Sanitize process name for GUI notification helper

As it turns out, the process name field /proc/PID/stat can
contain arbitrary characters. This is a problem, because
we call a notification helper, usually notify-send, using
system().

Aggressively strip all non-alphanumeric characters to fix
a shell code injection vulnerability.

Users who do not use GUI notifications (-n or -N) are
not affected.

kill.c

@@ -19,6 +19,7 @@
 #define BADNESS_AVOID -300
 
 extern int enable_debug;
+void sanitize(char* s);
 
 struct procinfo {
     int oom_score;
@@ -219,6 +220,8 @@ static void userspace_kill(DIR* procdir, int sig, int ignore_oom_score_adj,
     // that there is enough memory to spawn the notification helper.
     if (sig != 0) {
         char notif_args[PATH_MAX + 1000];
+        // maybe_notify() calls system(). We must sanitize the strings we pass.
+        sanitize(victim_name);
         snprintf(notif_args, sizeof(notif_args), "-i dialog-warning 'earlyoom' 'Killing process %d %s'", victim_pid, victim_name);
         maybe_notify(notif_command, notif_args);
     }

sanitize.c

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: MIT
+
+/* sanitize replaces everything in string "s" that is not [a-zA-Z0-9]
+ * with an underscore. The resulting string is safe to pass to a shell.
+ */
+void sanitize(char* s)
+{
+    char c;
+    for (int i = 0; s[i] != 0; i++) {
+        c = s[i];
+        if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9')) {
+            continue;
+        }
+        s[i] = '_';
+    }
+}