preproc: Fix accessing OOM address

In case if there is no environment variable present
we allocated empty string but when working with tokens
we test for second byte for special symbols, accessing
out of memory address (->text[1] for the reference).

http://bugzilla.nasm.us/show_bug.cgi?id=3392333

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/preproc.c b/preproc.c
index 8400773b..fdb9fc19 100644
--- a/preproc.c
+++ b/preproc.c
@@ -1269,9 +1269,13 @@ static char *detoken(Token * tlist, bool expand_locals)
                 if (!p) {
                     nasm_error(ERR_NONFATAL | ERR_PASS1,
                           "nonexistent environment variable `%s'", v);
-                    p = "";
-                }
-                t->text = nasm_strdup(p);
+                    /*
+                     * FIXME We better should investigate if accessing
+                     * ->text[1] without ->text[0] is safe enough.
+                     */
+                    t->text = nasm_zalloc(2);
+                } else
+                    t->text = nasm_strdup(p);
             }
             nasm_free(q);
         }