vfs, cachefiles: Mark a backing file in use with an inode flag

Use an inode flag, S_KERNEL_FILE, to mark that a backing file is in use by
the kernel to prevent cachefiles or other kernel services from interfering
with that file.

Alter rmdir to reject attempts to remove a directory marked with this flag.
This is used by cachefiles to prevent cachefilesd from removing them.

Using S_SWAPFILE instead isn't really viable as that has other effects in
the I/O paths.

Changes
=======
ver #3:
 - Check for the object pointer being NULL in the tracepoints rather than
   the caller.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819630256.215744.4815885535039369574.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906931596.143852.8642051223094013028.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967141000.1823006.12920680657559677789.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021541207.640689.564689725898537127.stgit@warthog.procyon.org.uk/ # v4

2 files changed, 44 insertions, 0 deletions

diff --git a/fs/cachefiles/Makefile b/fs/cachefiles/Makefile
index 463e3d608b75..e0b092ca077f 100644
--- a/fs/cachefiles/Makefile
+++ b/fs/cachefiles/Makefile
@@ -7,6 +7,7 @@ cachefiles-y := \
 	cache.o \
 	daemon.o \
 	main.o \
+	namei.o \
 	security.o
 
 cachefiles-$(CONFIG_CACHEFILES_ERROR_INJECTION) += error_inject.o
diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
new file mode 100644
index 000000000000..913f83f1c900
--- /dev/null
+++ b/fs/cachefiles/namei.c
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* CacheFiles path walking and related routines
+ *
+ * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/fs.h>
+#include "internal.h"
+
+/*
+ * Mark the backing file as being a cache file if it's not already in use.  The
+ * mark tells the culling request command that it's not allowed to cull the
+ * file or directory.  The caller must hold the inode lock.
+ */
+static bool __cachefiles_mark_inode_in_use(struct cachefiles_object *object,
+					   struct dentry *dentry)
+{
+	struct inode *inode = d_backing_inode(dentry);
+	bool can_use = false;
+
+	if (!(inode->i_flags & S_KERNEL_FILE)) {
+		inode->i_flags |= S_KERNEL_FILE;
+		trace_cachefiles_mark_active(object, inode);
+		can_use = true;
+	} else {
+		pr_notice("cachefiles: Inode already in use: %pd\n", dentry);
+	}
+
+	return can_use;
+}
+
+/*
+ * Unmark a backing inode.  The caller must hold the inode lock.
+ */
+static void __cachefiles_unmark_inode_in_use(struct cachefiles_object *object,
+					     struct dentry *dentry)
+{
+	struct inode *inode = d_backing_inode(dentry);
+
+	inode->i_flags &= ~S_KERNEL_FILE;
+	trace_cachefiles_mark_inactive(object, inode);
+}