk
├─sda1 8:1 0 200M 0 part /boot
├─sda2 8:2 0 60.9G 0 part
└─sda4 8:4 0 13.5G 0 part
nvme0n1 259:0 0 465.8G 0 disk
├─nvme0n1p1 259:1 0 200M 0 part
├─nvme0n1p2 259:2 0 80G 0 part /
└─nvme0n1p3 259:3 0 385.6G 0 part |
| Code: | # cat /proc/self/mountinfo # that's what 'df' reads
...
20 1 259:2 / / rw,noatime shared:1 - ext4 /dev/root rw
... |
| Code: | # cat /proc/mounts
...
/dev/root / ext4 rw,noatime 0 0
... |
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Etal
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Joined: 15 Jul 2005
Posts: 1931
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| Posted: Mon Oct 26, 2020 3:12 pm Post subject: |
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| Could it be depending on whether if it was mounted by the kernel or by userspace (in initrd)? |
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Zucca
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Joined: 14 Jun 2007
Posts: 3345
Location: Rasi, Finland
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| Posted: Mon Oct 26, 2020 7:09 pm Post subject: |
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I suspect (rather a wild guess) it's mounted in initrd from /dev/root, but after e?udev starts the dev nodes change and /dev/root goes missing/renamed but the earlier dev node name is already written into several places (mountinfo).
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Ionen
Developer
Joined: 06 Dec 2018
Posts: 2719
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| Posted: Mon Oct 26, 2020 7:54 pm Post subject: |
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/dev/root is a device the kernel uses to do the mount specified by root=... by itself (e.g. without relying on an initramfs). If you had an initramfs that did the mount with commonly-named devices, then you'd get the normal device, I assume debian does something like that.
You can see it in the kernel source, like in init/do_mounts.c inside mount_root() | Code: | #ifdef CONFIG_BLOCK
{
int err = create_dev("/dev/root", ROOT_DEV);
if (err < 0)
pr_emerg("Failed to create /dev/root: %d\n", err);
mount_block_root("/dev/root", root_mountflags);
}
#endif |
Later gone as it gets replaced by the real /dev, (e)udev, and all that stuff.
I don't have an initramfs/initrd at all (Edit: my kernel is even built without support) and I get /dev/root shown in df output.
Last edited by Ionen on Mon Oct 26, 2020 8:02 pm; edited 3 times in total |
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Tony0945
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Joined: 25 Jul 2006
Posts: 5127
Location: Illinois, USA
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| Posted: Mon Oct 26, 2020 8:00 pm Post subject: |
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It used to show the actual device but a bout a year ago showed the useless /dev/root. Wow, root is mount on /dev/root, whoda thunk it?
EDIT:
Seems to be failure to follow the symlink:
| Code: | tony@Casti ~ $ ls -l /dev/root
lrwxrwxrwx 1 root root 4 Oct 25 17:12 /dev/root -> sdb3 |
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Ionen
Developer
Joined: 06 Dec 2018
Posts: 2719
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| Posted: Mon Oct 26, 2020 8:06 pm Post subject: |
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| Tony0945 wrote: | EDIT:
Seems to be failure to follow the symlink:
| Code: | tony@Casti ~ $ ls -l /dev/root
lrwxrwxrwx 1 root root 4 Oct 25 17:12 /dev/root -> sdb3 |
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In my case the file doesn't even exist (devtmpfs+eudev+openrc+no initramfs) | Code: | $ ls -l /dev/root
ls: cannot access '/dev/root': No such file or directory |
But df still shows /dev/root, not that "personally" consider this a problem. I know what /dev/root means and I'm not using that output to find the root device. I could create the symlink with a simple udev rule if I wanted too. |
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Tony0945
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Joined: 25 Jul 2006
Posts: 5127
Location: Illinois, USA
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| Posted: Mon Oct 26, 2020 8:34 pm Post subject: |
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| Ionen wrote: | | In my case the file doesn't even exist (devtmpfs+eudev+openrc+no initramfs) |
Sounds like my system except I am using an old out of tree version of openrc. Someday I will analyze the later changes in OpenRC and see if they were bugfixes, enhancements, or just tinkering. I'm discounting for the moment claims that they were deliberate breakage to advance systemd, because of lack of evidence of intent.
Also an old eudev. Does this section of /lib/udev/rules.d/60-persistent-storage.rules do anything?
| Code: |
# add symlink to GPT root disk
ENV{ID_PART_ENTRY_SCHEME}=="gpt", ENV{ID_PART_GPT_AUTO_ROOT}=="1", SYMLINK+="gpt-auto-root"
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Tony0945
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Joined: 25 Jul 2006
Posts: 5127
Location: Illinois, USA
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| Posted: Mon Oct 26, 2020 8:38 pm Post subject: |
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Snippet from /etc/init.d/udev
| Code: | # This is here because some software expects /dev/root to exist.
# For more information, see this bug:
# https://bugs.gentoo.org/show_bug.cgi?id=438380
dev_root_link()
{
local RULESDIR=/run/udev/rules.d
[ -d $RULESDIR ] || mkdir -p $RULESDIR
eval $(udevadm info --export --export-prefix=ROOT_ --device-id-of-file=/ ||
true)
[ "$ROOT_MAJOR" -a "$ROOT_MINOR" ] || return 0
# btrfs filesystems have bogus major/minor numbers
[ "$ROOT_MAJOR" != 0 ] || return 0
echo 'ACTION=="add|change", SUBSYSTEM=="block", ENV{MAJOR}=="'$ROOT_MAJOR'", ENV{MINOR}=="'$ROOT_MINOR'", SYMLINK+="root"' > $RULESDIR/61-dev-root->
}
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Clear as mud.
EDIT: additional data
| Code: | tony@Casti ~ $ equery b /etc/init.d/udev
* Searching for /etc/init.d/udev ...
sys-fs/udev-init-scripts-27-r1 (/etc/init.d/udev)
tony@Casti ~ $ eix udev-init-scripts
[I] sys-fs/udev-init-scripts
Available versions: 27^t[1] 27-r1^t{tbz2}[1] [m]33^t [m]34^t [m]**9999*l^t
Installed versions: 27-r1^t{tbz2}[1](02:08:16 PM 01/27/2019)
Homepage: https://wiki.gentoo.org/wiki/No_homepage
Description: udev startup scripts for openrc
[1] "oldgentoo" /var/lib/layman/oldgentoo
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Ionen
Developer
Joined: 06 Dec 2018
Posts: 2719
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| Posted: Mon Oct 26, 2020 8:47 pm Post subject: |
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The gpt-auto-root thing was removed 3 years ago, dev_root_link() was moved out of init.d/udev to init.d/udev-trigger 6 years ago.
Edit:
For me seems the script runs, creates the udev rule, but is never triggered to actually make the symlink (maybe due to how I got other things setup). This process can also be disabled by a config option but is enabled by default. Missing /dev/root never caused me problems so I guess doesn't matter on my system anyway. | Code: | $ cat /run/udev/rules.d/61-dev-root-link.rules
ACTION=="add|change", SUBSYSTEM=="block", ENV{MAJOR}=="259", ENV{MINOR}=="2", SYMLINK+="root" |
Edit2: | Code: | # We can create a /dev/root symbolic link to point to the root device in
# some situations. This is on by default because some software relies on
# it,. However, this software should be fixed to not do this.
# For more information, see
# https://bugs.gentoo.org/show_bug.cgi?id=438380.
# If you are not using any affected software, you do not need this, so
# feel free to turn it off.
#rc_dev_root_symlink="YES" |
So I guess not having it is more "proper", might as well just change this to "no" (not that this have anything to do with what df will display, see earlier post instead)
Last edited by Ionen on Mon Oct 26, 2020 9:11 pm; edited 3 times in total |
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Tony0945
Watchman
Joined: 25 Jul 2006
Posts: 5127
Location: Illinois, USA
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| Posted: Mon Oct 26, 2020 9:05 pm Post subject: |
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| Yes, I run an old eudev as well as an old OpenRC. |
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mike155
Advocate
Joined: 17 Sep 2010
Posts: 4438
Location: Frankfurt, Germany
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| Posted: Mon Oct 26, 2020 10:54 pm Post subject: |
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@Zucca, @Etal, @Ionen, @Tony0945: thanks for your answers!
| Ionen wrote: | You can see it in the kernel source, like in init/do_mounts.c inside mount_root()
| Code: | #ifdef CONFIG_BLOCK
{
int err = create_dev("/dev/root", ROOT_DEV);
if (err < 0)
pr_emerg("Failed to create /dev/root: %d\n", err);
mount_block_root("/dev/root", root_mountflags);
}
#endif |
Later gone as it gets replaced by the real /dev, (e)udev, and all that stuff.
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You're right. I modified that code block ( s%/dev/root%/dev/rant% ) and recompiled the kernel. Amazingly enough, my kernel still boots. And now I get:
| Code: | # df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/rant 82045336 22391140 55443508 29% /
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And it's true: my Gentoo machine boots without initrd and my Debian machine boots with initrd.
What happens during initrd that changes '/dev/root' to the device name? Can I do it without initrd? |
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Ionen
Developer
Joined: 06 Dec 2018
Posts: 2719
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| Posted: Mon Oct 26, 2020 11:29 pm Post subject: |
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Well, a initramfs is technically your rootfs until it mounts the real one and switch (the primary reason it exists if need modules/lvm/cryptstuff/etc.. to mount it), it can have a normal /dev, mount program, etc... If it runs a mount command with normal devices, then the kernel's /dev/rant doesn't mean anything.
Without that the kernel has to mount your real rootfs before /dev even exists, so guess it just makes something up  I guess it could be handled more elegantly given it still knows "expected" device names but well.
Not that I've messed with initramfs much myself, I've never really needed them for how I do things. |
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Tony0945
Watchman
Joined: 25 Jul 2006
Posts: 5127
Location: Illinois, USA
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| Posted: Mon Oct 26, 2020 11:59 pm Post subject: |
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Well, I see the df just reads /proc/self/mountinfo in read_file_system_list in file lib/mountlist.c, so I shouldn't fault it for not following the symlink.
It would have to be patched tp check for /dev/root and then find the link target.
EDIT:
A lot easier to just make a local scripy to run df, then "ls -l /dev/root". A script wizard code run "ls" first, extract the value. run df in into a tempfile, use awk or sed to change the file then echo the file to stdout and delete it.
It ain't me It ain't me. I ain't no fortunate son. |
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Zucca
Moderator
Joined: 14 Jun 2007
Posts: 3345
Location: Rasi, Finland
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| Posted: Tue Oct 27, 2020 7:00 pm Post subject: |
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This one break the formatting: | Code: | | df | awk '{if ($1 ~ /^\//) {"realpath " $1 | getline realdev; close("realpath " $1); $1 = realdev} print}' |
... so one would need to modify $0 directly... via sub() maybe.
EDIT: It was quite easy | Code: | | df | awk '{if ($1 ~ /^\//) {"realpath " $1 | getline realdev; close("realpath " $1); sub($1,realdev)} print}' |
Pretty? No.
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Gentoo IRC channels reside on Libera.Chat.
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| Quote: | | I am NaN! I am a man! |
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Tony0945
Watchman
Joined: 25 Jul 2006
Posts: 5127
Location: Illinois, USA
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| Posted: Tue Oct 27, 2020 8:56 pm Post subject: |
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| pretty, yes! |
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