BR3028880: Fix NULL dereference on nonexistent environment variable

Frank reported we hit NULL dereference on nonexistent
environment variables. Fix it by leaving empty string
in text field of such token and yielding warning.

Reported-by: Frank Kotler <fbkotler@zytor.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

1 files changed, 7 insertions, 4 deletions

diff --git a/preproc.c b/preproc.c
index ef4f95c3..09ef32b6 100644
--- a/preproc.c
+++ b/preproc.c
@@ -1181,11 +1181,14 @@ static char *detoken(Token * tlist, bool expand_locals)
     list_for_each(t, tlist) {
         if (t->type == TOK_PREPROC_ID && t->text[1] == '!') {
             char *p = getenv(t->text + 2);
-            nasm_free(t->text);
-            if (p)
+            char *q = t->text;
+            if (p) {
                 t->text = nasm_strdup(p);
-            else
-                t->text = NULL;
+            } else {
+                t->text = nasm_strdup("");
+                error(ERR_WARNING | ERR_PASS1, "`%s' is empty", q + 2);
+            }
+            nasm_free(q);
         }
         /* Expand local macros here and not during preprocessing */
         if (expand_locals &&