[PATCH v2 0/1] exfat: Add support for FS_IOC_{GET,SET}FSLABEL

[Ethan Ferguson]

On 8/18/25 01:52, Christoph Hellwig wrote:
> On Sun, Aug 17, 2025 at 10:32:00AM -0400, Ethan Ferguson wrote:
>> Both e2fsprogs and btrfs-progs now use the FS_IOC_{GET,SET}FSLABEL
>> ioctls to change the label on a mounted filesystem.
> 
> Additionally userspace writes to blocks on mounted file systems are
> dangerous.  They can easily corrupt data when racing with updates
> performed by the kernel, and are impossible when the the
> CONFIG_BLK_DEV_WRITE_MOUNTED config option is disabled.

That's fair. I took a look at how btrfs guards against this, it seems
as if they use mnt_want_write_file to guard against bad writes, and
only write to the in-memory superblock, and commit the transaction
afterwards. However, this (during my testing with
CONFIG_BLK_DEV_WRITE_MOUNTED both on and off) still results in an
immediate disk flush.

My changes from this thread also seem to work with
CONFIG_BLK_DEV_WRITE_MOUNTED both disabled and enabled.

Maybe an alternative would be to only write to sbi->volume_label (with
mutex guarding), and only flush to disk on exfat_put_super? And to use
mnt_want_write_file as well.

Unfortunately, given that I'm pretty new to kernel development, I
wouldn't know that the best way to approach this. Any thoughts, or
pointers in the right direction, would be appreciated.

Thank you for the feedback!