Re: [PATCH v2 1/1] xfs: Fail remount with noattr2 on a v5 with v4 enabled

[Christoph Hellwig <hch@infradead.org>]

On Tue, Apr 29, 2025 at 02:32:08PM +0530, Nirjhar Roy (IBM) wrote:
> Bug: When we compile the kernel with CONFIG_XFS_SUPPORT_V4=y,
> remount with "-o remount,noattr2" on a v5 XFS does not
> fail explicitly.
> 
> Reproduction:
> mkfs.xfs -f /dev/loop0
> mount /dev/loop0 /mnt/scratch
> mount -o remount,noattr2 /dev/loop0 /mnt/scratch
> 
> However, with CONFIG_XFS_SUPPORT_V4=n, the remount
> correctly fails explicitly. This is because the way the
> following 2 functions are defined:
> 
> static inline bool xfs_has_attr2 (struct xfs_mount *mp)
> {
> 	return !IS_ENABLED(CONFIG_XFS_SUPPORT_V4) ||
> 		(mp->m_features & XFS_FEAT_ATTR2);
> }
> static inline bool xfs_has_noattr2 (const struct xfs_mount *mp)
> {
> 	return mp->m_features & XFS_FEAT_NOATTR2;
> }
> 
> xfs_has_attr2() returns true when CONFIG_XFS_SUPPORT_V4=n
> and hence, the the following if condition in
> xfs_fs_validate_params() succeeds and returns -EINVAL:
> 
> /*
>  * We have not read the superblock at this point, so only the attr2
>  * mount option can set the attr2 feature by this stage.
>  */
> 
> if (xfs_has_attr2(mp) && xfs_has_noattr2(mp)) {
> 	xfs_warn(mp, "attr2 and noattr2 cannot both be specified.");
> 	return -EINVAL;
> }
> 
> With CONFIG_XFS_SUPPORT_V4=y, xfs_has_attr2() always return
> false and hence no error is returned.
> 
> Fix: Check if the existing mount has crc enabled(i.e, of
> type v5 and has attr2 enabled) and the
> remount has noattr2, if yes, return -EINVAL.
> 
> I have tested xfs/{189,539} in fstests with v4
> and v5 XFS with both CONFIG_XFS_SUPPORT_V4=y/n and
> they both behave as expected.
> 
> This patch also fixes remount from noattr2 -> attr2 (on a v4 xfs).
> 
> Related discussion in [1]
> 
> [1] https://lore.kernel.org/all/Z65o6nWxT00MaUrW@dread.disaster.area/
> 
> Signed-off-by: Nirjhar Roy (IBM) <nirjhar.roy.lists@gmail.com>
> ---
>  fs/xfs/xfs_super.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> index b2dd0c0bf509..1fd45567ae00 100644
> --- a/fs/xfs/xfs_super.c
> +++ b/fs/xfs/xfs_super.c
> @@ -2114,6 +2114,22 @@ xfs_fs_reconfigure(
>  	if (error)
>  		return error;
>  
> +	/* attr2 -> noattr2 */
> +	if (xfs_has_noattr2(new_mp)) {
> +		if (xfs_has_crc(mp)) {
> +			xfs_warn(mp, "attr2 and noattr2 cannot both be specified.");

Nit: normal xfs style is to move the message to a new line when іt
overflows 80 characters:

		if (xfs_has_crc(mp)) {
			xfs_warn(mp,
	"attr2 and noattr2 cannot both be specified.");

> +			return -EINVAL;
> +		}
> +		else {

No need for an else after a return.  And for cases where there is an
else the kernel coding style keeps it on the same line as the closing
brace.

> +	/* Now that mp has been modified according to the remount options, we do a
> +	 * final option validation with xfs_finish_flags() just like it is done
> +	 * during mount. We cannot use xfs_finish_flags() on new_mp as it contains
> +	 * only the user given options.
> +	 */

Please keep comments to 80 characters.  Also the kernel coding style
keeps the

	/*

at the beginning of a comment on a separate line.