[CRIU] [PATCH 2/2 v4] fsnotify: Always provide the path for inotify watchees

[Cyrill Gorcunov]

Thanks a huge, Andrew! Attached.
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>From 8cc6da855c3ce60455869e901e81486062421e8e Mon Sep 17 00:00:00 2001
From: Cyrill Gorcunov <gorcunov at openvz.org>
Date: Tue, 13 Oct 2015 19:58:22 +0300
Subject: [PATCH] fsnotify: Always provide the path for inotify watchees

In debian-8 container we faced the problem -- systemd creates nested
mount namespaces and inotify watchee are resolved into a path which
is inaccessbile on restore, the same happens when pathes where
watchees are living are bind-overmounted. Thus when we try to
restore such watchees we can't open the paths.

Lets do a trick here (thanks a huge to Andrew Vagin for idea and
overall help) -- walk over all mount points which device match
the handle's device and open handle first and test if the path
provided is openable as well for tmpfs and devtmps which do not
save inodes between remounts. After all the inotify objects are
bound to inode so it's irrelevean via which path it's assigned.

https://jira.sw.ru/browse/PSBM-39957

Signed-off-by: Cyrill Gorcunov <gorcunov at openvz.org>
---
 fsnotify.c | 95 ++++++++++++++++++++++++++++++++++++++++++--------------------
 1 file changed, 65 insertions(+), 30 deletions(-)

diff --git a/fsnotify.c b/fsnotify.c
index 931e76785b41..8680f06d97a6 100644
--- a/fsnotify.c
+++ b/fsnotify.c
@@ -139,56 +139,91 @@ out:
 int check_open_handle(unsigned int s_dev, unsigned long i_ino,
 		FhEntry *f_handle)
 {
+	struct mount_info *m;
+	fh_t handle;
 	int fd = -1;
 	char *path;
 
-	fd = open_handle(s_dev, i_ino, f_handle);
-	if (fd >= 0) {
-		struct mount_info *mi;
+	decode_handle(&handle, f_handle);
+
+	/*
+	 * We gonna try to open the handle and then
+	 * depending on command line options and type
+	 * of the filesystem (tmpfs/devtmpfs do not
+	 * preserve their inodes between mounts) we
+	 * might need to find out an openable path
+	 * get used on restore as a watch destination.
+	 */
+	for (m = mntinfo; m; m = m->next) {
+		char buf[PATH_MAX], *__path;
+		int mntfd, openable_fd;
 
-		pr_debug("\tHandle 0x%x:0x%lx is openable\n", s_dev, i_ino);
+		if (m->s_dev != s_dev)
+			continue;
 
-		mi = lookup_mnt_sdev(s_dev);
-		if (mi == NULL) {
-			pr_err("Unable to lookup a mount by dev 0x%x\n", s_dev);
+		mntfd = __open_mountpoint(m, -1);
+		pr_debug("\t\tTrying via mntid %d root %s ns_mountpoint @%s (%d)\n",
+			 m->mnt_id, m->root, m->ns_mountpoint, mntfd);
+		if (mntfd < 0)
+			goto cant_open;
+
+		fd = userns_call(open_by_handle, UNS_FDOUT, &handle,
+				 sizeof(handle), mntfd);
+		close(mntfd);
+		if (fd < 0)
+			goto cant_open;
+
+		/*
+		 * On tmpfs/devtmps we have to always fetch
+		 * openable path, in turn on all others
+		 * it depends on command line option: if
+		 * we're requested to use irmap lets fetch
+		 * the path, otherwise simply save the bare
+		 * handler and that's it.
+		 */
+		if ((m->fstype->code != FSTYPE__TMPFS) &&
+		    (m->fstype->code != FSTYPE__DEVTMPFS)) {
+			if (!opts.force_irmap)
+				goto out_nopath;
+			else
+				goto force_irmap;
+		}
+
+		if (read_fd_link(fd, buf, sizeof(buf)) < 0) {
+			close_safe(&fd);
 			goto err;
 		}
+		close_safe(&fd);
 
 		/*
-		 * Inode numbers are not restored for tmpfs content, but we can
-		 * get file names, becasue tmpfs cache is not pruned.
+		 * Convert into a relative path.
 		 */
-		if ((mi->fstype->code == FSTYPE__TMPFS) ||
-				(mi->fstype->code == FSTYPE__DEVTMPFS)) {
-			char p[PATH_MAX];
+		__path = (buf[1] != '\0') ? buf + 1 : ".";
+		pr_debug("\t\t\tlink as %s\n", __path);
 
-			if (read_fd_link(fd, p, sizeof(p)) < 0)
-				goto err;
+		mntfd = mntns_get_root_by_mnt_id(m->mnt_id);
+		if (mntfd < 0)
+			goto err;
 
-			path = xstrdup(p);
+		openable_fd = openat(mntfd, __path, O_PATH);
+		if (openable_fd >= 0) {
+			close(openable_fd);
+
+			pr_debug("\t\t\topenable as %s\n", __path);
+			path = xstrdup(buf);
 			if (path == NULL)
 				goto err;
 
 			f_handle->has_mnt_id = true;
-			f_handle->mnt_id = mi->mnt_id;
-
+			f_handle->mnt_id = m->mnt_id;
 			goto out;
-		}
-
-		if (!opts.force_irmap)
-			/*
-			 * If we're not forced to do irmap, then
-			 * say we have no path for watch. Otherwise
-			 * do irmap scan even if the handle is
-			 * working.
-			 *
-			 * FIXME -- no need to open-by-handle if
-			 * we are in force-irmap and not on tempfs
-			 */
-			goto out_nopath;
+		} else
+			pr_debug("\t\t\tnot openable as %s (%m)\n", __path);
 	}
 
+cant_open:
 	pr_warn("\tHandle 0x%x:0x%lx cannot be opened\n", s_dev, i_ino);
+force_irmap:
 	path = irmap_lookup(s_dev, i_ino);
 	if (!path) {
 		pr_err("\tCan't dump that handle\n");
-- 
2.4.3