Re: [PATCH] libfrog: obtain the actual available device when the root device is /dev/root

[Dave Chinner <david@fromorbit.com>]

On Tue, Sep 09, 2025 at 07:29:02PM +0800, Wu Guanghao wrote:
> When starting a Fedora virtual machine using QEMU, if the device corresponding
> to the root directory is the entire disk or a disk partition, the device
> recorded in /proc/self/mounts will be /dev/root instead of the true device.

How does this /dev/root situation occur? My fedora VMs
show the real root device and not /dev/root in /proc/self/mounts,
so it's not clear to me why /dev/root is being reported here?

This smells of a custom boot sequence that is mounting the root
filesystem on a temporary initramfs /dev/root node (which then gets
removed once the initramfs is done) rather than using pivot_root to
move the real root fs into place once the real device nodes have
been initialised and the root fs mounted using them... 

> This can lead to the failure of executing commands such as xfs_growfs/xfs_info.
> 
> $ cat /proc/self/mounts
> /dev/root / xfs rw,seclabel,relatime,attr2,inode64,logbufs=8,logbsize=32k,noquota 0 0
> devtmpfs /dev devtmpfs rw,seclabel,relatime,size=4065432k,nr_inodes=1016358,mode=755 0 0
> ...

> 
> $ mount
> /dev/sda3 on / type xfs (rw,relatime,seclabel,attr2,inode64,logbufs=8,logbsize=32k,noquota)
> devtmpfs on /dev type devtmpfs (rw,relatime,seclabel,size=4065432k,nr_inodes=1016358,mode=755)
> ...
>
> $ xfs_growfs /
> xfs_growfs: / is not a mounted XFS filesystem
> 
> $ xfs_growfs /dev/sda3
> xfs_growfs: /dev/sda3 is not a mounted XFS filesystem
> 
> $ xfs_info /
> /: cannot find mount point.#
> 
> So, if the root device is found to be /dev/root, we need to obtain the
> corresponding real device first.

IMO, having a bogus device in /proc/self/mounts output is the
problem that needs to be fixed here. Working around a broken
/dev/root device in every userspace utility that reads
/proc/self/mounts does not feel like the right way to address this
problem.

-Dave.
-- 
Dave Chinner
david@fromorbit.com