linux-kernel - Re: [PATCH 2/6] fs: FS_IOC

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Message-ID: <20240205235930.GP616564@frogsfrogsfrogs>
Date: Mon, 5 Feb 2024 15:59:30 -0800
From: "Darrick J. Wong" <djwong@...nel.org>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Dave Chinner <david@...morbit.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Dave Chinner <dchinner@...hat.com>, Theodore Ts'o <tytso@....edu>,
	linux-fsdevel@...r.kernel.or
Subject: Re: [PATCH 2/6] fs: FS_IOC_GETUUID

On Mon, Feb 05, 2024 at 05:49:30PM -0500, Kent Overstreet wrote:
> On Tue, Feb 06, 2024 at 09:17:58AM +1100, Dave Chinner wrote:
> > On Mon, Feb 05, 2024 at 03:05:13PM -0500, Kent Overstreet wrote:
> > > Add a new generic ioctls for querying the filesystem UUID.
> > > 
> > > These are lifted versions of the ext4 ioctls, with one change: we're not
> > > using a flexible array member, because UUIDs will never be more than 16
> > > bytes.
> > > 
> > > This patch adds a generic implementation of FS_IOC_GETFSUUID, which
> > > reads from super_block->s_uuid; FS_IOC_SETFSUUID is left for individual
> > > filesystems to implement.
> > > 
> > > Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> > > Cc: Christian Brauner <brauner@...nel.org>
> > > Cc: Jan Kara <jack@...e.cz>
> > > Cc: Dave Chinner <dchinner@...hat.com>
> > > Cc: "Darrick J. Wong" <djwong@...nel.org>
> > > Cc: Theodore Ts'o <tytso@....edu>
> > > Cc: linux-fsdevel@...r.kernel.or
> > > Signed-off-by: Kent Overstreet <kent.overstreet@...ux.dev>
> > > ---
> > >  fs/ioctl.c              | 16 ++++++++++++++++
> > >  include/uapi/linux/fs.h | 16 ++++++++++++++++
> > >  2 files changed, 32 insertions(+)
> > > 
> > > diff --git a/fs/ioctl.c b/fs/ioctl.c
> > > index 76cf22ac97d7..858801060408 100644
> > > --- a/fs/ioctl.c
> > > +++ b/fs/ioctl.c
> > > @@ -763,6 +763,19 @@ static int ioctl_fssetxattr(struct file *file, void __user *argp)
> > >  	return err;
> > >  }
> > >  
> > > +static int ioctl_getfsuuid(struct file *file, void __user *argp)
> > > +{
> > > +	struct super_block *sb = file_inode(file)->i_sb;
> > > +
> > > +	if (WARN_ON(sb->s_uuid_len > sizeof(sb->s_uuid)))
> > > +		sb->s_uuid_len = sizeof(sb->s_uuid);
> > 
> > A "get"/read only ioctl should not be change superblock fields -
> > this is not the place for enforcing superblock filed constraints.
> > Make a helper function super_set_uuid(sb, uuid, uuid_len) for the
> > filesystems to call that does all the validity checking and then
> > sets the superblock fields appropriately.
> 
> *nod* good thought...
> 
> > > +struct fsuuid2 {
> > > +	__u32       fsu_len;
> > > +	__u32       fsu_flags;
> > > +	__u8        fsu_uuid[16];
> > > +};
> > 
> > Nobody in userspace will care that this is "version 2" of the ext4
> > ioctl. I'd just name it "fs_uuid" as though the ext4 version didn't
> > ever exist.
> 
> I considered that - but I decided I wanted the explicit versioning,
> because too often we live with unfixed mistakes because versioning is
> ugly, or something?
> 
> Doing a new revision of an API should be a normal, frequent thing, and I
> want to start making it a convention.
> 
> > 
> > > +
> > >  /* extent-same (dedupe) ioctls; these MUST match the btrfs ioctl definitions */
> > >  #define FILE_DEDUPE_RANGE_SAME		0
> > >  #define FILE_DEDUPE_RANGE_DIFFERS	1
> > > @@ -215,6 +229,8 @@ struct fsxattr {
> > >  #define FS_IOC_FSSETXATTR		_IOW('X', 32, struct fsxattr)
> > >  #define FS_IOC_GETFSLABEL		_IOR(0x94, 49, char[FSLABEL_MAX])
> > >  #define FS_IOC_SETFSLABEL		_IOW(0x94, 50, char[FSLABEL_MAX])
> > > +#define FS_IOC_GETFSUUID		_IOR(0x94, 51, struct fsuuid2)
> > > +#define FS_IOC_SETFSUUID		_IOW(0x94, 52, struct fsuuid2)
> > 
> > 0x94 is the btrfs ioctl space, not the VFS space - why did you
> > choose that? That said, what is the VFS ioctl space identifier? 'v',
> > perhaps?
> 
> "Promoting ioctls from fs to vfs without revising and renaming
> considered harmful"... this is a mess that could have been avoided if we
> weren't taking the lazy route.
> 
> And 'v' doesn't look like it to me, I really have no idea what to use
> here. Does anyone?

I thought it was 'f' but apparently that's ext?

--D