preproc.c: Make %substr robust

Make %substr robust to handle -1,-1 parameters
and restore old behavior when number of characters
in substring is greater then length of string itself.

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

1 files changed, 5 insertions, 4 deletions

diff --git a/preproc.c b/preproc.c
index c6349922..70e550d0 100644
--- a/preproc.c
+++ b/preproc.c
@@ -3449,13 +3449,14 @@ issue_error:
 
         len = nasm_unquote(t->text, NULL);
 
-        /* check the values provided, on error -- empty string */
+        /* make start and count being in range */
+        if (start < 0)
+            start = 0;
         if (count < 0)
             count = len + count + 1 - start;
         if (start + count > (int64_t)len)
-            start = -1;
-
-        if (!len || count < 0 || start < 0)
+            count = len - start;
+        if (!len || count < 0 || start >=(int64_t)len)
             start = -1, count = 0; /* empty string */
 
         macro_start = nasm_malloc(sizeof(*macro_start));