Re: [PATCH 1/2] xfs: quietly ignore deprecated mount options

[Oleksandr Natalenko <oleksandr@natalenko.name>]

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Hello.

On úterý 14. října 2025 1:32:29, středoevropský letní čas Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> Apparently we can never deprecate mount options in this project, because
> it will invariably turn out that some foolish userspace depends on some
> behavior and break.  From Oleksandr Natalenko:
> 
> > In v6.18, the attr2 XFS mount option is removed. This may silently
> > break system boot if the attr2 option is still present in /etc/fstab
> > for rootfs.
> >
> > Consider Arch Linux that is being set up from scratch with / being
> > formatted as XFS. The genfstab command that is used to generate
> > /etc/fstab produces something like this by default:
> >
> > /dev/sda2 on / type xfs (rw,relatime,attr2,discard,inode64,logbufs=8,logbsize=32k,noquota)
> >
> > Once the system is set up and rebooted, there's no deprecation warning
> > seen in the kernel log:
> >
> > # cat /proc/cmdline
> > root=UUID=77b42de2-397e-47ee-a1ef-4dfd430e47e9 rootflags=discard rd.luks.options=discard quiet
> >
> > # dmesg | grep -i xfs
> > [    2.409818] SGI XFS with ACLs, security attributes, realtime, scrub, repair, quota, no debug enabled
> > [    2.415341] XFS (sda2): Mounting V5 Filesystem 77b42de2-397e-47ee-a1ef-4dfd430e47e9
> > [    2.442546] XFS (sda2): Ending clean mount
> >
> > Although as per the deprecation intention, it should be there.
> >
> > Vlastimil (in Cc) suggests this is because xfs_fs_warn_deprecated()
> > doesn't produce any warning by design if the XFS FS is set to be
> > rootfs and gets remounted read-write during boot. This imposes two
> > problems:
> >
> > 1) a user doesn't see the deprecation warning; and
> > 2) with v6.18 kernel, the read-write remount fails because of unknown
> >    attr2 option rendering system unusable:
> >
> > systemd[1]: Switching root.
> > systemd-remount-fs[225]: /usr/bin/mount for / exited with exit status 32.
> >
> > # mount -o rw /
> > mount: /: fsconfig() failed: xfs: Unknown parameter 'attr2'.
> >
> > Thorsten (in Cc) suggested reporting this as a user-visible regression.
> >
> > From my PoV, although the deprecation is in place for 5 years already,
> > it may not be visible enough as the warning is not emitted for rootfs.
> > Considering the amount of systems set up with XFS on /, this may
> > impose a mass problem for users.
> >
> > Vlastimil suggested making attr2 option a complete noop instead of
> > removing it.
> 
> IOWs, the initrd mounts the root fs with (I assume) no mount options,
> and mount -a remounts with whatever options are in fstab.  However,
> XFS doesn't complain about deprecated mount options during a remount, so
> technically speaking we were not warning all users in all combinations
> that they were heading for a cliff.
> 
> Gotcha!!
> 
> Now, how did 'attr2' get slurped up on so many systems?  The old code
> would put that in /proc/mounts if the filesystem happened to be in attr2
> mode, even if user hadn't mounted with any such option.  IOWs, this is
> because someone thought it would be a good idea to advertise system
> state via /proc/mounts.
> 
> The easy way to fix this is to reintroduce the four mount options but
> map them to a no-op option that ignores them, and hope that nobody's
> depending on attr2 to appear in /proc/mounts.  (Hint: use the fsgeometry
> ioctl).
> 
> Lessons learned:
> 
>  1. Don't expose system state via /proc/mounts; the only strings that
>     ought to be there are options *explicitly* provided by the user.
>  2. Never tidy, it's not worth the stress and irritation.
> 
> Reported-by: oleksandr@natalenko.name
> Reported-by: vbabka@suse.cz
> Cc: <stable@vger.kernel.org> # v6.18-rc1
> Fixes: b9a176e54162f8 ("xfs: remove deprecated mount options")
> Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
> ---
>  fs/xfs/xfs_super.c |   13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index e85a156dc17d16..e1df41991fccc3 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -102,7 +102,7 @@ static const struct constant_table dax_param_enums[] = {
>   * Table driven mount option parser.
>   */
>  enum {
> -	Opt_logbufs, Opt_logbsize, Opt_logdev, Opt_rtdev,
> +	Opt_quietlyignore, Opt_logbufs, Opt_logbsize, Opt_logdev, Opt_rtdev,
>  	Opt_wsync, Opt_noalign, Opt_swalloc, Opt_sunit, Opt_swidth, Opt_nouuid,
>  	Opt_grpid, Opt_nogrpid, Opt_bsdgroups, Opt_sysvgroups,
>  	Opt_allocsize, Opt_norecovery, Opt_inode64, Opt_inode32,
> @@ -115,6 +115,14 @@ enum {
>  };
>  
>  static const struct fs_parameter_spec xfs_fs_parameters[] = {
> +	/*
> +	 * These mount options were advertised in /proc/mounts even if the
> +	 * filesystem had not been mounted with that option.  Quietly ignore
> +	 * them to avoid breaking scripts that captured /proc/mounts.
> +	 */
> +	fsparam_flag("attr",		Opt_quietlyignore),

Should have been "attr2" here I suppose.

Thanks.

> +	fsparam_flag("noattr2",		Opt_quietlyignore),
> +
>  	fsparam_u32("logbufs",		Opt_logbufs),
>  	fsparam_string("logbsize",	Opt_logbsize),
>  	fsparam_string("logdev",	Opt_logdev),
> @@ -1408,6 +1416,8 @@ xfs_fs_parse_param(
>  		return opt;
>  
>  	switch (opt) {
> +	case Opt_quietlyignore:
> +		return 0;
>  	case Opt_logbufs:
>  		parsing_mp->m_logbufs = result.uint_32;
>  		return 0;
> @@ -1528,7 +1538,6 @@ xfs_fs_parse_param(
>  		xfs_mount_set_dax_mode(parsing_mp, result.uint_32);
>  		return 0;
>  #endif
> -	/* Following mount options will be removed in September 2025 */
>  	case Opt_max_open_zones:
>  		parsing_mp->m_max_open_zones = result.uint_32;
>  		return 0;
> 

-- 
Oleksandr Natalenko, MSE

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