src/api.c: support /proc mounted with subset=pid

/proc filesystem can be mounted with subset=pid as one of its mount
options.  This option hides all the top-level files and directories,
those are not related to processes. The cgroup v1 filesystem depends
on the /proc/cgroups to populate the cgroups controllers and will fail
during the cgroup_init() phase, when not available, whereas cgroup v2
considers this as a deprecated file and recommends reading the list of
controller from <unified mount point>/cgroup.controllers[1].

Support this valid /proc mount point only when the system is booted with
the unified mode and will fail to initialize in the case we find the
cgroup v1 mounted, i.e, the system booted with legacy or hybrid mode.

[1] https://docs.kernel.org/admin-guide/cgroup-v2.html#deprecated-v1-core-features

Fixes: https://github.com/libcgroup/libcgroup/issues/234
Reported-by: @0n-s (github username)
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
(cherry picked from commit c83cb5a2d2ff02bb42f1db4a559ccf4d432b0419)

diff --git a/src/api.c b/src/api.c
index 0874ca1d898a81d0594d27f8f98e5a2902e49f01..8b7c665f356aa2845da9709379d0b027772363c2 100644 (file)
--- a/src/api.c
+++ b/src/api.c
@@ -1299,6 +1299,75 @@ out:
        return ret;
 }
 
+/*
+ * Parses the mount options of the given mount point and checks for the
+ * option in the list of mount options.
+ * @mnt: Mount point name to search for mount points.
+ * @mnt_opt: Mount option to be searched.
+ * @is_set: Set to 1, when mount option is found, 0 otherwise.
+ *
+ * Returns 0, in case of success and ECGOTHER on failure.
+ */
+static int check_mount_point_opt(const char *mnt, const char *mnt_opt,
+                                int * const is_set)
+{
+       struct mntent *ent, *temp_ent = NULL;
+       char mntent_buffer[4 * FILENAME_MAX];
+       char *mntopt = NULL;
+       char mnt_opt_delim;
+       FILE *proc_mount;
+       int ret = 0;
+
+       if (!mnt || !mnt_opt || !is_set)
+               return ECGINVAL;
+
+       proc_mount = setmntent(mnt, "r");
+       if (!proc_mount) {
+               cgroup_err("Error: cannot open %s:%s ", mnt, strerror(errno));
+               last_errno = errno;
+               ret = ECGOTHER;
+               goto err;
+       }
+
+       temp_ent = (struct mntent *) malloc(sizeof(struct mntent));
+       if (!temp_ent) {
+               last_errno = errno;
+               ret = ECGOTHER;
+               goto err;
+       }
+
+       ent = getmntent_r(proc_mount, temp_ent, mntent_buffer,
+                         sizeof(mntent_buffer));
+       if (!ent) {
+               last_errno = errno;
+               ret = ECGOTHER;
+               goto err;
+       }
+
+       *is_set = 0;
+       while ((mntopt = hasmntopt(ent, mnt_opt))) {
+               mnt_opt_delim = mntopt[strlen(mnt_opt)];
+               if (mnt_opt_delim == '\0' || mnt_opt_delim == ',') {
+                       *is_set = 1;
+                       break;
+               }
+       }
+
+       if (*is_set == 0) {
+               cgroup_dbg("Debug: %s, not found in the list of ", mnt_opt);
+               cgroup_dbg("mount options of %s\n", mnt);
+       }
+
+err:
+       if (proc_mount)
+               endmntent(proc_mount);
+
+       if (temp_ent)
+               free(temp_ent);
+
+       return ret;
+}
+
 /**
  * cgroup_init(), initializes the MOUNT_POINT.
  *
@@ -1322,6 +1391,8 @@ int cgroup_init(void)
        int err;
        char *buf = NULL;
        char mntent_buffer[4 * FILENAME_MAX];
+       char mnt_opt[] = "subset=pid";
+       int mnt_opt_set;
 
        cgroup_set_default_logger(-1);
 
@@ -1339,13 +1410,25 @@ int cgroup_init(void)
        memset(&cg_mount_table, 0, sizeof(cg_mount_table));
 
        proc_cgroup = fopen("/proc/cgroups", "re");
-
        if (!proc_cgroup) {
-               cgroup_err("Error: cannot open /proc/cgroups: %s\n",
-                               strerror(errno));
-               last_errno = errno;
-               ret = ECGOTHER;
-               goto unlock_exit;
+               cgroup_warn("Warning: cannot open /proc/cgroups: %s\n",
+                           strerror(errno));
+               ret = check_mount_point_opt("/proc/self/mounts", mnt_opt,
+                                           &mnt_opt_set);
+               if (ret)
+                       goto unlock_exit;
+
+               if (!mnt_opt_set) {
+                       ret = ECGINVAL;
+                       goto unlock_exit;
+               }
+
+               /*
+                * /proc, mounted with subset=pid is valid. cgroup v2 doesn't
+                * depend on /proc/cgroups to parse the available controllers.
+                */
+               ret = 0;
+               goto cgroup_unified;
        }
 
        /*
@@ -1385,6 +1468,7 @@ int cgroup_init(void)
        }
        controllers[i] = NULL;
 
+cgroup_unified:
        proc_mount = fopen("/proc/self/mounts", "re");
        if (proc_mount == NULL) {
                cgroup_err("Error: cannot open /proc/self/mounts: %s\n",
@@ -1406,6 +1490,14 @@ int cgroup_init(void)
                                        mntent_buffer,
                                        sizeof(mntent_buffer))) != NULL) {
                if (strcmp(ent->mnt_type, "cgroup") == 0) {
+                       if (controllers[0] == NULL) {
+                               cgroup_err("Error: cgroup v1 requires ");
+                               cgroup_err("/proc/cgroups, check if /proc is ");
+                               cgroup_err("mounted with subset=pid option.\n");
+                               ret = ECGINVAL;
+                               goto unlock_exit;
+                       }
+
                        ret = cgroup_process_v1_mnt(controllers, ent,
                                                    &found_mnt);
                        if (ret)