linux-ext4 - Re: [PATCH 1/6] fs: super_block->s_uuid

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Date: Mon, 5 Feb 2024 17:56:23 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Dave Chinner <david@...morbit.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-btrfs@...r.kernel.org, linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 1/6] fs: super_block->s_uuid_len

On Tue, Feb 06, 2024 at 08:58:49AM +1100, Dave Chinner wrote:
> On Mon, Feb 05, 2024 at 03:05:12PM -0500, Kent Overstreet wrote:
> > Some weird old filesytems have UUID-like things that we wish to expose
> > as UUIDs, but are smaller; add a length field so that the new
> > FS_IOC_(GET|SET)UUID ioctls can handle them in generic code.
> > 
> > Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> > ---
> >  fs/super.c         | 1 +
> >  include/linux/fs.h | 1 +
> >  2 files changed, 2 insertions(+)
> > 
> > diff --git a/fs/super.c b/fs/super.c
> > index d35e85295489..ed688d2a58a7 100644
> > --- a/fs/super.c
> > +++ b/fs/super.c
> > @@ -375,6 +375,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
> >  	s->s_time_gran = 1000000000;
> >  	s->s_time_min = TIME64_MIN;
> >  	s->s_time_max = TIME64_MAX;
> > +	s->s_uuid_len = sizeof(s->s_uuid);
> 
> So if the filesystem doesn't copy a uuid into sb->s_uuid, then we
> allow those 16 bytes to be pulled from userspace?
> 
> Shouldn't this only get set when the filesystem copies it's uuid
> to the superblock?
> 
> And then in the get uuid  ioctl, if s_uuid_len is zero we can return
> -ENOENT to indicate the filesystem doesn't have a UUID, rather that
> require userspace to determine a filesystem doesn't have a valid
> UUID somehow...

*nod* this all falls out of your super_set_uuid() suggestion