core: serialize/deserialize IP accounting across daemon reload/reexec

Make sure the current IP accounting counters aren't lost during
reload/reexec.

Note that we destroy all BPF file objects during a reload: the BPF
programs, the access and the accounting maps. The former two need to be
regenerated anyway with the newly loaded configuration data, but the
latter one needs to survive reloads/reexec. In this implementation I
opted to only save/restore the accounting map content instead of the map
itself. While this opens a (theoretic) window where IP traffic is still
accounted to the old map after we read it out, and we thus miss a few
bytes this has the benefit that we can alter the map layout between
versions should the need arise.

src/core/cgroup.c

@@ -2224,6 +2224,7 @@ int unit_get_ip_accounting(
                 CGroupIPAccountingMetric metric,
                 uint64_t *ret) {
 
+        uint64_t value;
         int fd, r;
 
         assert(u);
@@ -2239,9 +2240,17 @@ int unit_get_ip_accounting(
                 return -ENODATA;
 
         if (IN_SET(metric, CGROUP_IP_INGRESS_BYTES, CGROUP_IP_EGRESS_BYTES))
-                r = bpf_firewall_read_accounting(fd, ret, NULL);
+                r = bpf_firewall_read_accounting(fd, &value, NULL);
         else
-                r = bpf_firewall_read_accounting(fd, NULL, ret);
+                r = bpf_firewall_read_accounting(fd, NULL, &value);
+        if (r < 0)
+                return r;
+
+        /* Add in additional metrics from a previous runtime. Note that when reexecing/reloading the daemon we compile
+         * all BPF programs and maps anew, but serialize the old counters. When deserializing we store them in the
+         * ip_accounting_extra[] field, and add them in here transparently. */
+
+        *ret = value + u->ip_accounting_extra[metric];
 
         return r;
 }
@@ -2275,6 +2284,8 @@ int unit_reset_ip_accounting(Unit *u) {
         if (u->ip_accounting_egress_map_fd >= 0)
                 q = bpf_firewall_reset_accounting(u->ip_accounting_egress_map_fd);
 
+        zero(u->ip_accounting_extra);
+
         return r < 0 ? r : q;
 }
 

src/core/unit.c

@@ -2770,7 +2770,15 @@ static int unit_serialize_cgroup_mask(FILE *f, const char *key, CGroupMask mask)
         return r;
 }
 
+static const char *ip_accounting_metric_field[_CGROUP_IP_ACCOUNTING_METRIC_MAX] = {
+        [CGROUP_IP_INGRESS_BYTES] = "ip-accounting-ingress-bytes",
+        [CGROUP_IP_INGRESS_PACKETS] = "ip-accounting-ingress-packets",
+        [CGROUP_IP_EGRESS_BYTES] = "ip-accounting-egress-bytes",
+        [CGROUP_IP_EGRESS_PACKETS] = "ip-accounting-egress-packets",
+};
+
 int unit_serialize(Unit *u, FILE *f, FDSet *fds, bool serialize_jobs) {
+        CGroupIPAccountingMetric m;
         int r;
 
         assert(u);
@@ -2831,6 +2839,14 @@ int unit_serialize(Unit *u, FILE *f, FDSet *fds, bool serialize_jobs) {
 
         bus_track_serialize(u->bus_track, f, "ref");
 
+        for (m = 0; m < _CGROUP_IP_ACCOUNTING_METRIC_MAX; m++) {
+                uint64_t v;
+
+                r = unit_get_ip_accounting(u, m, &v);
+                if (r >= 0)
+                        unit_serialize_item_format(u, f, ip_accounting_metric_field[m], "%" PRIu64, v);
+        }
+
         if (serialize_jobs) {
                 if (u->job) {
                         fprintf(f, "job\n");
@@ -2937,6 +2953,7 @@ int unit_deserialize(Unit *u, FILE *f, FDSet *fds) {
 
         for (;;) {
                 char line[LINE_MAX], *l, *v;
+                CGroupIPAccountingMetric m;
                 size_t k;
 
                 if (!fgets(line, sizeof(line), f)) {
@@ -3147,6 +3164,21 @@ int unit_deserialize(Unit *u, FILE *f, FDSet *fds) {
                         continue;
                 }
 
+                /* Check if this is an IP accounting metric serialization field */
+                for (m = 0; m < _CGROUP_IP_ACCOUNTING_METRIC_MAX; m++)
+                        if (streq(l, ip_accounting_metric_field[m]))
+                                break;
+                if (m < _CGROUP_IP_ACCOUNTING_METRIC_MAX) {
+                        uint64_t c;
+
+                        r = safe_atou64(v, &c);
+                        if (r < 0)
+                                log_unit_debug(u, "Failed to parse IP accounting value %s, ignoring.", v);
+                        else
+                                u->ip_accounting_extra[m] = c;
+                        continue;
+                }
+
                 if (unit_can_serialize(u)) {
                         if (rt) {
                                 r = exec_runtime_deserialize_item(u, rt, l, v, fds);

src/core/unit.h

@@ -34,6 +34,7 @@ typedef struct UnitStatusMessageFormats UnitStatusMessageFormats;
 #include "install.h"
 #include "list.h"
 #include "unit-name.h"
+#include "cgroup.h"
 
 typedef enum KillOperation {
         KILL_TERMINATE,
@@ -224,6 +225,8 @@ struct Unit {
         BPFProgram *ip_bpf_ingress;
         BPFProgram *ip_bpf_egress;
 
+        uint64_t ip_accounting_extra[_CGROUP_IP_ACCOUNTING_METRIC_MAX];
+
         /* How to start OnFailure units */
         JobMode on_failure_job_mode;