[PATCH 3/3] fanotify: support reporting events with fid on btrfs sub-volumes

[Amir Goldstein <amir73il@gmail.com>]

Setting fanotify marks that report events with fid on btrfs sub-volumes
is not supported, because in the case of btrfs sub-volumes, there is no
uniform fsid that can be cached in the sb connector.

If filesystem supports the cheap ->get_fsid() operation, do not use the
cached fsid in connector and query fsid from inode on every event.
This allows setting fanotify marks that report events with fid on btrfs
sub-volumes.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/notify/fanotify/fanotify.c      | 28 +++++++++++++++++++++-------
 fs/notify/fanotify/fanotify_user.c | 14 ++++++++++----
 2 files changed, 31 insertions(+), 11 deletions(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 9dac7f6e72d2..25282ca0173d 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -535,6 +535,21 @@ static struct inode *fanotify_dfid_inode(u32 event_mask, const void *data,
 	return dir;
 }
 
+/*
+ * If @inode is available and if filesystem supports ->get_fsid(), return the
+ * fsid associated with the inode.  Otherwise, return the pre cached fsid.
+ */
+static __kernel_fsid_t fanotify_inode_fsid(struct inode *inode,
+					   __kernel_fsid_t *fsid)
+{
+	__kernel_fsid_t __fsid;
+
+	if (inode && inode_get_fsid(inode, &__fsid) == 0)
+		return __fsid;
+
+	return *fsid;
+}
+
 static struct fanotify_event *fanotify_alloc_path_event(const struct path *path,
 							unsigned int *hash,
 							gfp_t gfp)
@@ -586,8 +601,8 @@ static struct fanotify_event *fanotify_alloc_fid_event(struct inode *id,
 		return NULL;
 
 	ffe->fae.type = FANOTIFY_EVENT_TYPE_FID;
-	ffe->fsid = *fsid;
-	*hash ^= fanotify_hash_fsid(fsid);
+	ffe->fsid = fanotify_inode_fsid(id, fsid);
+	*hash ^= fanotify_hash_fsid(&ffe->fsid);
 	fanotify_encode_fh(&ffe->object_fh, id, fanotify_encode_fh_len(id),
 			   hash, gfp);
 
@@ -627,8 +642,8 @@ static struct fanotify_event *fanotify_alloc_name_event(struct inode *dir,
 		return NULL;
 
 	fne->fae.type = FANOTIFY_EVENT_TYPE_FID_NAME;
-	fne->fsid = *fsid;
-	*hash ^= fanotify_hash_fsid(fsid);
+	fne->fsid = fanotify_inode_fsid(dir, fsid);
+	*hash ^= fanotify_hash_fsid(&fne->fsid);
 	info = &fne->info;
 	fanotify_info_init(info);
 	if (dir_fh_len) {
@@ -691,9 +706,10 @@ static struct fanotify_event *fanotify_alloc_error_event(
 	fee->fae.type = FANOTIFY_EVENT_TYPE_FS_ERROR;
 	fee->error = report->error;
 	fee->err_count = 1;
-	fee->fsid = *fsid;
 
 	inode = report->inode;
+	fee->fsid = fanotify_inode_fsid(inode, fsid);
+	*hash ^= fanotify_hash_fsid(&fee->fsid);
 	fh_len = fanotify_encode_fh_len(inode);
 
 	/* Bad fh_len. Fallback to using an invalid fh. Should never happen. */
@@ -702,8 +718,6 @@ static struct fanotify_event *fanotify_alloc_error_event(
 
 	fanotify_encode_fh(&fee->object_fh, inode, fh_len, NULL, 0);
 
-	*hash ^= fanotify_hash_fsid(fsid);
-
 	return &fee->fae;
 }
 
diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 0eb9622e8a9f..ed67e5f973ab 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -1570,16 +1570,22 @@ static int fanotify_test_fsid(struct dentry *dentry, __kernel_fsid_t *fsid)
 		return -ENODEV;
 
 	/*
-	 * Make sure dentry is not of a filesystem subvolume (e.g. btrfs)
-	 * which uses a different fsid than sb root.
+	 * If dentry is on a filesystem subvolume (e.g. btrfs), which uses a
+	 * different fsid than sb root, then make sure that filesytem supports
+	 * getting fsid from inode.
 	 */
 	err = vfs_get_fsid(dentry->d_sb->s_root, &root_fsid);
 	if (err)
 		return err;
 
 	if (root_fsid.val[0] != fsid->val[0] ||
-	    root_fsid.val[1] != fsid->val[1])
-		return -EXDEV;
+	    root_fsid.val[1] != fsid->val[1]) {
+		if (!dentry->d_sb->s_op->get_fsid)
+			return -EXDEV;
+
+		/* Cache root fsid in case inode is not available on event */
+		*fsid = root_fsid;
+	}
 
 	return 0;
 }
-- 
2.34.1