Handle already-mounted cgroups in cgroup_init()

Add handling for EBUSY when mounting cgroup2 filesystem, which occurs
when cgroups are already mounted. This can happen after switch_root
when cgroups were moved from the initramfs, or in container environments.

Verify the existing mount is actually cgroup2 before proceeding, and
track whether we mounted to avoid unmounting on error if we didn't.

src/cgroup.c

@@ -34,6 +34,8 @@
 #endif
 #include <sys/mount.h>
 #include <sys/sysinfo.h>		/* get_nprocs_conf() */
+#include <sys/vfs.h>
+#include <linux/magic.h>
 
 #include "cgroup.h"
 #include "finit.h"
@@ -841,6 +843,7 @@ void cgroup_config(void)
 void cgroup_init(uev_ctx_t *ctx)
 {
 	int opts = MS_NODEV | MS_NOEXEC | MS_NOSUID;
+	int mounted = 0;
 	char buf[80];
 	FILE *fp;
 	int fd;
@@ -851,14 +854,36 @@ void cgroup_init(uev_ctx_t *ctx)
 #endif
 
 	if (mount("none", FINIT_CGPATH, "cgroup2", opts, NULL)) {
-		if (errno == ENOENT)
+		if (errno == EBUSY) {
+			/*
+			 * Already mounted - this happens after switch_root
+			 * when cgroups were moved from the initramfs.
+			 * Verify it's actually cgroup2 before proceeding.
+			 */
+			struct statfs sfs;
+
+			if (statfs(FINIT_CGPATH, &sfs) || sfs.f_type != CGROUP2_SUPER_MAGIC) {
+				logit(LOG_ERR, "Mount point %s busy but not cgroup2", FINIT_CGPATH);
+				avail = 0;
+				return;
+			}
+			dbg("cgroup2 already mounted at %s, reusing", FINIT_CGPATH);
+		} else if (errno == ENOENT) {
 			logit(LOG_INFO, "Kernel does not support cgroups v2, disabling.");
-		else if (errno == EPERM) /* Probably inside an unprivileged container */
+			avail = 0;
+			return;
+		} else if (errno == EPERM) {
+			/* Probably inside an unprivileged container */
 			logit(LOG_INFO, "Not allowed to mount cgroups v2, disabling.");
-		else
+			avail = 0;
+			return;
+		} else {
 			err(1, "Failed mounting cgroup v2");
-		avail = 0;
-		return;
+			avail = 0;
+			return;
+		}
+	} else {
+		mounted = 1;
 	}
 	avail = 1;
 
@@ -867,7 +892,8 @@ void cgroup_init(uev_ctx_t *ctx)
 	if (!fp) {
 		err(1, "Failed opening %s", FINIT_CGPATH "/cgroup.controllers");
 	abort:
-		umount(FINIT_CGPATH);
+		if (mounted)
+			umount(FINIT_CGPATH);
 		avail = 0;
 		return;
 	}