util: add ability to execute programs in a specified userns

It's required for dumping tmpfs, where we use tar to save content.
If we need to execute tar from a proper userns to get right uid-s and
gid-s for files.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/util.h

@@ -170,6 +170,8 @@ extern void *shmalloc(size_t bytes);
 extern void shfree_last(void *ptr);
 
 extern int cr_system(int in, int out, int err, char *cmd, char *const argv[]);
+extern int cr_system_userns(int in, int out, int err, char *cmd,
+				char *const argv[], int userns_pid);
 extern int cr_daemon(int nochdir, int noclose, int *keep_fd, int close_fd);
 extern int is_root_user(void);
 

util.c

@@ -37,6 +37,7 @@
 #include "image.h"
 #include "vma.h"
 #include "mem.h"
+#include "namespaces.h"
 
 #include "cr_options.h"
 #include "servicefd.h"
@@ -506,6 +507,12 @@ void shfree_last(void *ptr)
  * If "out" or "err" are negative, a log file descriptor will be used.
  */
 int cr_system(int in, int out, int err, char *cmd, char *const argv[])
+{
+	return cr_system_userns(in, out, err, cmd, argv, -1);
+}
+
+int cr_system_userns(int in, int out, int err, char *cmd,
+			char *const argv[], int userns_pid)
 {
 	sigset_t blockmask, oldmask;
 	int ret = -1, status;
@@ -523,6 +530,15 @@ int cr_system(int in, int out, int err, char *cmd, char *const argv[])
 		pr_perror("fork() failed");
 		goto out;
 	} else if (pid == 0) {
+		if (userns_pid > 0) {
+			if (switch_ns(userns_pid, &user_ns_desc, NULL))
+				goto out_chld;
+			if (setuid(0) || setgid(0)) {
+				pr_perror("Unable to set uid or gid");
+				goto out_chld;
+			}
+		}
+
 		if (out < 0)
 			out = log_get_fd();
 		if (err < 0)