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/proc/mounts doesn't store bind flag

DISCUSSION Phillip Susi 2 participants 4 messages Nov 1, 2013 – Nov 4, 2013 Util-Linux package

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Phillip Susi

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#0 lore raw

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When symlinking /etc/mtab to /proc/mounts, bind mounts no longer have the bind option stored. Is this a bug or working as intended?

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#1 lore raw
Quoted 3 lines · Phillip Susi

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When symlinking /etc/mtab to /proc/mounts, bind mounts no longer have the bind option stored. Is this a bug or working as intended?

This is expected thing. All utils that expect "bind" flag are broken.

There is nothing like "bind" mountpoint (and kernel does not maintain such information at all), all mountpoints are equal, there is nothing like a primary (original) mountpoint.

You have to think about it as about hardlinks for files. The bind operation creates just another path to the filesystem. That's all.

  mount /dev/sda1 /A
  mount --bind /A /B
  umount /A

then /B is still valid (BTW, this is impossible to describe in the traditional /etc/mtab where was always relation between /A and /B).

Karel

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 Karel Zak  <[email protected]>
 http://karelzak.blogspot.com
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#2 lore raw

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That is pretty much what I thought, but why is the bind flag stored when you do use mtab then? Shouldn't they give the same behavior, so bind should not be stored in mtab? In particular it changes the way you have to remount a bind mount ro, which is an unexpected user visible change.

Quoted 15 lines · Karel Zak
Quoted 3 lines · Phillip Susi

-----BEGIN PGP SIGNED MESSAGE----- When symlinking /etc/mtab to /proc/mounts, bind mounts no longer have the bind option stored. Is this a bug or working as intended?

This is expected thing. All utils that expect "bind" flag are broken.

There is nothing like "bind" mountpoint (and kernel does not maintain such information at all), all mountpoints are equal, there is nothing like a primary (original) mountpoint.

You have to think about it as about hardlinks for files. The bind operation creates just another path to the filesystem. That's all.

Quoted 8 lines

mount /dev/sda1 /A mount --bind /A /B umount /A

then /B is still valid (BTW, this is impossible to describe in the traditional /etc/mtab where was always relation between /A and /B).

Karel

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maintainer
#3 lore raw
Quoted 5 lines · Phillip Susi

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That is pretty much what I thought, but why is the bind flag stored when you do use mtab then? Shouldn't they give the same behavior, so

Historical reasons... 10 years ago nobody cares about the sense of the flag in the mtab ;-(

And it's also difficult for userspace to generate the right mount source device if a directory is specified.

I'm not sure if I want to change the mtab behavior if for example old coreutils df(1) or some backup tools depend on this odd behavior.

It seems better to keep it without a change and provide a better solution only for new systems without mtab.

Quoted 3 lines

bind should not be stored in mtab? In particular it changes the way you have to remount a bind mount ro, which is an unexpected user visible change.

Yes. The update to systems without mtab has some disadvantages, but it's still acceptable compare to mtab disadvantages.

Fedora did the change years (2?) ago and the number of the bug reports related to this problem is almost zero. The question is what will happen on server (e.g. RHEL), we will see :-)

Karel

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 Karel Zak  <[email protected]>
 http://karelzak.blogspot.com