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.
