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Date:   Wed, 20 Jul 2022 19:27:02 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Theodore Ts'o <tytso@mit.edu>
Cc:     "Darrick J. Wong" <djwong@kernel.org>,
        Jeremy Bongio <bongiojp@gmail.com>, linux-ext4@vger.kernel.org,
        linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v4] Add ioctls to get/set the ext4 superblock uuid.
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On Wed, Jul 20, 2022 at 02:00:25PM -0400, Theodore Ts'o wrote:
> On Wed, Jul 20, 2022 at 03:11:21PM +0100, Matthew Wilcox wrote:
> > Uhhh.  So what are the semantics of len?  That is, on SET, what does
> > a filesystem do if userspace says "Here's 8 bytes" but the filesystem
> > usually uses 16 bytes?  What does the same filesystem do if userspace
> > offers it 32 bytes?  If the answer is "returns -EINVAL", how does
> > userspace discover what size of volume ID is acceptable to a particular
> > filesystem?
> > 
> > And then, on GET, does 'len' just mean "here's the length of the buffer,
> > put however much will fit into it"?  Should filesystems update it to
> > inform userspace how much was transferred?
> 
> What I'd suggest is that for GET, the length field when called should
> be the length of the buffer, and if the length is too small, we should
> return some error --- probably EINVAL or ENOSPC.  If the buffer size
> length is larger than what is needed, having the file system update it
> with the size of the UUID that was returned.
> 
> And this would be how the userspace can discover size of the UUID.  In
> practice, though, the human user is going to be suppliyng the UUID,
> which means the *human* is going to have to understand that "oh, this
> is a VFAT file system, so I need to give 32-bit UUID formatted as
> DEAD-BEAF" or "oh, this is a ntfs file system, so I need to enter into
> the command line a UUID formatted as the text string
> A24E62F14E62BDA3".  (The user might also end up having to ntfs or vfat
> specific uuid changing tool; that's unclear at this point.)

I think you covered all my questions there except for what happens
if the user tried to set ext4 to 0xDEADBEEF; that should return
-EINVAL?  They could specify 0xDEADBEEF'00000000'00000000'00000000 or
0x00000000'00000000'00000000'DEADBEEF, but it'd be up to them to choose
which one they wanted rather than have the filesystem pad it out for them?

> As far as Jeremy's patch is concerned, I don't think we need to change
> anything forthe SET ioctl, but for the GET util, it would be better in
> the extremely unlikely case where the user pass in a length larger
> than 16 bytes (say, 32), that we return the 16 byte UUID, and update
> the length field to be 16 bytes.
> 
> I don't think it's strictly necessary, since in practice there is no
> reason why a file system's volume identifier would ever be larger than
> 16 bytes --- the chances that we might need an extra 240 bytes to
> specify a multiverse identifier seems.... unlikely.  :-)

Yes, 128 bits is sufficiently unique for this use case.