[PATCH 06/25] vfs: Introduce a non-repeating system-unique superblock ID [ver #13]

[David Howells]

Introduce a non-repeating system-unique superblock ID that can be used to
tag superblock notification messages.  The ID is time-based to make it
harder to use it as a covert communications channel.

Make it so that this ID can be fetched by the fsinfo() system call.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 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 256a87b62eed..6afeabfe9451 100644
--- a/fs/fsinfo.c
+++ b/fs/fsinfo.c
@@ -92,6 +92,7 @@ static int fsinfo_generic_ids(struct path *path, struct fsinfo_ids *p)
 	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 27b3e41394c8..61819e8e5469 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -43,6 +43,8 @@ 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",
@@ -187,6 +189,27 @@ 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
@@ -270,6 +293,7 @@ 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 71ce3b054c42..f1c74596cd77 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1527,6 +1527,9 @@ 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 e98384e8fb46..90926024e1c5 100644
--- a/samples/vfs/test-fsinfo.c
+++ b/samples/vfs/test-fsinfo.c
@@ -179,6 +179,7 @@ static void dump_attr_IDS(union reply *r, 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_attr_LIMITS(union reply *r, int size)