fanotify: encode file identifier for FAN_REPORT_FID

When user requests the flag FAN_REPORT_FID in fanotify_init(),
a unique file identifier of the event target object will be reported
with the event.

The file identifier includes the filesystem's fsid (i.e. from statfs(2))
and an NFS file handle of the file (i.e. from name_to_handle_at(2)).

The file identifier makes holding the path reference and passing a file
descriptor to user redundant, so those are disabled in a group with
FAN_REPORT_FID.

Encode fid and store it in event for a group with FAN_REPORT_FID.
Up to 12 bytes of file handle on 32bit arch (16 bytes on 64bit arch)
are stored inline in fanotify_event struct. Larger file handles are
stored in an external allocated buffer.

On failure to encode fid, we print a warning and queue the event
without the fid information.

[JK: Fold part of later patched into this one to use
exportfs_encode_inode_fh() right away]

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

1 files changed, 75 insertions, 3 deletions

diff --git a/fs/notify/fanotify/fanotify.h b/fs/notify/fanotify/fanotify.h
index 898b5b2bc1c7..271482fb9611 100644
--- a/fs/notify/fanotify/fanotify.h
+++ b/fs/notify/fanotify/fanotify.h
@@ -2,12 +2,50 @@
 #include <linux/fsnotify_backend.h>
 #include <linux/path.h>
 #include <linux/slab.h>
+#include <linux/exportfs.h>
 
 extern struct kmem_cache *fanotify_mark_cache;
 extern struct kmem_cache *fanotify_event_cachep;
 extern struct kmem_cache *fanotify_perm_event_cachep;
 
 /*
+ * 3 dwords are sufficient for most local fs (64bit ino, 32bit generation).
+ * For 32bit arch, fid increases the size of fanotify_event by 12 bytes and
+ * fh_* fields increase the size of fanotify_event by another 4 bytes.
+ * For 64bit arch, fid increases the size of fanotify_fid by 8 bytes and
+ * fh_* fields are packed in a hole after mask.
+ */
+#if BITS_PER_LONG == 32
+#define FANOTIFY_INLINE_FH_LEN	(3 << 2)
+#else
+#define FANOTIFY_INLINE_FH_LEN	(4 << 2)
+#endif
+
+struct fanotify_fid {
+	__kernel_fsid_t fsid;
+	union {
+		unsigned char fh[FANOTIFY_INLINE_FH_LEN];
+		unsigned char *ext_fh;
+	};
+};
+
+static inline void *fanotify_fid_fh(struct fanotify_fid *fid,
+				    unsigned int fh_len)
+{
+	return fh_len <= FANOTIFY_INLINE_FH_LEN ? fid->fh : fid->ext_fh;
+}
+
+static inline bool fanotify_fid_equal(struct fanotify_fid *fid1,
+				      struct fanotify_fid *fid2,
+				      unsigned int fh_len)
+{
+	return fid1->fsid.val[0] == fid2->fsid.val[0] &&
+		fid1->fsid.val[1] == fid2->fsid.val[1] &&
+		!memcmp(fanotify_fid_fh(fid1, fh_len),
+			fanotify_fid_fh(fid2, fh_len), fh_len);
+}
+
+/*
  * Structure for normal fanotify events. It gets allocated in
  * fanotify_handle_event() and freed when the information is retrieved by
  * userspace
@@ -16,13 +54,47 @@ struct fanotify_event {
 	struct fsnotify_event fse;
 	u32 mask;
 	/*
-	 * We hold ref to this path so it may be dereferenced at any point
-	 * during this object's lifetime
+	 * Those fields are outside fanotify_fid to pack fanotify_event nicely
+	 * on 64bit arch and to use fh_type as an indication of whether path
+	 * or fid are used in the union:
+	 * FILEID_ROOT (0) for path, > 0 for fid, FILEID_INVALID for neither.
 	 */
-	struct path path;
+	u8 fh_type;
+	u8 fh_len;
+	u16 pad;
+	union {
+		/*
+		 * We hold ref to this path so it may be dereferenced at any
+		 * point during this object's lifetime
+		 */
+		struct path path;
+		/*
+		 * With FAN_REPORT_FID, we do not hold any reference on the
+		 * victim object. Instead we store its NFS file handle and its
+		 * filesystem's fsid as a unique identifier.
+		 */
+		struct fanotify_fid fid;
+	};
 	struct pid *pid;
 };
 
+static inline bool fanotify_event_has_path(struct fanotify_event *event)
+{
+	return event->fh_type == FILEID_ROOT;
+}
+
+static inline bool fanotify_event_has_fid(struct fanotify_event *event)
+{
+	return event->fh_type != FILEID_ROOT &&
+		event->fh_type != FILEID_INVALID;
+}
+
+static inline bool fanotify_event_has_ext_fh(struct fanotify_event *event)
+{
+	return fanotify_event_has_fid(event) &&
+		event->fh_len > FANOTIFY_INLINE_FH_LEN;
+}
+
 /*
  * Structure for permission fanotify events. It gets allocated and freed in
  * fanotify_handle_event() since we wait there for user response. When the