hush: fix 'x=; echo ${x:-"$@"}' producing 'BUG in varexp2' message

function                                             old     new   delta
expand_string_to_string                              126     128      +2

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 14 insertions, 7 deletions

diff --git a/shell/hush.c b/shell/hush.c
index 534fabbd0..da10a09a8 100644
--- a/shell/hush.c
+++ b/shell/hush.c
@@ -6529,13 +6529,20 @@ static char *expand_string_to_string(const char *str, int EXP_flags, int do_unba
         argv[0] = (char*)str;
         argv[1] = NULL;
         list = expand_variables(argv, EXP_flags | EXP_FLAG_SINGLEWORD);
-        if (HUSH_DEBUG)
-                if (!list[0] || list[1])
-                        bb_error_msg_and_die("BUG in varexp2");
-        /* actually, just move string 2*sizeof(char*) bytes back */
-        overlapping_strcpy((char*)list, list[0]);
-        if (do_unbackslash)
-                unbackslash((char*)list);
+        if (!list[0]) {
+                /* Example where it happens:
+                 * x=; echo ${x:-"$@"}
+                 */
+                ((char*)list)[0] = '\0';
+        } else {
+                if (HUSH_DEBUG)
+                        if (list[1])
+                                bb_error_msg_and_die("BUG in varexp2");
+                /* actually, just move string 2*sizeof(char*) bytes back */
+                overlapping_strcpy((char*)list, list[0]);
+                if (do_unbackslash)
+                        unbackslash((char*)list);
+        }
         debug_printf_expand("string_to_string=>'%s'\n", (char*)list);
         return (char*)list;
 }