dump: handle overmounted devices (v2)

"init" in LXC opens /dev/null and then mounts devtmpfs in /dev,
so crtools can not resolve the path to the origin /dev/null.

crtools with the option --evasive-devices will check the origin
device and a new device are the same and if it's true, crtools will
dump a new path.

v2: add a description for the option
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

crtools.c

@@ -88,6 +88,7 @@ int main(int argc, char *argv[])
 			{ "close", required_argument, 0, 43 },
 			{ "log-pid", no_argument, 0, 44},
 			{ "version", no_argument, 0, 'V'},
+			{ "evasive-devices", no_argument, 0, 45},
 			{ },
 		};
 
@@ -165,6 +166,9 @@ int main(int argc, char *argv[])
 		case 44:
 			opts.log_file_per_pid = 1;
 			break;
+		case 45:
+			opts.evasive_devices = true;
+			break;
 		case 'V':
 			pr_msg("Version: %d.%d\n", CRIU_VERSION_MAJOR, CRIU_VERSION_MINOR);
 			return 0;
@@ -252,6 +256,7 @@ usage:
 	pr_msg("  -x|--ext-unix-sk      allow external unix connections\n");
 	pr_msg("     --%s  checkpoint/restore established TCP connections\n", SK_EST_PARAM);
 	pr_msg("  -r|--root [PATH]	change the root filesystem (when run in mount namespace)\n");
+	pr_msg("  --evasive-devices	use any path to a device file if the original one is inaccessible\n");
 
 	pr_msg("\n* Logging:\n");
 	pr_msg("  -o|--log-file [NAME]  log file name (relative path is relative to --images-dir)\n");

files-reg.c

@@ -285,6 +285,10 @@ static int check_path_remap(char *rpath, const struct stat *ost, int lfd, u32 id
 	}
 
 	if ((pst.st_ino != ost->st_ino) || (pst.st_dev != ost->st_dev)) {
+		if (opts.evasive_devices &&
+		    (S_ISCHR(ost->st_mode) || S_ISBLK(ost->st_mode)) &&
+		    pst.st_rdev == ost->st_rdev)
+			return 0;
 		/*
 		 * FIXME linked file, but the name we see it by is reused
 		 * by somebody else.

include/crtools.h

@@ -84,6 +84,7 @@ struct cr_options {
 	bool			restore_detach;
 	bool			ext_unix_sk;
 	bool			tcp_established_ok;
+	bool			evasive_devices;
 	unsigned int		namespaces_flags;
 	bool			log_file_per_pid;
 	char			*output;

test/zdtm.sh

@@ -174,7 +174,7 @@ run_test()
 	echo Dump $PID
 	mkdir -p $ddump
 	save_fds $PID  $ddump/dump.fd
-	setsid $CRTOOLS dump -D $ddump -o dump.log -v 4 -t $PID $args $ARGS || {
+	setsid $CRTOOLS dump --evasive-devices -D $ddump -o dump.log -v 4 -t $PID $args $ARGS || {
 		echo WARNING: process $tname is left running for your debugging needs
 		return 1
 	}