[2/6] fs: FS_IOC_GETUUID

Commit Message

Kent Overstreet Feb. 5, 2024, 8:05 p.m. UTC

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 <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: Dave Chinner <[e-mail]>
Cc: "Darrick J. Wong" <[e-mail]>
Cc: Theodore Ts'o <[e-mail]>
Cc: [e-mail]
Signed-off-by: Kent Overstreet <[e-mail]>
---
 fs/ioctl.c              | 16 ++++++++++++++++
 include/uapi/linux/fs.h | 16 ++++++++++++++++
 2 files changed, 32 insertions(+)

Comments

Dave Chinner Feb. 5, 2024, 10:17 p.m. UTC | #1

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 <[e-mail]>
> Cc: Christian Brauner <[e-mail]>
> Cc: Jan Kara <[e-mail]>
> Cc: Dave Chinner <[e-mail]>
> Cc: "Darrick J. Wong" <[e-mail]>
> Cc: Theodore Ts'o <[e-mail]>
> Cc: [e-mail]
> Signed-off-by: Kent Overstreet <[e-mail]>
> ---
>  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.

> +
> +	struct fsuuid2 u = { .fsu_len = sb->s_uuid_len, };
> +	memcpy(&u.fsu_uuid[0], &sb->s_uuid, sb->s_uuid_len);

	if (!u.fsu_len)
		return -ENOENT;
	memcpy(&u.fsu_uuid[0], &sb->s_uuid, u.fsu_len);

> +
> +	return copy_to_user(argp, &u, sizeof(u)) ? -EFAULT : 0;
> +}
> +
>  /*
>   * do_vfs_ioctl() is not for drivers and not intended to be EXPORT_SYMBOL()'d.
>   * It's just a simple helper for sys_ioctl and compat_sys_ioctl.
> @@ -845,6 +858,9 @@ static int do_vfs_ioctl(struct file *filp, unsigned int fd,
>  	case FS_IOC_FSSETXATTR:
>  		return ioctl_fssetxattr(filp, argp);
>  
> +	case FS_IOC_GETFSUUID:
> +		return ioctl_getfsuuid(filp, argp);
> +
>  	default:
>  		if (S_ISREG(inode->i_mode))
>  			return file_ioctl(filp, cmd, argp);
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 48ad69f7722e..0389fea87db5 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -64,6 +64,20 @@ struct fstrim_range {
>  	__u64 minlen;
>  };
>  
> +/*
> + * We include a length field because some filesystems (vfat) have an identifier
> + * that we do want to expose as a UUID, but doesn't have the standard length.
> + *
> + * We use a fixed size buffer beacuse this interface will, by fiat, never
> + * support "UUIDs" longer than 16 bytes; we don't want to force all downstream
> + * users to have to deal with that.
> + */
> +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.

> +
>  /* 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?

-Dave.

Kent Overstreet Feb. 5, 2024, 10:49 p.m. UTC | #2

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 <[e-mail]>
> > Cc: Christian Brauner <[e-mail]>
> > Cc: Jan Kara <[e-mail]>
> > Cc: Dave Chinner <[e-mail]>
> > Cc: "Darrick J. Wong" <[e-mail]>
> > Cc: Theodore Ts'o <[e-mail]>
> > Cc: [e-mail]
> > Signed-off-by: Kent Overstreet <[e-mail]>
> > ---
> >  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?

Darrick J. Wong Feb. 5, 2024, 11:59 p.m. UTC | #3

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 <[e-mail]>
> > > Cc: Christian Brauner <[e-mail]>
> > > Cc: Jan Kara <[e-mail]>
> > > Cc: Dave Chinner <[e-mail]>
> > > Cc: "Darrick J. Wong" <[e-mail]>
> > > Cc: Theodore Ts'o <[e-mail]>
> > > Cc: [e-mail]
> > > Signed-off-by: Kent Overstreet <[e-mail]>
> > > ---
> > >  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

Amir Goldstein Feb. 6, 2024, 8:24 a.m. UTC | #4

On Tue, Feb 6, 2024 at 12:49 AM Kent Overstreet
<[e-mail]> 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.
> > >

It's fine to have a generic implementation, but the filesystem should
have the option to opt-in for a specific implementation.

There are several examples, even with xfs and btrfs where ->s_uuid
does not contain the filesystem's UUID or there is more than one
uuid and ->s_uuid is not the correct one to expose to the user.

A model like ioctl_[gs]etflags() looks much more appropriate
and could be useful for network filesystems/FUSE as well.

> > > Signed-off-by: Kent Overstreet <[e-mail]>
> > > Cc: Christian Brauner <[e-mail]>
> > > Cc: Jan Kara <[e-mail]>
> > > Cc: Dave Chinner <[e-mail]>
> > > Cc: "Darrick J. Wong" <[e-mail]>
> > > Cc: Theodore Ts'o <[e-mail]>
> > > Cc: [e-mail]
> > > Signed-off-by: Kent Overstreet <[e-mail]>
> > > ---
> > >  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?
>

All the other hoisted FS_IOC_* use the original fs ioctl namespace they
came from. Although it is not an actual hoist, I'd use:

struct fsuuid128 {
       __u32       fsu_len;
       __u32       fsu_flags;
       __u8        fsu_uuid[16];
};

#define FS_IOC_GETFSUUID              _IOR('f', 45, struct fsuuid128)
#define FS_IOC_SETFSUUID              _IOW('f', 46, struct fsuuid128)

Technically, could also overload EXT4_IOC_[GS]ETFSUUID numbers
because of the different type:

#define FS_IOC_GETFSUUID              _IOR('f', 44, struct fsuuid128)
#define FS_IOC_SETFSUUID              _IOW('f', 44, struct fsuuid128)

and then ext4 can follow up with this patch, because as far as I can tell,
the ext4 implementation is already compatible with the new ioctls.

Thanks,
Amir.

--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -1613,8 +1613,10 @@ static long __ext4_ioctl(struct file *filp,
unsigned int cmd, unsigned long arg)
                return ext4_ioctl_setlabel(filp,
                                           (const void __user *)arg);

+       case FS_IOC_GETFSUUID:
         case EXT4_IOC_GETFSUUID:
                 return ext4_ioctl_getuuid(EXT4_SB(sb), (void __user *)arg);
+       case FS_IOC_SETFSUUID:
         case EXT4_IOC_SETFSUUID:
                 return ext4_ioctl_setuuid(filp, (const void __user *)arg);

Kent Overstreet Feb. 6, 2024, 9 a.m. UTC | #5

On Tue, Feb 06, 2024 at 10:24:45AM +0200, Amir Goldstein wrote:
> On Tue, Feb 6, 2024 at 12:49 AM Kent Overstreet
> <[e-mail]> 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.
> > > >
> 
> It's fine to have a generic implementation, but the filesystem should
> have the option to opt-in for a specific implementation.
> 
> There are several examples, even with xfs and btrfs where ->s_uuid
> does not contain the filesystem's UUID or there is more than one
> uuid and ->s_uuid is not the correct one to expose to the user.

Yeah, some of you were smoking some good stuff from the stories I've
been hearing...

> A model like ioctl_[gs]etflags() looks much more appropriate
> and could be useful for network filesystems/FUSE as well.

A filesystem needs to store two UUIDs (that identify the filesystem as a
whole).

 - Your internal UUID, which can never change because it's referenced in
   various other on disk data structures
 - Your external UUID, which identifies the filesystem to the outside
   world. Users want to be able to change this - which is why it has to
   be distinct from the internal UUID.

The internal UUID must never be exposed to the outside world, and that
includes the VFS; storing your private UUID in sb->s_uuid is wrong -
separation of concerns.

yes, I am aware of fscrypt, and yes, someone's going to have to fix
that.

This interface is only for the external/public UUID.

Patch

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);
+
+	struct fsuuid2 u = { .fsu_len = sb->s_uuid_len, };
+	memcpy(&u.fsu_uuid[0], &sb->s_uuid, sb->s_uuid_len);
+
+	return copy_to_user(argp, &u, sizeof(u)) ? -EFAULT : 0;
+}
+
 /*
  * do_vfs_ioctl() is not for drivers and not intended to be EXPORT_SYMBOL()'d.
  * It's just a simple helper for sys_ioctl and compat_sys_ioctl.
@@ -845,6 +858,9 @@  static int do_vfs_ioctl(struct file *filp, unsigned int fd,
 	case FS_IOC_FSSETXATTR:
 		return ioctl_fssetxattr(filp, argp);
 
+	case FS_IOC_GETFSUUID:
+		return ioctl_getfsuuid(filp, argp);
+
 	default:
 		if (S_ISREG(inode->i_mode))
 			return file_ioctl(filp, cmd, argp);
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 48ad69f7722e..0389fea87db5 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -64,6 +64,20 @@  struct fstrim_range {
 	__u64 minlen;
 };
 
+/*
+ * We include a length field because some filesystems (vfat) have an identifier
+ * that we do want to expose as a UUID, but doesn't have the standard length.
+ *
+ * We use a fixed size buffer beacuse this interface will, by fiat, never
+ * support "UUIDs" longer than 16 bytes; we don't want to force all downstream
+ * users to have to deal with that.
+ */
+struct fsuuid2 {
+	__u32       fsu_len;
+	__u32       fsu_flags;
+	__u8        fsu_uuid[16];
+};
+
 /* 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)
 
 /*
  * Inode flags (FS_IOC_GETFLAGS / FS_IOC_SETFLAGS)