[PATCH v2 5/7] fs: FS_IOC_GETSYSFSNAME

[Kent Overstreet <kent.overstreet@linux.dev>]

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).

Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: "Darrick J. Wong" <djwong@kernel.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 fs/ioctl.c              | 17 +++++++++++++++++
 include/linux/fs.h      |  1 +
 include/uapi/linux/fs.h | 10 ++++++++++
 3 files changed, 28 insertions(+)

diff --git a/fs/ioctl.c b/fs/ioctl.c
index 046c30294a82..7c37765bd04e 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_get_fs_sysfs_path(struct file *file, void __user *argp)
+{
+	struct super_block *sb = file_inode(file)->i_sb;
+
+	if (!strlen(sb->s_sysfs_name))
+		return -ENOIOCTLCMD;
+
+	struct fssysfspath u = {};
+
+	snprintf(u.name, sizeof(u.name), "%s/%s", sb->s_type->name, sb->s_sysfs_name);
+
+	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_GETFSSYSFSPATH:
+		return ioctl_get_fs_sysfs_path(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 acdc56987cb1..b96c1d009718 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 16a6ecadfd8d..c0f7bd4b6350 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -77,6 +77,10 @@ struct fsuuid2 {
 	__u8	uuid[16];
 };
 
+struct fssysfspath {
+	__u8			name[128];
+};
+
 /* extent-same (dedupe) ioctls; these MUST match the btrfs ioctl definitions */
 #define FILE_DEDUPE_RANGE_SAME		0
 #define FILE_DEDUPE_RANGE_DIFFERS	1
@@ -206,6 +210,12 @@ struct fsxattr {
 /* Returns the external filesystem UUID, the same one blkid returns */
 #define FS_IOC_GETFSUUID		_IOR(0x12, 142, struct fsuuid2)
 
+/*
+ * Returns the path component under /sys/fs/ that refers to this filesystem;
+ * also /sys/kernel/debug/ for filesystems with debugfs exports
+ */
+#define FS_IOC_GETFSSYSFSPATH		_IOR(0x12, 143, struct fssysfspath)
+
 #define BMAP_IOCTL 1		/* obsolete - kept for compatibility */
 #define FIBMAP	   _IO(0x00,1)	/* bmap access */
 #define FIGETBSZ   _IO(0x00,2)	/* get the block size used for bmap */
-- 
2.43.0