On Wed, Jul 20, 2022 at 12:09 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Wed, Jul 20, 2022 at 11:47:08AM -0700, Darrick J. Wong wrote: > > 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. > > Or userspace passes in 0 for the len and the filesystem returns -EINVAL > and sets ->len to what the valid size would be? There's a few ways of > solving this. This solution seems more intuitive to me. If EXT4_IOCTL_GETFSUUID is called with fsu_len set to 0, then fsu_len will be set to the required UUID length and return with an error code. I discussed this solution when first developing the ioctl, but I left it out since for ext4 I don't have a use case. However since other filesystems will likely implement this ioctl, it makes sense to add. I'll send out a new manpage with that detail added and update the code.
