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Date: Tue, 19 Jul 2022 20:30:57 -0700 From: "Darrick J. Wong" <djwong@...nel.org> To: Matthew Wilcox <willy@...radead.org> Cc: Jeremy Bongio <bongiojp@...il.com>, Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org, linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org Subject: Re: [PATCH v4] Add ioctls to get/set the ext4 superblock uuid. On Wed, Jul 20, 2022 at 04:18:51AM +0100, Matthew Wilcox wrote: > On Tue, Jul 19, 2022 at 04:41:31PM -0700, Jeremy Bongio wrote: > > +/* > > + * Structure for EXT4_IOC_GETFSUUID/EXT4_IOC_SETFSUUID > > + */ > > +struct fsuuid { > > + __u32 fsu_len; > > + __u32 fsu_flags; > > + __u8 fsu_uuid[]; > > +}; > > A UUID has a defined size (128 bits): > https://en.wikipedia.org/wiki/Universally_unique_identifier > > Why are we defining flags and len? @flags because XFS actually need to add a superblock feature bit (meta_uuid) to change the UUID while the fs is mounted. That kind of change can break backwards compatiblity, so we might want to make *absolutely sure* that the sysadmin is aware of this: # xfs_io -c 'setfsuuid 42f3d4d6-d5bb-4e91-a187-2ed0f3c080b2 --to-hell-with-backwards-compatibility' /mnt @len because some filesystems like vfat have volume identifiers that aren't actually UUIDs (they're u32); some day someone might want to port vfat to implement at least the GETFSUUID part (they already have FAT_IOCTL_GET_VOLUME_ID); and given the amount of confusion that results when buffer lengths are implied (see [GS]ETFSLABEL) I'd rather this pair of ioctls be explicit about the buffer length now rather than deal with the fallout of omitting it now and regretting it later. --D
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