[lxc-devel] [PATCH] Add option to lxc-attach to select specific namespaces

Christian Seiler christian at iwakd.de
Fri May 18 10:55:27 UTC 2012


This patch adds the -s/--namespaces option to lxc-attach that works
analogously to lxc-unshare, allowing the user to select the namespaces the
process should be attached to.

Signed-off-by: Christian Seiler <christian at iwakd.de>
Cc: Stéphane Graber <stgraber at ubuntu.com>
Cc: Daniel Lezcano <daniel.lezcano at free.fr>
Cc: Serge Hallyn <serge.hallyn at canonical.com>
---
 doc/lxc-attach.sgml.in |   92 +++++++++++++++++++++++++++++++++++++++++++++---
 src/lxc/attach.c       |   12 +++++-
 src/lxc/attach.h       |    2 +-
 src/lxc/lxc_attach.c   |   36 +++++++++++++++++--
 src/lxc/lxc_unshare.c  |   46 ------------------------
 src/lxc/namespace.c    |   46 ++++++++++++++++++++++++
 src/lxc/namespace.h    |    3 ++
 7 files changed, 180 insertions(+), 57 deletions(-)

diff --git a/doc/lxc-attach.sgml.in b/doc/lxc-attach.sgml.in
index 7092f16..4e462f9 100644
--- a/doc/lxc-attach.sgml.in
+++ b/doc/lxc-attach.sgml.in
@@ -49,7 +49,8 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
   <refsynopsisdiv>
     <cmdsynopsis><command>lxc-attach <replaceable>-n
     name</replaceable> <optional>-a
-    arch</optional> <optional>-e</optional>
+    arch</optional> <optional>-e</optional> <optional>-s
+    namespaces</optional>
     <optional>-- command</optional></command></cmdsynopsis>
   </refsynopsisdiv>
 
@@ -122,6 +123,28 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 	</listitem>
       </varlistentry>
 
+      <varlistentry>
+	<term>
+	  <option>-s, --namespaces <replaceable>namespaces</replaceable></option>
+	</term>
+	<listitem>
+	  <para>
+	    Specify the namespaces to attach to, as a pipe-separated liste,
+	    e.g. <replaceable>NETWORK|IPC</replaceable>. Allowed values are
+	    <replaceable>MOUNT</replaceable>, <replaceable>PID</replaceable>,
+	    <replaceable>UTSNAME</replaceable>, <replaceable>IPC</replaceable>
+	    and <replaceable>NETWORK</replaceable>. This allows one to change
+	    the context of the process to e.g. the network namespace of the
+	    container while retaining the other namespaces as those of the
+	    host.
+	  </para>
+	  <para>
+	    <emphasis>Important:</emphasis> This option implies
+	    <option>-e</option>.
+	  </para>
+	</listitem>
+      </varlistentry>
+
     </variablelist>
 
   </refsect1>
@@ -144,19 +167,78 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
       </para>
       <para>
         To deactivate the network link eth1 of a running container that
-        does not have the NET_ADMIN capability, use the <option>-e</option>
-        option to use increased capabilities:
+        does not have the NET_ADMIN capability, use either the
+        <option>-e</option> option to use increased capabilities,
+        assuming the <command>ip</command> tool is installed:
         <programlisting>
           lxc-attach -n container -e -- /sbin/ip link delete eth1
         </programlisting>
+        Or, alternatively, use the <option>-s</option> to use the
+        tools installed on the host outside the container:
+        <programlisting>
+          lxc-attach -n container -s NETWORK -- /sbin/ip link delete eth1
+        </programlisting>
       </para>
   </refsect1>
 
   <refsect1>
+    <title>Compatibility</title>
+    <para>
+      Attaching completely (including the pid and mount namespaces) to a
+      container requires a patched kernel, please see the lxc website for
+      details. <command>lxc-attach</command> will fail in that case if
+      used with an unpatched kernel.
+    </para>
+    <para>
+      Nevertheless, it will succeed on an unpatched kernel of version 3.0
+      or higher if the <option>-s</option> option is used to restrict the
+      namespaces that the process is to be attached to to one or more of 
+      <replaceable>NETWORK</replaceable>, <replaceable>IPC</replaceable>
+      and <replaceable>UTSNAME</replaceable>.
+    </para>
+  </refsect1>
+
+  <refsect1>
+    <title>Notes</title>
+    <para>
+      The Linux <replaceable>/proc</replaceable> and
+      <replaceable>/sys</replaceable> filesystems contain information
+      about some quantities that are affected by namespaces, such as
+      the directories named after process ids in
+      <replaceable>/proc</replaceable> or the network interface infromation
+      in <replaceable>/sys/class/net</replaceable>. The namespace of the
+      process mounting the pseudo-filesystems determines what information
+      is shown, <emphasis>not</emphasis> the namespace of the process
+      accessing <replaceable>/proc</replaceable> or
+      <replaceable>/sys</replaceable>.
+    </para>
+    <para>
+      If one uses the <option>-s</option> option to only attach to
+      the pid namespace of a container, but not its mount namespace
+      (which will contain the <replaceable>/proc</replaceable> of the
+      container and not the host), the contents of <option>/proc</option>
+      will reflect that of the host and not the container. Analogously,
+      the same issue occurs when reading the contents of
+      <replaceable>/sys/class/net</replaceable> and attaching to just
+      the network namespace.
+    </para>
+    <para>
+      A workaround is to use <command>lxc-unshare</command> to unshare
+      the mount namespace after using <command>lxc-attach</command> with
+      <replaceable>-s PID</replaceable> and/or <replaceable>-s
+      NETWORK</replaceable> and then unmount and then mount again both
+      pseudo-filesystems within that new mount namespace, before
+      executing a program/script that relies on this information to be
+      correct.
+    </para>
+  </refsect1>
+
+  <refsect1>
     <title>Security</title>
     <para>
-      The <option>-e</option> should be used with care, as it may break
-      the isolation of the containers if used improperly.
+      The <option>-e</option> and <option>-s</option> options should
+      be used with care, as it may break the isolation of the containers
+      if used improperly.
     </para>
   </refsect1>
 
diff --git a/src/lxc/attach.c b/src/lxc/attach.c
index a95b3d3..fcf47da 100644
--- a/src/lxc/attach.c
+++ b/src/lxc/attach.c
@@ -121,10 +121,11 @@ out_error:
 	return NULL;
 }
 
-int lxc_attach_to_ns(pid_t pid)
+int lxc_attach_to_ns(pid_t pid, int which)
 {
 	char path[MAXPATHLEN];
 	char *ns[] = { "pid", "mnt", "net", "ipc", "uts" };
+	int flags[] = { CLONE_NEWPID, CLONE_NEWNS, CLONE_NEWNET, CLONE_NEWIPC, CLONE_NEWUTS };
 	const int size = sizeof(ns) / sizeof(char *);
 	int fd[size];
 	int i;
@@ -136,6 +137,13 @@ int lxc_attach_to_ns(pid_t pid)
 	}
 
 	for (i = 0; i < size; i++) {
+		/* ignore if we are not supposed to attach
+		 * to that namespace
+		 */
+		if (which != -1 && !(which & flags[i])) {
+			fd[i] = -1;
+			continue;
+		}
 		snprintf(path, MAXPATHLEN, "/proc/%d/ns/%s", pid, ns[i]);
 		fd[i] = open(path, O_RDONLY);
 		if (fd[i] < 0) {
@@ -145,7 +153,7 @@ int lxc_attach_to_ns(pid_t pid)
 	}
 
 	for (i = 0; i < size; i++) {
-		if (setns(fd[i], 0)) {
+		if (fd[i] > 0 && setns(fd[i], 0)) {
 			SYSERROR("failed to set namespace '%s'", ns[i]);
 			return -1;
 		}
diff --git a/src/lxc/attach.h b/src/lxc/attach.h
index 2d46c83..d96fdae 100644
--- a/src/lxc/attach.h
+++ b/src/lxc/attach.h
@@ -33,7 +33,7 @@ struct lxc_proc_context_info {
 
 extern struct lxc_proc_context_info *lxc_proc_get_context_info(pid_t pid);
 
-extern int lxc_attach_to_ns(pid_t other_pid);
+extern int lxc_attach_to_ns(pid_t other_pid, int which);
 extern int lxc_attach_drop_privs(struct lxc_proc_context_info *ctx);
 
 #endif
diff --git a/src/lxc/lxc_attach.c b/src/lxc/lxc_attach.c
index 955e9f4..db4de97 100644
--- a/src/lxc/lxc_attach.c
+++ b/src/lxc/lxc_attach.c
@@ -40,20 +40,24 @@
 #include "start.h"
 #include "sync.h"
 #include "log.h"
+#include "namespace.h"
 
 lxc_log_define(lxc_attach_ui, lxc);
 
 static const struct option my_longopts[] = {
 	{"elevated-privileges", no_argument, 0, 'e'},
 	{"arch", required_argument, 0, 'a'},
+	{"namespaces", required_argument, 0, 's'},
 	LXC_COMMON_OPTIONS
 };
 
 static int elevated_privileges = 0;
 static signed long new_personality = -1;
+static int namespace_flags = -1;
 
 static int my_parser(struct lxc_arguments* args, int c, char* arg)
 {
+	int ret;
 	switch (c) {
 	case 'e': elevated_privileges = 1; break;
 	case 'a':
@@ -63,6 +67,17 @@ static int my_parser(struct lxc_arguments* args, int c, char* arg)
 			return -1;
 		}
 		break;
+	case 's':
+		namespace_flags = 0;
+		ret = lxc_fill_namespace_flags(arg, &namespace_flags);
+		if (ret)
+			return -1;
+		if (namespace_flags & CLONE_NEWUSER) {
+			lxc_error(args, "attaching to user namespaces is "
+			                "currently not supported: %08x", namespace_flags);
+			return -1;
+		}
+		break;
 	}
 
 	return 0;
@@ -83,7 +98,16 @@ Options :\n\
                     WARNING: This may leak privleges into the container.\n\
                     Use with care.\n\
   -a, --arch=ARCH   Use ARCH for program instead of container's own\n\
-                    architecture.\n",
+                    architecture.\n\
+  -s, --namespaces=FLAGS\n\
+                    Don't attach to all the namespaces of the container\n\
+                    but just to the following OR'd list of flags:\n\
+                    MOUNT, PID, UTSNAME, IPC, NETWORK\n\
+                    WARNING: Using -s implies -e, it may therefore\n\
+                    leak privileges into the container. Use with care.",
+	/* NOTE: -s option does not have USER here, because setns() doesn't
+	 * support user namespaces yet
+	 */
 	.options  = my_longopts,
 	.parser   = my_parser,
 	.checker  = NULL,
@@ -111,7 +135,13 @@ int main(int argc, char *argv[])
 			   my_args.progname, my_args.quiet);
 	if (ret)
 		return ret;
-
+	
+	/* if we do not attach to all namespaces, we will assume
+	 * elevated privileges by default anyway.
+	 */
+	if (namespace_flags != -1)
+		elevated_privileges = 1;
+	
 	init_pid = get_init_pid(my_args.name);
 	if (init_pid < 0) {
 		ERROR("failed to get the init pid");
@@ -178,7 +208,7 @@ int main(int argc, char *argv[])
 
 		curdir = get_current_dir_name();
 
-		ret = lxc_attach_to_ns(init_pid);
+		ret = lxc_attach_to_ns(init_pid, namespace_flags);
 		if (ret < 0) {
 			ERROR("failed to enter the namespace");
 			return -1;
diff --git a/src/lxc/lxc_unshare.c b/src/lxc/lxc_unshare.c
index 0baccb0..9d8c8ca 100644
--- a/src/lxc/lxc_unshare.c
+++ b/src/lxc/lxc_unshare.c
@@ -85,52 +85,6 @@ static uid_t lookup_user(const char *optarg)
 	return uid;
 }
 
-static char *namespaces_list[] = {
-	"MOUNT", "PID", "UTSNAME", "IPC",
-	"USER", "NETWORK"
-};
-static int cloneflags_list[] = {
-	CLONE_NEWNS, CLONE_NEWPID, CLONE_NEWUTS, CLONE_NEWIPC,
-	CLONE_NEWUSER, CLONE_NEWNET
-};
-
-static int lxc_namespace_2_cloneflag(char *namespace)
-{
-	int i, len;
-	len = sizeof(namespaces_list)/sizeof(namespaces_list[0]);
-	for (i = 0; i < len; i++)
-		if (!strcmp(namespaces_list[i], namespace))
-			return cloneflags_list[i];
-
-	ERROR("invalid namespace name %s", namespace);
-	return -1;
-}
-
-static int lxc_fill_namespace_flags(char *flaglist, int *flags)
-{
-	char *token, *saveptr = NULL;
-	int aflag;
-
-	if (!flaglist) {
-		ERROR("need at least one namespace to unshare");
-		return -1;
-	}
-
-	token = strtok_r(flaglist, "|", &saveptr);
-	while (token) {
-
-		aflag = lxc_namespace_2_cloneflag(token);
-		if (aflag < 0)
-			return -1;
-
-		*flags |= aflag;
-
-		token = strtok_r(NULL, "|", &saveptr);
-	}
-	return 0;
-}
-
-
 struct start_arg {
 	char ***args;
 	int *flags;
diff --git a/src/lxc/namespace.c b/src/lxc/namespace.c
index 3e6fc3a..e3c7a09 100644
--- a/src/lxc/namespace.c
+++ b/src/lxc/namespace.c
@@ -69,3 +69,49 @@ pid_t lxc_clone(int (*fn)(void *), void *arg, int flags)
 
 	return ret;
 }
+
+static char *namespaces_list[] = {
+	"MOUNT", "PID", "UTSNAME", "IPC",
+	"USER", "NETWORK"
+};
+static int cloneflags_list[] = {
+	CLONE_NEWNS, CLONE_NEWPID, CLONE_NEWUTS, CLONE_NEWIPC,
+	CLONE_NEWUSER, CLONE_NEWNET
+};
+
+int lxc_namespace_2_cloneflag(char *namespace)
+{
+	int i, len;
+	len = sizeof(namespaces_list)/sizeof(namespaces_list[0]);
+	for (i = 0; i < len; i++)
+		if (!strcmp(namespaces_list[i], namespace))
+			return cloneflags_list[i];
+
+	ERROR("invalid namespace name %s", namespace);
+	return -1;
+}
+
+int lxc_fill_namespace_flags(char *flaglist, int *flags)
+{
+	char *token, *saveptr = NULL;
+	int aflag;
+
+	if (!flaglist) {
+		ERROR("need at least one namespace to unshare/attach");
+		return -1;
+	}
+
+	token = strtok_r(flaglist, "|", &saveptr);
+	while (token) {
+
+		aflag = lxc_namespace_2_cloneflag(token);
+		if (aflag < 0)
+			return -1;
+
+		*flags |= aflag;
+
+		token = strtok_r(NULL, "|", &saveptr);
+	}
+	return 0;
+}
+
diff --git a/src/lxc/namespace.h b/src/lxc/namespace.h
index 5442dd3..04e81bb 100644
--- a/src/lxc/namespace.h
+++ b/src/lxc/namespace.h
@@ -50,4 +50,7 @@
 
 extern pid_t lxc_clone(int (*fn)(void *), void *arg, int flags);
 
+extern int lxc_namespace_2_cloneflag(char *namespace);
+extern int lxc_fill_namespace_flags(char *flaglist, int *flags);
+
 #endif
-- 
1.7.2.5





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