Check inode number to see if we are in init namespace (#35306)

This is a more comprehensive fix compared to #35273. Also adds a minimal
test only.

Based on Luca's #35273 but generalizes the code a bit.

In v258 we really should get rid of the old heuristics around userns and
cgroupns detection, but given we are late in the v257 cycle this keeps
them in.

4 files changed, 81 insertions, 8 deletions

diff --git a/src/basic/missing_namespace.h b/src/basic/missing_namespace.h
new file mode 100644
index 0000000000..318c0143b8
--- /dev/null
+++ b/src/basic/missing_namespace.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+/* Root namespace inode numbers, as per include/linux/proc_ns.h in the kernel source tree, since v3.8:
+ * https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=98f842e675f96ffac96e6c50315790912b2812be */
+
+#define PROC_IPC_INIT_INO    ((ino_t) UINT32_C(0xEFFFFFFF))
+#define PROC_UTS_INIT_INO    ((ino_t) UINT32_C(0xEFFFFFFE))
+#define PROC_USER_INIT_INO   ((ino_t) UINT32_C(0xEFFFFFFD))
+#define PROC_PID_INIT_INO    ((ino_t) UINT32_C(0xEFFFFFFC))
+#define PROC_CGROUP_INIT_INO ((ino_t) UINT32_C(0xEFFFFFFB))
+#define PROC_TIME_INIT_INO   ((ino_t) UINT32_C(0xEFFFFFFA))
diff --git a/src/basic/namespace-util.c b/src/basic/namespace-util.c
index 5e8908216c..5d02ac6025 100644
--- a/src/basic/namespace-util.c
+++ b/src/basic/namespace-util.c
@@ -12,6 +12,7 @@
 #include "fileio.h"
 #include "missing_fs.h"
 #include "missing_magic.h"
+#include "missing_namespace.h"
 #include "missing_sched.h"
 #include "missing_syscall.h"
 #include "mountpoint-util.h"
@@ -23,17 +24,17 @@
 #include "user-util.h"
 
 const struct namespace_info namespace_info[_NAMESPACE_TYPE_MAX + 1] = {
-        [NAMESPACE_CGROUP] =  { "cgroup", "ns/cgroup", CLONE_NEWCGROUP,                          },
-        [NAMESPACE_IPC]    =  { "ipc",    "ns/ipc",    CLONE_NEWIPC,                             },
-        [NAMESPACE_NET]    =  { "net",    "ns/net",    CLONE_NEWNET,                             },
+        [NAMESPACE_CGROUP] =  { "cgroup", "ns/cgroup", CLONE_NEWCGROUP, PROC_CGROUP_INIT_INO     },
+        [NAMESPACE_IPC]    =  { "ipc",    "ns/ipc",    CLONE_NEWIPC,    PROC_IPC_INIT_INO        },
+        [NAMESPACE_NET]    =  { "net",    "ns/net",    CLONE_NEWNET,    0                        },
         /* So, the mount namespace flag is called CLONE_NEWNS for historical
          * reasons. Let's expose it here under a more explanatory name: "mnt".
          * This is in-line with how the kernel exposes namespaces in /proc/$PID/ns. */
-        [NAMESPACE_MOUNT]  =  { "mnt",    "ns/mnt",    CLONE_NEWNS,                              },
-        [NAMESPACE_PID]    =  { "pid",    "ns/pid",    CLONE_NEWPID,                             },
-        [NAMESPACE_USER]   =  { "user",   "ns/user",   CLONE_NEWUSER,                            },
-        [NAMESPACE_UTS]    =  { "uts",    "ns/uts",    CLONE_NEWUTS,                             },
-        [NAMESPACE_TIME]   =  { "time",   "ns/time",   CLONE_NEWTIME,                            },
+        [NAMESPACE_MOUNT]  =  { "mnt",    "ns/mnt",    CLONE_NEWNS,     0                        },
+        [NAMESPACE_PID]    =  { "pid",    "ns/pid",    CLONE_NEWPID,    PROC_PID_INIT_INO        },
+        [NAMESPACE_USER]   =  { "user",   "ns/user",   CLONE_NEWUSER,   PROC_USER_INIT_INO       },
+        [NAMESPACE_UTS]    =  { "uts",    "ns/uts",    CLONE_NEWUTS,    PROC_UTS_INIT_INO        },
+        [NAMESPACE_TIME]   =  { "time",   "ns/time",   CLONE_NEWTIME,   PROC_TIME_INIT_INO       },
         { /* Allow callers to iterate over the array without using _NAMESPACE_TYPE_MAX. */       },
 };
 
@@ -479,6 +480,28 @@ int namespace_open_by_type(NamespaceType type) {
         return fd;
 }
 
+int namespace_is_init(NamespaceType type) {
+        int r;
+
+        assert(type >= 0);
+        assert(type <= _NAMESPACE_TYPE_MAX);
+
+        if (namespace_info[type].root_inode == 0)
+                return -EBADR; /* Cannot answer this question */
+
+        const char *p = pid_namespace_path(0, type);
+
+        struct stat st;
+        r = RET_NERRNO(stat(p, &st));
+        if (r == -ENOENT)
+                /* If the /proc/ns/<type> API is not around in /proc/ then ns is off in the kernel and we are in the init ns */
+                return proc_mounted() == 0 ? -ENOSYS : true;
+        if (r < 0)
+                return r;
+
+        return st.st_ino == namespace_info[type].root_inode;
+}
+
 int is_our_namespace(int fd, NamespaceType request_type) {
         int clone_flag;
 
diff --git a/src/basic/namespace-util.h b/src/basic/namespace-util.h
index 105bab6fdb..e92d407864 100644
--- a/src/basic/namespace-util.h
+++ b/src/basic/namespace-util.h
@@ -24,6 +24,7 @@ extern const struct namespace_info {
         const char *proc_name;
         const char *proc_path;
         unsigned int clone_flag;
+        ino_t root_inode;
 } namespace_info[_NAMESPACE_TYPE_MAX + 1];
 
 int pidref_namespace_open(
@@ -74,6 +75,8 @@ int parse_userns_uid_range(const char *s, uid_t *ret_uid_shift, uid_t *ret_uid_r
 
 int namespace_open_by_type(NamespaceType type);
 
+int namespace_is_init(NamespaceType type);
+
 int is_our_namespace(int fd, NamespaceType type);
 
 int is_idmapping_supported(const char *path);
diff --git a/src/basic/virt.c b/src/basic/virt.c
index fd0c353791..9dcafb9dea 100644
--- a/src/basic/virt.c
+++ b/src/basic/virt.c
@@ -585,6 +585,14 @@ static int running_in_cgroupns(void) {
         if (!cg_ns_supported())
                 return false;
 
+        r = namespace_is_init(NAMESPACE_CGROUP);
+        if (r < 0)
+                log_debug_errno(r, "Failed to test if in root cgroup namespace, ignoring: %m");
+        else if (r > 0)
+                return false;
+
+        // FIXME: We really should drop the heuristics below.
+
         r = cg_all_unified();
         if (r < 0)
                 return r;
@@ -645,6 +653,16 @@ static int running_in_cgroupns(void) {
         }
 }
 
+static int running_in_pidns(void) {
+        int r;
+
+        r = namespace_is_init(NAMESPACE_PID);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to test if in root PID namespace, ignoring: %m");
+
+        return !r;
+}
+
 static Virtualization detect_container_files(void) {
         static const struct {
                 const char *file_path;
@@ -790,12 +808,21 @@ check_files:
 
         r = running_in_cgroupns();
         if (r > 0) {
+                log_debug("Running in a cgroup namespace, assuming unknown container manager.");
                 v = VIRTUALIZATION_CONTAINER_OTHER;
                 goto finish;
         }
         if (r < 0)
                 log_debug_errno(r, "Failed to detect cgroup namespace: %m");
 
+        /* Finally, the root pid namespace has an hardcoded inode number of 0xEFFFFFFC since kernel 3.8, so
+         * if all else fails we can check the inode number of our pid namespace and compare it. */
+        if (running_in_pidns() > 0) {
+                log_debug("Running in a pid namespace, assuming unknown container manager.");
+                v = VIRTUALIZATION_CONTAINER_OTHER;
+                goto finish;
+        }
+
         /* If none of that worked, give up, assume no container manager. */
         v = VIRTUALIZATION_NONE;
         goto finish;
@@ -863,6 +890,14 @@ int running_in_userns(void) {
         _cleanup_free_ char *line = NULL;
         int r;
 
+        r = namespace_is_init(NAMESPACE_USER);
+        if (r < 0)
+                log_debug_errno(r, "Failed to test if in root user namespace, ignoring: %m");
+        else if (r > 0)
+                return false;
+
+        // FIXME: We really should drop the heuristics below.
+
         r = userns_has_mapping("/proc/self/uid_map");
         if (r != 0)
                 return r;