ebpf: fixed dns uprobes

We were not deleting DNS entries from the hash map, so when it reached
the maximum capacity (12k entries), we couldn't allocate new entries,
resulting in events not being sent to userspace.

(cherry picked from commit 1518cb3)

Author: gustavo-iniguez-goya

ebpf_prog/opensnitch-dns.c

@@ -1,4 +1,24 @@
-#define KBUILD_MODNAME "dummy"
+/*   Copyright (C) 2022      calesanz
+//                 2023-2024 Gustavo Iñiguez Goya
+//
+//   This file is part of OpenSnitch.
+//
+//   OpenSnitch is free software: you can redistribute it and/or modify
+//   it under the terms of the GNU General Public License as published by
+//   the Free Software Foundation, either version 3 of the License, or
+//   (at your option) any later version.
+//
+//   OpenSnitch is distributed in the hope that it will be useful,
+//   but WITHOUT ANY WARRANTY; without even the implied warranty of
+//   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+//   GNU General Public License for more details.
+//
+//   You should have received a copy of the GNU General Public License
+//   along with OpenSnitch.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+
+#define KBUILD_MODNAME "opensnitch-dns"
 
 #include <linux/in.h>
 #include <linux/in6.h>
@@ -51,7 +71,7 @@ struct addrinfo_args_cache {
 // define temporary array for data
 struct bpf_map_def SEC("maps/addrinfo_args_hash") addrinfo_args_hash = {
     .type = BPF_MAP_TYPE_HASH,
-    .max_entries = MAPSIZE,
+    .max_entries = 256, // max entries at any time
     .key_size = sizeof(u32),
     .value_size = sizeof(struct addrinfo_args_cache),
 };
@@ -61,7 +81,7 @@ struct bpf_map_def SEC("maps/events") events = {
     .type = BPF_MAP_TYPE_PERF_EVENT_ARRAY,
     .key_size = sizeof(u32),
     .value_size = sizeof(u32),
-    .max_entries = MAPSIZE,
+    .max_entries = 256, // max cpus
 };
 
 /**
@@ -170,7 +190,7 @@ int ret_addrinfo(struct pt_regs *ctx) {
         struct addrinfo *res;
         bpf_probe_read(&res, sizeof(res), res_p);
         if (res == NULL) {
-            return 0;
+            goto out;
         }
         bpf_probe_read(&data.addr_type, sizeof(data.addr_type),
                        &res->ai_family);
@@ -186,7 +206,7 @@ int ret_addrinfo(struct pt_regs *ctx) {
 
             bpf_probe_read_user(&data.ip, sizeof(data.ip), &ipv6->sin6_addr);
         } else {
-			return 1;
+			goto out;
 		}
 
         bpf_probe_read_kernel_str(&data.host, sizeof(data.host),
@@ -198,9 +218,15 @@ int ret_addrinfo(struct pt_regs *ctx) {
 
 		struct addrinfo * next;
         bpf_probe_read(&next, sizeof(next), &res->ai_next);
+        if (next == NULL){
+            goto out;
+        }
 		res_p = &next;
     }
 
+out:
+    bpf_map_delete_elem(&addrinfo_args_hash, &tid);
+
     return 0;
 }