mount: detect the newinstance option for devpts (v2)

The devpts instance was mounted w/o the newinstance option if,
the device number is equal to the root /dev/pts.

I think this condition is strong enough to not mount devpts in a
temporary place.

v2: move the host.bla-bla-bla in kerndat.c
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/fs-magic.h

@@ -29,4 +29,8 @@
 # define SOCKFS_MAGIC		0x534f434b
 #endif
 
+#ifndef DEVPTS_SUPER_MAGIC
+#define DEVPTS_SUPER_MAGIC	0x1cd1
+#endif
+
 #endif /* __CR_FS_MAGIC_H__ */

include/kerndat.h

@@ -21,4 +21,7 @@ extern int tcp_max_rshare;
 extern int kern_last_cap;
 extern u64 zero_page_pfn;
 
+struct stat;
+extern struct stat *kerndat_get_devpts_stat(void);
+
 #endif /* __CR_KERNDAT_H__ */

kerndat.c

@@ -9,6 +9,7 @@
 #include "log.h"
 #include "bug.h"
 #include "kerndat.h"
+#include "fs-magic.h"
 #include "mem.h"
 #include "compiler.h"
 #include "sysctl.h"
@@ -52,6 +53,32 @@ static int kerndat_get_shmemdev(void)
 	return 0;
 }
 
+struct stat *kerndat_get_devpts_stat()
+{
+	static struct stat st = {};
+	struct statfs fst;
+
+	if (st.st_dev != 0)
+		return &st;
+
+	if (statfs("/dev/pts", &fst)) {
+		pr_perror("Unable to statefs /dev/pts");
+		return NULL;
+	}
+	if (fst.f_type != DEVPTS_SUPER_MAGIC) {
+		pr_err("devpts isn't mount on the host\n");
+		return NULL;
+	}
+
+	/* The root /dev/pts is mounted w/o newinstance, isn't it? */
+	if (stat("/dev/pts", &st)) {
+		pr_perror("Unable to stat /dev/pts");
+		return NULL;
+	}
+
+	return &st;
+}
+
 /*
  * Check whether pagemap reports soft dirty bit. Kernel has
  * this functionality under CONFIG_MEM_SOFT_DIRTY option.

mount.c

@@ -24,6 +24,7 @@
 #include "image.h"
 #include "namespaces.h"
 #include "protobuf.h"
+#include "kerndat.h"
 #include "protobuf/mnt.pb-c.h"
 
 /*
@@ -578,6 +579,46 @@ out:
 	return NULL;
 }
 
+/* Is it mounted w or w/o the newinstance option */
+static int devpts_dump(struct mount_info *pm)
+{
+	static const char newinstance[] = ",newinstance";
+	struct stat *host_st;
+	struct stat st;
+	DIR *fdir = NULL;
+	char *buf;
+	int len;
+
+	host_st = kerndat_get_devpts_stat();
+	if (host_st == NULL)
+		return -1;
+
+	fdir = open_mountpoint(pm);
+	if (fdir == NULL)
+		return -1;
+
+	if (fstat(dirfd(fdir), &st)) {
+		pr_perror("Unable to statfs %d:%s",
+				pm->mnt_id, pm->mountpoint);
+		close_mountpoint(fdir);
+		return -1;
+	}
+	close_mountpoint(fdir);
+
+	if (host_st->st_dev == st.st_dev)
+		return 0;
+
+	len = strlen(pm->options);
+	buf = xrealloc(pm->options, len + sizeof(newinstance));
+	if (buf == NULL)
+		return -1;
+	memcpy(buf, newinstance, sizeof(newinstance));
+
+	pm->options = buf;
+
+	return 0;
+}
+
 static int tmpfs_dump(struct mount_info *pm)
 {
 	int ret = -1;
@@ -694,6 +735,7 @@ static struct fstype fstypes[] = {
 		.restore = tmpfs_restore,
 	}, {
 		.name = "devpts",
+		.dump = devpts_dump,
 		.code = FSTYPE__DEVPTS,
 	}, {
 		.name = "simfs",