Introduce an (effectively) non-repeating system-unique superblock ID that
can be used to determine that two object are in the same superblock without
risking reuse of the ID in the meantime (as is possible with device IDs).

The ID is time-based to make it harder to use it as a covert communications
channel.

Also make it so that this ID can be fetched by the fsinfo() system call.
The ID added so that superblock notification messages will also be able to
be tagged with it.

Signed-off-by: David Howells <dhowells@redhat.com>
---

 fs/fsinfo.c               |    1 +
 fs/super.c                |   24 ++++++++++++++++++++++++
 include/linux/fs.h        |    3 +++
 samples/vfs/test-fsinfo.c |    1 +
 4 files changed, 29 insertions(+)

diff --git a/fs/fsinfo.c b/fs/fsinfo.c
index 55710d6da327..f8e85762fc47 100644
--- a/fs/fsinfo.c
+++ b/fs/fsinfo.c

@@ -92,6 +92,7 @@ static int fsinfo_generic_ids(struct path *path, struct fsinfo_context *ctx)
 	p->f_fstype	= sb->s_magic;
 	p->f_dev_major	= MAJOR(sb->s_dev);
 	p->f_dev_minor	= MINOR(sb->s_dev);
+	p->f_sb_id	= sb->s_unique_id;

 
 	memcpy(&p->f_fsid, &buf.f_fsid, sizeof(p->f_fsid));
 	strlcpy(p->f_fs_name, path->dentry->d_sb->s_type->name,

diff --git a/fs/super.c b/fs/super.c
index cd352530eca9..a63073e6127e 100644
--- a/fs/super.c
+++ b/fs/super.c

@@ -44,6 +44,8 @@ static int thaw_super_locked(struct super_block *sb);
 
 static LIST_HEAD(super_blocks);
 static DEFINE_SPINLOCK(sb_lock);
+static u64 sb_last_identifier;
+static u64 sb_identifier_offset;
 
 static char *sb_writers_name[SB_FREEZE_LEVELS] = {
 	"sb_writers",

@@ -188,6 +190,27 @@ static void destroy_unused_super(struct super_block *s)
 	destroy_super_work(&s->destroy_work);
 }
 
+/*
+ * Generate a unique identifier for a superblock.
+ */
+static void generate_super_id(struct super_block *s)
+{
+	u64 id = ktime_to_ns(ktime_get());
+
+	spin_lock(&sb_lock);
+
+	id += sb_identifier_offset;
+	if (id <= sb_last_identifier) {
+		id = sb_last_identifier + 1;
+		sb_identifier_offset = sb_last_identifier - id;
+	}
+
+	sb_last_identifier = id;
+	spin_unlock(&sb_lock);
+
+	s->s_unique_id = id;
+}
+
 /**
  *	alloc_super	-	create new superblock
  *	@type:	filesystem type superblock should belong to

@@ -273,6 +296,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 		goto fail;
 	if (list_lru_init_memcg(&s->s_inode_lru, &s->s_shrink))
 		goto fail;
+	generate_super_id(s);
 	return s;
 
 fail:

diff --git a/include/linux/fs.h b/include/linux/fs.h
index f74a4ee36eb3..e5db22d536a3 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h

@@ -1550,6 +1550,9 @@ struct super_block {
 
 	spinlock_t		s_inode_wblist_lock;
 	struct list_head	s_inodes_wb;	/* writeback inodes */
+
+	/* Superblock event notifications */
+	u64			s_unique_id;
 } __randomize_layout;
 
 /* Helper functions so that in most cases filesystems will

diff --git a/samples/vfs/test-fsinfo.c b/samples/vfs/test-fsinfo.c
index 6fbf0ce099b2..d6ec5713364f 100644
--- a/samples/vfs/test-fsinfo.c
+++ b/samples/vfs/test-fsinfo.c

@@ -140,6 +140,7 @@ static void dump_fsinfo_generic_ids(void *reply, unsigned int size)
 	printf("\tdev          : %02x:%02x\n", f->f_dev_major, f->f_dev_minor);
 	printf("\tfs           : type=%x name=%s\n", f->f_fstype, f->f_fs_name);
 	printf("\tfsid         : %llx\n", (unsigned long long)f->f_fsid);
+	printf("\tsbid         : %llx\n", (unsigned long long)f->f_sb_id);
 }
 
 static void dump_fsinfo_generic_limits(void *reply, unsigned int size)