Re: [PATCH 4/6] fs: FS_IOC_GETSYSFSNAME

[Kent Overstreet]

On Mon, Feb 05, 2024 at 02:27:32PM -0800, Darrick J. Wong wrote:
> On Mon, Feb 05, 2024 at 03:05:15PM -0500, Kent Overstreet wrote:
> > Add a new ioctl for getting the sysfs name of a filesystem - the path
> > under /sys/fs.
> > 
> > This is going to let us standardize exporting data from sysfs across
> > filesystems, e.g. time stats.
> > 
> > The returned path will always be of the form "$FSTYP/$SYSFS_IDENTIFIER",
> > where the sysfs identifier may be a UUID (for bcachefs) or a device name
> > (xfs).
> > 
> > Signed-off-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
> > Cc: Christian Brauner <brauner@xxxxxxxxxx>
> > Cc: Jan Kara <jack@xxxxxxx>
> > Cc: Dave Chinner <dchinner@xxxxxxxxxx>
> > Cc: "Darrick J. Wong" <djwong@xxxxxxxxxx>
> > Cc: Theodore Ts'o <tytso@xxxxxxx>
> > Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
> > ---
> >  fs/ioctl.c              | 17 +++++++++++++++++
> >  include/linux/fs.h      |  1 +
> >  include/uapi/linux/fs.h |  5 +++++
> >  3 files changed, 23 insertions(+)
> > 
> > diff --git a/fs/ioctl.c b/fs/ioctl.c
> > index 858801060408..cb3690811d3d 100644
> > --- a/fs/ioctl.c
> > +++ b/fs/ioctl.c
> > @@ -776,6 +776,20 @@ static int ioctl_getfsuuid(struct file *file, void __user *argp)
> >  	return copy_to_user(argp, &u, sizeof(u)) ? -EFAULT : 0;
> >  }
> >  
> > +static int ioctl_getfssysfsname(struct file *file, void __user *argp)
> 
> ackpthspacesplease.
> 
> "ioctl_get_fs_sysfs_name"?

It did feel a bit trolling writing that :)

> 
> > +{
> > +	struct super_block *sb = file_inode(file)->i_sb;
> > +
> > +	if (!strlen(sb->s_sysfs_name))
> > +		return -ENOIOCTLCMD;
> > +
> > +	struct fssysfsname u = {};
> > +
> > +	snprintf(u.name, sizeof(u.name), "%s/%s", sb->s_type->name, sb->s_sysfs_name);
> 
> Does this actually guarantee that there will be a trailing null in the
> output?  It's really stupid that GETFSLABEL can return an unterminated
> string if the label is exactly the size of the char array.

It's snprintf, so yes.

(queue another "why are we using raw char arrays everywhere in 2024"
rant, I have to double check this stuff too).

> 
> > +
> > +	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.
> > @@ -861,6 +875,9 @@ static int do_vfs_ioctl(struct file *filp, unsigned int fd,
> >  	case FS_IOC_GETFSUUID:
> >  		return ioctl_getfsuuid(filp, argp);
> >  
> > +	case FS_IOC_GETFSSYSFSNAME:
> 
> File System Ioctl Get File System System File System Name.
> 
> Yuck.
> 
> FS_IOC_GETSYSFSPATH?
> 
> Also, do we want to establish that this works for /sys/fs and
> /sys/kernel/debug at the same time?

Yeah, I'll add a comment to that effect.

> 
> > +		return ioctl_getfssysfsname(filp, argp);
> > +
> >  	default:
> >  		if (S_ISREG(inode->i_mode))
> >  			return file_ioctl(filp, cmd, argp);
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index ff41ea6c3a9c..7f23f593f17c 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -1258,6 +1258,7 @@ struct super_block {
> >  	char			s_id[32];	/* Informational name */
> >  	uuid_t			s_uuid;		/* UUID */
> >  	u8			s_uuid_len;	/* Default 16, possibly smaller for weird filesystems */
> > +	char			s_sysfs_name[UUID_STRING_LEN + 1];
> >  
> >  	unsigned int		s_max_links;
> >  
> > diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> > index 0389fea87db5..6dd14a453277 100644
> > --- a/include/uapi/linux/fs.h
> > +++ b/include/uapi/linux/fs.h
> > @@ -78,6 +78,10 @@ struct fsuuid2 {
> >  	__u8        fsu_uuid[16];
> >  };
> >  
> > +struct fssysfsname {
> > +	__u8			name[64];
> > +};
> > +
> >  /* extent-same (dedupe) ioctls; these MUST match the btrfs ioctl definitions */
> >  #define FILE_DEDUPE_RANGE_SAME		0
> >  #define FILE_DEDUPE_RANGE_DIFFERS	1
> > @@ -231,6 +235,7 @@ struct fsxattr {
> >  #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)
> > +#define FS_IOC_GETFSSYSFSNAME		_IOR(0x94, 53, struct fssysfsname)
> 
> 0x94 is btrfs, don't add things to their "name" space.

Can we please document this somewhere!?

What, dare I ask, is the "namespace" I should be using?