`/etc/fstab: Read-only file system` when starting a NixOS machine with Vagrant v2.4.7 (regression from v2.4.6)

[amezin]

Debug output

https://gist.github.com/amezin/fcd5612f13e2aff765b9f0349131c239

Expected behavior

Machine should start successfully, without errors, like on the previous v2.4.6

Actual behavior

The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!

echo '#VAGRANT-BEGIN
# The contents below are automatically generated by Vagrant. Do not modify.
#VAGRANT-END' >> /etc/fstab

Stdout from the command:



Stderr from the command:

bash: line 6: /etc/fstab: Read-only file system

vagrant up exits with code 1

Reproduction information

Vagrant version

v2.4.7

Host operating system

Arch Linux

Guest operating system

NixOS 24.11

Steps to reproduce

1. vagrant up with the provided Vagrantfile

Vagrantfile

Vagrant.configure("2") do |config|
  config.vm.box = "gnome-shell-box/nixos"
  config.vm.provider :libvirt do |libvirt|
    libvirt.qemu_use_session = true
  end
end

You can find box's base Vagrantfile here: https://github.com/ddterm/gnome-shell-box/blob/0.0.11/Vagrantfile

[amezin]

vagrant ssh -c 'stat /etc/fstab' with Vagrant v2.4.7:

  File: /etc/fstab -> /etc/static/fstab
  Size: 17        	Blocks: 8          IO Block: 4096   symbolic link
Device: 0,29	Inode: 365580      Links: 1
Access: (0777/lrwxrwxrwx)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2025-06-27 06:38:21.094000000 +0000
Modify: 2025-06-27 06:38:20.449000000 +0000
Change: 2025-06-27 06:38:20.449000000 +0000
 Birth: 2025-06-27 06:38:20.449000000 +0000

vagrant ssh -c 'stat /etc/fstab' with Vagrant v2.4.6:

  File: /etc/fstab
  Size: 596       	Blocks: 8          IO Block: 4096   regular file
Device: 0,29	Inode: 365793      Links: 1
Access: (0444/-r--r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2025-06-27 06:47:58.038000000 +0000
Modify: 2025-06-27 06:47:58.166000000 +0000
Change: 2025-06-27 06:47:58.166000000 +0000
 Birth: 2025-06-27 06:47:58.038000000 +0000

[taru-garg-hashicorp]

Hi @amezin,

Thanks for the report on the issue, I wonder if it could be a regression due to #12607, could you try setting the following parameter to something like below ?

config.vm.allow_fstab_modification = false

Given it is a read-only file system I think it shouldn't matter if it is set to false unless you want Vagrant to modify /etc/fstab in which case the error is correct, and Vagrant would need ability to write to the file system to be able to make changes.

Cheers!

[amezin]

Given it is a read-only file system I think it shouldn't matter if it is set to false

No, it matters

and Vagrant would need ability to write to the file system to be able to make changes

Vagrant 2.4.6 successfully modifies the file. Only the new v2.4.7 can't.

Because /etc/fstab is not itself on a read-only file system. It's a symlink to a file on a read-only file system. Vagrant 2.4.6 simply replaces /etc/fstab with a modified file. Vagrant 2.4.7 tries to write to the symlink target - /etc/static/fstab - instead.