Re: [PATCH 2/6] fs: FS_IOC_GETUUID

[Kent Overstreet <kent.overstreet@linux.dev>]

On Tue, Feb 06, 2024 at 09:17:58AM +1100, Dave Chinner wrote:
> On Mon, Feb 05, 2024 at 03:05:13PM -0500, Kent Overstreet wrote:
> > Add a new generic ioctls for querying the filesystem UUID.
> > 
> > These are lifted versions of the ext4 ioctls, with one change: we're not
> > using a flexible array member, because UUIDs will never be more than 16
> > bytes.
> > 
> > This patch adds a generic implementation of FS_IOC_GETFSUUID, which
> > reads from super_block->s_uuid; FS_IOC_SETFSUUID is left for individual
> > filesystems to implement.
> > 
> > Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
> > Cc: Christian Brauner <brauner@kernel.org>
> > Cc: Jan Kara <jack@suse.cz>
> > Cc: Dave Chinner <dchinner@redhat.com>
> > Cc: "Darrick J. Wong" <djwong@kernel.org>
> > Cc: Theodore Ts'o <tytso@mit.edu>
> > Cc: linux-fsdevel@vger.kernel.or
> > Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
> > ---
> >  fs/ioctl.c              | 16 ++++++++++++++++
> >  include/uapi/linux/fs.h | 16 ++++++++++++++++
> >  2 files changed, 32 insertions(+)
> > 
> > diff --git a/fs/ioctl.c b/fs/ioctl.c
> > index 76cf22ac97d7..858801060408 100644
> > --- a/fs/ioctl.c
> > +++ b/fs/ioctl.c
> > @@ -763,6 +763,19 @@ static int ioctl_fssetxattr(struct file *file, void __user *argp)
> >  	return err;
> >  }
> >  
> > +static int ioctl_getfsuuid(struct file *file, void __user *argp)
> > +{
> > +	struct super_block *sb = file_inode(file)->i_sb;
> > +
> > +	if (WARN_ON(sb->s_uuid_len > sizeof(sb->s_uuid)))
> > +		sb->s_uuid_len = sizeof(sb->s_uuid);
> 
> A "get"/read only ioctl should not be change superblock fields -
> this is not the place for enforcing superblock filed constraints.
> Make a helper function super_set_uuid(sb, uuid, uuid_len) for the
> filesystems to call that does all the validity checking and then
> sets the superblock fields appropriately.

*nod* good thought...

> > +struct fsuuid2 {
> > +	__u32       fsu_len;
> > +	__u32       fsu_flags;
> > +	__u8        fsu_uuid[16];
> > +};
> 
> Nobody in userspace will care that this is "version 2" of the ext4
> ioctl. I'd just name it "fs_uuid" as though the ext4 version didn't
> ever exist.

I considered that - but I decided I wanted the explicit versioning,
because too often we live with unfixed mistakes because versioning is
ugly, or something?

Doing a new revision of an API should be a normal, frequent thing, and I
want to start making it a convention.

> 
> > +
> >  /* extent-same (dedupe) ioctls; these MUST match the btrfs ioctl definitions */
> >  #define FILE_DEDUPE_RANGE_SAME		0
> >  #define FILE_DEDUPE_RANGE_DIFFERS	1
> > @@ -215,6 +229,8 @@ struct fsxattr {
> >  #define FS_IOC_FSSETXATTR		_IOW('X', 32, struct fsxattr)
> >  #define FS_IOC_GETFSLABEL		_IOR(0x94, 49, char[FSLABEL_MAX])
> >  #define FS_IOC_SETFSLABEL		_IOW(0x94, 50, char[FSLABEL_MAX])
> > +#define FS_IOC_GETFSUUID		_IOR(0x94, 51, struct fsuuid2)
> > +#define FS_IOC_SETFSUUID		_IOW(0x94, 52, struct fsuuid2)
> 
> 0x94 is the btrfs ioctl space, not the VFS space - why did you
> choose that? That said, what is the VFS ioctl space identifier? 'v',
> perhaps?

"Promoting ioctls from fs to vfs without revising and renaming
considered harmful"... this is a mess that could have been avoided if we
weren't taking the lazy route.

And 'v' doesn't look like it to me, I really have no idea what to use
here. Does anyone?