ebpf: delete pid from exec maps if it exists

We track new processes execution by intercepting the enter and exit
of the functions, but sometimes the exit hook is not called, so the
corresponding entry was not being removed from the map.
In this situation the map becomes full and accepts no new entries.

Now the entry is deleted from the map once the process exits, if it
still exists in the map.

(cherry picked from commit 9446d19)

ebpf_prog/opensnitch-procs.c

@@ -39,6 +39,9 @@ static __always_inline void new_event(struct data_t* data)
     bpf_get_current_comm(&data->comm, sizeof(data->comm));
 };
 
+/*
+ * send to userspace the result of the execve* call.
+ */
 static __always_inline void __handle_exit_execve(struct trace_sys_exit_execve *ctx)
 {
     u64 pid_tgid = bpf_get_current_pid_tgid();
@@ -50,7 +53,7 @@ static __always_inline void __handle_exit_execve(struct trace_sys_exit_execve *c
     bpf_perf_event_output(ctx, &events, BPF_F_CURRENT_CPU, proc, sizeof(*proc));
 
 out:
-	bpf_map_delete_elem(&execMap, &pid_tgid);
+    bpf_map_delete_elem(&execMap, &pid_tgid);
 }
 
 // https://0xax.gitbooks.io/linux-insides/content/SysCall/linux-syscall-4.html
@@ -68,6 +71,8 @@ int tracepoint__sched_sched_process_exit(struct pt_regs *ctx)
     data->type = EVENT_SCHED_EXIT;
     bpf_perf_event_output(ctx, &events, BPF_F_CURRENT_CPU, data, sizeof(*data));
 
+    u64 pid_tgid = bpf_get_current_pid_tgid();
+    bpf_map_delete_elem(&execMap, &pid_tgid);
     return 0;
 };