On Wed, Jul 20, 2022 at 07:27:02PM +0100, Matthew Wilcox wrote: > 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. I'd suggest something different -- calling the getfsuuid ioctl with a null argument should return the filesystem's volid/uuid size as the return value. If userspace supplies a non-null argument, then fsu_len has to match the filesystem's volid/uuid size or else you get EINVAL. --D > > 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.
