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, 78 insertions, 6 deletions

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index d8e3b6e50844..dd33227e518a 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -13,6 +13,7 @@
 #include <linux/wait.h>
 #include <linux/audit.h>
 #include <linux/sched/mm.h>
+#include <linux/statfs.h>
 
 #include "fanotify.h"
 
@@ -25,10 +26,18 @@ static bool should_merge(struct fsnotify_event *old_fsn,
 	old = FANOTIFY_E(old_fsn);
 	new = FANOTIFY_E(new_fsn);
 
-	if (old_fsn->inode == new_fsn->inode && old->pid == new->pid &&
-	    old->path.mnt == new->path.mnt &&
-	    old->path.dentry == new->path.dentry)
-		return true;
+	if (old_fsn->inode != new_fsn->inode || old->pid != new->pid ||
+	    old->fh_type != new->fh_type || old->fh_len != new->fh_len)
+		return false;
+
+	if (fanotify_event_has_path(old)) {
+		return old->path.mnt == new->path.mnt &&
+			old->path.dentry == new->path.dentry;
+	} else if (fanotify_event_has_fid(old)) {
+		return fanotify_fid_equal(&old->fid, &new->fid, old->fh_len);
+	}
+
+	/* Do not merge events if we failed to encode fid */
 	return false;
 }
 
@@ -143,6 +152,60 @@ static u32 fanotify_group_event_mask(struct fsnotify_iter_info *iter_info,
 		~marks_ignored_mask;
 }
 
+static int fanotify_encode_fid(struct fanotify_event *event,
+			       const struct path *path, gfp_t gfp)
+{
+	struct fanotify_fid *fid = &event->fid;
+	int dwords, bytes = 0;
+	struct kstatfs stat;
+	int err, type;
+
+	stat.f_fsid.val[0] = stat.f_fsid.val[1] = 0;
+	fid->ext_fh = NULL;
+	dwords = 0;
+	err = -ENOENT;
+	type = exportfs_encode_inode_fh(d_inode(path->dentry), NULL, &dwords,
+					NULL);
+	if (!dwords)
+		goto out_err;
+
+	err = vfs_statfs(path, &stat);
+	if (err)
+		goto out_err;
+
+	bytes = dwords << 2;
+	if (bytes > FANOTIFY_INLINE_FH_LEN) {
+		/* Treat failure to allocate fh as failure to allocate event */
+		err = -ENOMEM;
+		fid->ext_fh = kmalloc(bytes, gfp);
+		if (!fid->ext_fh)
+			goto out_err;
+	}
+
+	type = exportfs_encode_inode_fh(d_inode(path->dentry),
+					fanotify_fid_fh(fid, bytes), &dwords,
+					NULL);
+	err = -EINVAL;
+	if (!type || type == FILEID_INVALID || bytes != dwords << 2)
+		goto out_err;
+
+	fid->fsid = stat.f_fsid;
+	event->fh_len = bytes;
+
+	return type;
+
+out_err:
+	pr_warn_ratelimited("fanotify: failed to encode fid (fsid=%x.%x, "
+			    "type=%d, bytes=%d, err=%i)\n",
+			    stat.f_fsid.val[0], stat.f_fsid.val[1],
+			    type, bytes, err);
+	kfree(fid->ext_fh);
+	fid->ext_fh = NULL;
+	event->fh_len = 0;
+
+	return FILEID_INVALID;
+}
+
 struct fanotify_event *fanotify_alloc_event(struct fsnotify_group *group,
 						 struct inode *inode, u32 mask,
 						 const struct path *path)
@@ -181,10 +244,16 @@ init: __maybe_unused
 		event->pid = get_pid(task_pid(current));
 	else
 		event->pid = get_pid(task_tgid(current));
-	if (path) {
+	event->fh_len = 0;
+	if (path && FAN_GROUP_FLAG(group, FAN_REPORT_FID)) {
+		/* Report the event without a file identifier on encode error */
+		event->fh_type = fanotify_encode_fid(event, path, gfp);
+	} else if (path) {
+		event->fh_type = FILEID_ROOT;
 		event->path = *path;
 		path_get(&event->path);
 	} else {
+		event->fh_type = FILEID_INVALID;
 		event->path.mnt = NULL;
 		event->path.dentry = NULL;
 	}
@@ -281,7 +350,10 @@ static void fanotify_free_event(struct fsnotify_event *fsn_event)
 	struct fanotify_event *event;
 
 	event = FANOTIFY_E(fsn_event);
-	path_put(&event->path);
+	if (fanotify_event_has_path(event))
+		path_put(&event->path);
+	else if (fanotify_event_has_ext_fh(event))
+		kfree(event->fid.ext_fh);
 	put_pid(event->pid);
 	if (fanotify_is_perm_event(event->mask)) {
 		kmem_cache_free(fanotify_perm_event_cachep,