[lxc-devel] [lxc/master] Do mount rewriting

[tych0 on Github]

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From 5f4e44a22df60c25005c95b9326fd989efd80569 Mon Sep 17 00:00:00 2001
From: Tycho Andersen <tycho.andersen at canonical.com>
Date: Mon, 31 Oct 2016 10:07:25 -0600
Subject: [PATCH 1/2] c/r: explicitly emit bind mounts as criu arguments

We switched to --ext-mount-map auto because of "system" (liblxc) added
mounts like the cgmanager socket that weren't in the config file. This had
the added advantage that we could drop all the mount processing code,
because we no longer needed an --ext-mount-map argument.

The problem here is that mounts can move between hosts. While
--ext-mount-map auto does its best to detect this situation, it explicitly
disallows moves that change the path name. In LXD, we bind mount
/var/lib/lxd/shmounts/$container to /dev/.lxd-mounts for each container,
and so when a container is renamed in a migration, the name changes.
--ext-mount-map auto won't detect this, and so the migration fails.

We *could* implement mount rewriting in CRIU, but my experience with cgroup
and apparmor rewriting is that this is painful and error prone. Instead, it
is much easier to go back to explicitly listing --ext-mount-map arguments
from the config file, and allow the source of the bind to change. We leave
--ext-mount-map auto to catch any stragling (or future) system added
mounts.

I believe this should fix Launchpad Bug 1580765

Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>
---
 src/lxc/criu.c | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/src/lxc/criu.c b/src/lxc/criu.c
index 0940967..867139b 100644
--- a/src/lxc/criu.c
+++ b/src/lxc/criu.c
@@ -46,6 +46,12 @@
 #include "network.h"
 #include "utils.h"
 
+#if IS_BIONIC
+#include <../include/lxcmntent.h>
+#else
+#include <mntent.h>
+#endif
+
 #define CRIU_VERSION		"2.0"
 
 #define CRIU_GITID_VERSION	"2.0"
@@ -123,6 +129,8 @@ static void exec_criu(struct criu_opts *opts)
 	int static_args = 23, argc = 0, i, ret;
 	int netnr = 0;
 	struct lxc_list *it;
+	FILE *mnts;
+	struct mntent mntent;
 
 	char buf[4096], tty_info[32];
 	size_t pos;
@@ -200,6 +208,8 @@ static void exec_criu(struct criu_opts *opts)
 	if (opts->user->action_script)
 		static_args += 2;
 
+	static_args += 2 * lxc_list_len(&opts->c->lxc_conf->mount_list);
+
 	ret = snprintf(log, PATH_MAX, "%s/%s.log", opts->user->directory, opts->action);
 	if (ret < 0 || ret >= PATH_MAX) {
 		ERROR("logfile name too long\n");
@@ -315,6 +325,36 @@ static void exec_criu(struct criu_opts *opts)
 		DECLARE_ARG(opts->user->action_script);
 	}
 
+	mnts = write_mount_file(&opts->c->lxc_conf->mount_list);
+	if (!mnts)
+		goto err;
+
+	while (getmntent_r(mnts, &mntent, buf, sizeof(buf))) {
+		char *fmt, *key, *val;
+		char arg[2 * PATH_MAX + 2];
+
+		if (strcmp(opts->action, "dump") == 0) {
+			fmt = "/%s:%s";
+			key = mntent.mnt_dir;
+			val = mntent.mnt_dir;
+		} else {
+			fmt = "%s:%s";
+			key = mntent.mnt_dir;
+			val = mntent.mnt_fsname;
+		}
+
+		ret = snprintf(arg, sizeof(arg), fmt, key, val);
+		if (ret < 0 || ret >= sizeof(arg)) {
+			fclose(mnts);
+			ERROR("snprintf failed");
+			goto err;
+		}
+
+		DECLARE_ARG("--ext-mount-map");
+		DECLARE_ARG(arg);
+	}
+	fclose(mnts);
+
 	if (strcmp(opts->action, "dump") == 0 || strcmp(opts->action, "pre-dump") == 0) {
 		char pid[32], *freezer_relative;
 

From ed408e6674bd5372d6213c5d672422bf4d2862a7 Mon Sep 17 00:00:00 2001
From: Tycho Andersen <tycho.andersen at canonical.com>
Date: Mon, 31 Oct 2016 19:48:38 +0000
Subject: [PATCH 2/2] log: bump LXC_LOG_BUFFER_SIZE to 4096

We need to log longer lines due to CRIU arguments.

Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>
---
 src/lxc/log.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/lxc/log.h b/src/lxc/log.h
index 7a36600..3c0bbaa 100644
--- a/src/lxc/log.h
+++ b/src/lxc/log.h
@@ -46,7 +46,7 @@
 #endif
 
 #define LXC_LOG_PREFIX_SIZE	32
-#define LXC_LOG_BUFFER_SIZE	1024
+#define LXC_LOG_BUFFER_SIZE	4096
 
 /* This attribute is required to silence clang warnings */
 #if defined(__GNUC__)