Tighten checking for variable validity

In the future we might want to allow additional syntax (for example
"unset VAR". But let's check that the data we're getting does not contain
anything unexpected.

src/basic/exec-util.c

@@ -279,6 +279,11 @@ static int gather_environment_generate(int fd, void *arg) {
         STRV_FOREACH_PAIR(x, y, new) {
                 char *p;
 
+                if (!env_name_is_valid(*x)) {
+                        log_warning("Invalid variable assignment \"%s=...\", ignoring.", *x);
+                        continue;
+                }
+
                 p = strjoin(*x, "=", *y);
                 if (!p)
                         return -ENOMEM;

src/basic/fileio.c

@@ -773,6 +773,16 @@ static int merge_env_file_push(
 
         assert(env);
 
+        if (!value) {
+                log_error("%s:%u: invalid syntax (around \"%s\"), ignoring.", strna(filename), line, key);
+                return 0;
+        }
+
+        if (!env_name_is_valid(key)) {
+                log_error("%s:%u: invalid variable name \"%s\", ignoring.", strna(filename), line, key);
+                return 0;
+        }
+
         expanded_value = replace_env(value, *env,
                                      REPLACE_ENV_USE_ENVIRONMENT|REPLACE_ENV_ALLOW_BRACELESS);
         if (!expanded_value)