detect-virt: check the inode number of the pid namespace

The indoe number of root pid namespace is hardcoded in the kernel to
0xEFFFFFFC since 3.8, so check the inode number of our pid namespace
if all else fails. If it's not 0xEFFFFFFC then we are in a pid
namespace, hence a container environment.

Fixes https://github.com/systemd/systemd/issues/35249

src/basic/virt.c

@@ -24,6 +24,10 @@
 #include "uid-range.h"
 #include "virt.h"
 
+/* Root namespace inode number, 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_PID_INIT_INO UINT64_C(0xEFFFFFFC)
+
 enum {
       SMBIOS_VM_BIT_SET,
       SMBIOS_VM_BIT_UNSET,
@@ -645,6 +649,20 @@ static int running_in_cgroupns(void) {
         }
 }
 
+static int running_in_pidns(void) {
+        _cleanup_close_ int pidns_fd = -EBADF;
+        struct stat st;
+
+        pidns_fd = namespace_open_by_type(NAMESPACE_PID);
+        if (pidns_fd < 0)
+                return log_debug_errno(pidns_fd, "Failed to open PID namespace, ignoring: %m");
+
+        if (fstat(pidns_fd, &st) < 0)
+                return log_debug_errno(errno, "Failed to fstat pid namespace fd, ignoring: %m");
+
+        return (uint64_t) st.st_ino != PROC_PID_INIT_INO;
+}
+
 static Virtualization detect_container_files(void) {
         static const struct {
                 const char *file_path;
@@ -796,6 +814,13 @@ check_files:
         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) {
+                v = VIRTUALIZATION_CONTAINER_OTHER;
+                goto finish;
+        }
+
         /* If none of that worked, give up, assume no container manager. */
         v = VIRTUALIZATION_NONE;
         goto finish;

test/TEST-74-AUX-UTILS/test.sh

@@ -4,6 +4,7 @@ set -e
 
 TEST_DESCRIPTION="Tests for auxiliary utilities"
 NSPAWN_ARGUMENTS="--private-network"
+TEST_NO_NSPAWN=1
 
 # shellcheck source=test/test-functions
 . "${TEST_BASE_DIR:?}/test-functions"

test/units/TEST-74-AUX-UTILS.detect-virt.sh

@@ -5,3 +5,5 @@ set -o pipefail
 
 SYSTEMD_IN_CHROOT=1 systemd-detect-virt --chroot
 (! SYSTEMD_IN_CHROOT=0 systemd-detect-virt --chroot)
+
+unshare --mount-proc --fork --user --pid systemd-detect-virt --container