bindings: allow users to switch between virtualization and non-virtualization mode

When LXCFS has a bug and provides wrong or inconsistent values user
often want to turn off virtualization until we have figured out a fix
and rollout an upgrade to reload the shared library. Allow them to
toggle between virtualization mode and non-virtualization mode by
sending SIGUSR2 to lxcfs:

 Kernel supports pidfds
 api_extensions:
 - cgroups
 - sys_cpu_online
 - proc_cpuinfo
 - proc_diskstats
 - proc_loadavg
 - proc_meminfo
 - proc_stat
 - proc_swaps
 - proc_uptime
 - shared_pidns
 - cpuview_daemon
 - loadavg_daemon
 - pidfds
 Switched into non-virtualization mode
 Switched into virtualization mode
 Switched into non-virtualization mode
 Switched into virtualization mode
 Switched into non-virtualization mode

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

diff --git a/src/bindings.c b/src/bindings.c
index fb3dcb74c592e8a403d781ffd3843f6c9f56e642..e642e6494502b623c17c55cbe818a72cd75b4d45 100644 (file)
--- a/src/bindings.c
+++ b/src/bindings.c
@@ -50,11 +50,12 @@
 #include "utils.h"
 
 static bool can_use_pidfd;
-static bool reload_successful;
+
+static volatile sig_atomic_t reload_successful;
 
 bool liblxcfs_functional(void)
 {
-       return reload_successful;
+       return reload_successful != 0;
 }
 
 /* Define pivot_root() if missing from the C library */
@@ -721,6 +722,54 @@ static bool cgfs_setup_controllers(void)
        return true;
 }
 
+static void sigusr2_handler(int signo, siginfo_t *info, void *extra)
+{
+       int ret;
+
+       if (reload_successful) {
+               reload_successful = 0;
+
+               /* write() is async signal safe */
+               ret = write(STDERR_FILENO,
+                           "Switched into non-virtualization mode\n",
+                           STRLITERALLEN("Switched into non-virtualization mode\n"));
+               if (ret < 0)
+                       goto please_compiler;
+       } else {
+               reload_successful = 1;
+
+               /* write() is async signal safe */
+               ret = write(STDERR_FILENO, "Switched into virtualization mode\n",
+                           STRLITERALLEN("Switched into virtualization mode\n"));
+               if (ret < 0)
+                       goto please_compiler;
+       }
+
+please_compiler:
+       /*
+        * The write() syscall is a function whose return value needs to be
+        * checked. Otherwise the compiler will warn. This is how we
+        * please our master. Another one could be to use
+        * syscall(__NR_write, ...) directly but whatever.
+        */
+       return;
+}
+
+static int set_sigusr2_handler(void)
+{
+       int ret;
+       struct sigaction action = {
+               .sa_flags = SA_SIGINFO,
+               .sa_sigaction = sigusr2_handler,
+       };
+
+       ret = sigaction(SIGUSR2, &action, NULL);
+       if (ret)
+               return log_error_errno(-1, errno, "Failed to set SIGUSR2 signal handler");
+
+       return 0;
+}
+
 static void __attribute__((constructor)) lxcfs_init(void)
 {
        __do_close_prot_errno int init_ns = -EBADF, root_fd = -EBADF,
@@ -788,11 +837,14 @@ static void __attribute__((constructor)) lxcfs_init(void)
        else if (fchdir(root_fd) < 0)
                lxcfs_info("%s - Failed to change to root directory", strerror(errno));
 
-       reload_successful = true;
+       if (set_sigusr2_handler())
+               goto broken_upgrade;
+
+       reload_successful = 1;
        return;
 
 broken_upgrade:
-       reload_successful = false;
+       reload_successful = 0;
        lxcfs_info("Failed to run constructor %s to reload liblxcfs", __func__);
 }