cgfs: explicitly check for NULL

Somehow this implementation of a cgroupfs backend decided to use the hierarchy
numbers it detects in /proc/cgroups and /proc/self/cgroups as indices for
the hierarchy struct. Controller numbering usually starts at 1 but may start at
0 if:

    a) the controller is not mounted on a cgroups v1 hierarchy;
    b) the controller is bound to the cgroups v2 single unified hierarchy; or
    c) the controller is disabled

To avoid having to rework our fallback backend significantly, we should
explicitly check for each controller if hierarchy[i] != NULL.

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

diff --git a/src/lxc/cgfs.c b/src/lxc/cgfs.c
index 250b811611443b628da63246d74f2421adf52d06..6bf0fca006867e63bdc450015840014c6ce724df 100644 (file)
--- a/src/lxc/cgfs.c
+++ b/src/lxc/cgfs.c
@@ -495,7 +495,7 @@ static bool find_hierarchy_mountpts( struct cgroup_meta_data *meta_data, char **
                        goto out;
 
                h = NULL;
-               for (k = 1; k <= meta_data->maximum_hierarchy; k++) {
+               for (k = 0; k <= meta_data->maximum_hierarchy; k++) {
                        if (meta_data->hierarchies[k] &&
                            meta_data->hierarchies[k]->subsystems[0] &&
                            lxc_string_in_array(meta_data->hierarchies[k]->subsystems[0], (const char **)subsystems)) {
@@ -637,6 +637,8 @@ static struct cgroup_hierarchy *lxc_cgroup_find_hierarchy(struct cgroup_meta_dat
        size_t i;
        for (i = 0; i <= meta_data->maximum_hierarchy; i++) {
                struct cgroup_hierarchy *h = meta_data->hierarchies[i];
+               if (!h)
+                       continue;
                if (h && lxc_string_in_array(subsystem, (const char **)h->subsystems))
                        return h;
        }
@@ -864,6 +866,8 @@ static struct cgroup_process_info *lxc_cgroupfs_create(const char *name, const c
        /* find mount points we can use */
        for (info_ptr = base_info; info_ptr; info_ptr = info_ptr->next) {
                h = info_ptr->hierarchy;
+               if (!h)
+                       continue;
                mp = lxc_cgroup_find_mount_point(h, info_ptr->cgroup_path, true);
                if (!mp) {
                        ERROR("Could not find writable mount point for cgroup hierarchy %d while trying to create cgroup.", h->index);
@@ -951,6 +955,9 @@ static struct cgroup_process_info *lxc_cgroupfs_create(const char *name, const c
                for (i = 0, info_ptr = base_info; info_ptr; info_ptr = info_ptr->next, i++) {
                        char *parts2[3];
 
+                       if (!info_ptr->hierarchy)
+                               continue;
+
                        if (lxc_string_in_array("ns", (const char **)info_ptr->hierarchy->subsystems))
                                continue;
                        current_entire_path = NULL;
@@ -1045,6 +1052,8 @@ static struct cgroup_process_info *lxc_cgroupfs_create(const char *name, const c
 
        /* we're done, now update the paths */
        for (i = 0, info_ptr = base_info; info_ptr; info_ptr = info_ptr->next, i++) {
+               if (!info_ptr->hierarchy)
+                       continue;
                /* ignore legacy 'ns' subsystem here, lxc_cgroup_create_legacy
                 * will take care of it
                 * Since we do a continue in above loop, new_cgroup_paths[i] is
@@ -1082,6 +1091,9 @@ static int lxc_cgroup_create_legacy(struct cgroup_process_info *base_info, const
        int r;
 
        for (info_ptr = base_info; info_ptr; info_ptr = info_ptr->next) {
+               if (!info_ptr->hierarchy)
+                       continue;
+
                if (!lxc_string_in_array("ns", (const char **)info_ptr->hierarchy->subsystems))
                        continue;
                /*
@@ -1166,6 +1178,9 @@ static int lxc_cgroupfs_enter(struct cgroup_process_info *info, pid_t pid, bool
 
        snprintf(pid_buf, 32, "%lu", (unsigned long)pid);
        for (info_ptr = info; info_ptr; info_ptr = info_ptr->next) {
+               if (!info_ptr->hierarchy)
+                       continue;
+
                char *cgroup_path = (enter_sub && info_ptr->cgroup_path_sub) ?
                        info_ptr->cgroup_path_sub :
                        info_ptr->cgroup_path;
@@ -1407,6 +1422,10 @@ static bool cgroupfs_mount_cgroup(void *hdata, const char *root, int type)
        for (info = base_info; info; info = info->next) {
                size_t subsystem_count, i;
                struct cgroup_mount_point *mp = info->designated_mount_point;
+
+               if (!info->hierarchy)
+                       continue;
+
                if (!mountpoint_is_accessible(mp))
                        mp = lxc_cgroup_find_mount_point(info->hierarchy, info->cgroup_path, true);
 
@@ -1887,6 +1906,8 @@ find_info_for_subsystem(struct cgroup_process_info *info, const char *subsystem)
        struct cgroup_process_info *info_ptr;
        for (info_ptr = info; info_ptr; info_ptr = info_ptr->next) {
                struct cgroup_hierarchy *h = info_ptr->hierarchy;
+               if (!h)
+                       continue;
                if (lxc_string_in_array(subsystem, (const char **)h->subsystems))
                        return info_ptr;
        }