Re: [PATCH 1/3] btrfs: derive f_fsid from on-disk fsuuid and dev_t

[Thiago Macieira <thiago.macieira@intel.com>]

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On Friday, 11 September 2026 07:51:16 Pacific Daylight Time Anand Suveer Jain 
wrote:
> I wish I had more clarity on how OpenConnect uses the FSID before I
> comment. Do you know? Can you shed some light on this?

It's openconnect's option --key-password-from-fsid. It uses the FSID encoded 
in hex format as the key's passphrase.

To reproduce, take any key without a password and do:
  openssl rsa -in unencrypted-key.pem -out encrypted-key.pem
And paste your FSID as the password.

This is not real security, we all agree. David W can probably remember better 
why this option exists, but I would suspect it was an IT mandate that a) the 
key not be left unencrypted on disk and b) be specific to a given machine, 
avoiding reuse by being copied to another. Ideally, we'd use TPM these days, 
but there are still a lot of systems without it where Linux runs, and 
especially a lot of legacy set ups.

But very simply, it's the fact that the FSID has been used as a stable 
identifier and no longer is if the device in question is not itself stable. 

> Also, a couple of quick checks:
> 
>  Is the underlying device MAJ:MIN changing before and after the upgrade?

Unknown. It's LVM and I don't know if the order of activation of LVs is 
guaranteed. I wouldn't rely on it. I'll tell you next time I reboot if the 
order is the same as this:

brw-rw---- 1 root disk 254,   0 Sep  7 20:28 cr-auto-1
brw-rw---- 1 root disk 254,   1 Sep  7 20:28 system-root
brw-rw---- 1 root disk 254,   2 Sep  7 20:28 system-swap
brw-rw---- 1 root disk 254,   3 Sep  7 20:28 system-pool_tmeta
brw-rw---- 1 root disk 254,   4 Sep  7 20:28 system-pool_tdata
brw-rw---- 1 root disk 254,   5 Sep  7 20:28 system-pool-tpool
brw-rw---- 1 root disk 254,   6 Sep  7 20:28 system-pool
brw-rw---- 1 root disk 254,   7 Sep  7 20:28 system-stuff
brw-rw---- 1 root disk 254,   8 Sep  7 20:28 system-home
brw-rw---- 1 root disk 254,   9 Sep  7 20:28 system-stuff2

Either way, the point is that it changed from 7.1 when the MAJ:MIN was not 
mixed in.

>  Are the original and cloned Btrfs filesystems mounted simultaneously?

No cloning is involved.

> Patch v3 (1/2) keeps the FSID persistent from the on-disk superblock and
> stops exposing the internal random FSID. However, if OpenConnect relies
> on statfs.f_fsid, patch v3 (2/2) now derives f_fsid from the underlying
> MAJ:MIN.


-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Principal Engineer - Intel DCG - Platform & Sys. Eng.

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