shmem: allow reporting fanotify events with file handles on tmpfs

mainline inclusion
from mainline-5.13-rc1
commit 59cda49e
category: feature
bugzilla: 187162, https://gitee.com/openeuler/kernel/issues/I5FYKV
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=59cda49ecf6c9a32fae4942420701b6e087204f6

-------------------------------------------------

Since kernel v5.1, fanotify_init(2) supports the flag FAN_REPORT_FID
for identifying objects using file handle and fsid in events.

fanotify_mark(2) fails with -ENODEV when trying to set a mark on
filesystems that report null f_fsid in stasfs(2).

Use the digest of uuid as f_fsid for tmpfs to uniquely identify tmpfs
objects as best as possible and allow setting an fanotify mark that
reports events with file handles on tmpfs.

Link: https://lore.kernel.org/r/20210322173944.449469-3-amir73il@gmail.comAcked-by: NHugh Dickins <hughd@google.com>
Reviewed-by: NChristian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: NAmir Goldstein <amir73il@gmail.com>
Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: NBaokun Li <libaokun1@huawei.com>
Reviewed-by: Nchengzhihao <chengzhihao1@huawei.com>
Reviewed-by: NZhang Yi <yi.zhang@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

mm/shmem.c

@@ -2869,6 +2869,9 @@ static int shmem_statfs(struct dentry *dentry, struct kstatfs *buf)
 		buf->f_ffree = sbinfo->free_inodes;
 	}
 	/* else leave those fields 0 like simple_statfs */
+
+	buf->f_fsid = uuid_to_fsid(dentry->d_sb->s_uuid.b);
+
 	return 0;
 }