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Message-ID: <CAOQ4uxjvEL4P4vV5SKpHVS5DtOwKpxAn4n4+Kfqawcu+H-MC5g@mail.gmail.com>
Date: Tue, 6 Feb 2024 10:24:45 +0200
From: Amir Goldstein <amir73il@...il.com>
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>,
"Darrick J. Wong" <djwong@...nel.org>, "Theodore Ts'o" <tytso@....edu>, linux-fsdevel@...r.kernel.or,
Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [PATCH 2/6] fs: FS_IOC_GETUUID
On Tue, Feb 6, 2024 at 12:49 AM Kent Overstreet
<kent.overstreet@...ux.dev> 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 <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?
>
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);
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