ash: dont allow e.g. exec <&10 to attach to stript's fd!

function                                             old     new   delta
is_hidden_fd                                           -      61     +61
redirect                                            1135    1164     +29
popstring                                            134     140      +6
printf_main                                          635     637      +2
evalvar                                             1374    1376      +2
echo_main                                            294     296      +2
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 5/0 up/down: 102/0)             Total: 102 bytes

2 files changed, 12 insertions, 6 deletions

diff --git a/coreutils/echo.c b/coreutils/echo.c
index 36cb6b3af..decca095f 100644
--- a/coreutils/echo.c
+++ b/coreutils/echo.c
@@ -46,8 +46,11 @@ int echo_main(int argc UNUSED_PARAM, char **argv)
          * even if libc receives EBADF on write attempts, it feels determined
          * to output data no matter what. So it will try later,
          * and possibly will clobber future output. Not good. */
-        if (dup2(1, 1) != 1)
-                return -1;
+// TODO: check fcntl() & O_ACCMODE == O_WRONLY or O_RDWR?
+        if (fcntl(1, F_GETFL) == -1)
+                return 1; /* match coreutils 6.10 (sans error msg to stderr) */
+        //if (dup2(1, 1) != 1) - old way
+        //      return 1;
 
         arg = *++argv;
         if (!arg)
@@ -58,8 +61,8 @@ int echo_main(int argc UNUSED_PARAM, char **argv)
         char eflag = 0;
 
         /* We must check that stdout is not closed. */
-        if (dup2(1, 1) != 1)
-                return -1;
+        if (fcntl(1, F_GETFL) == -1)
+                return 1;
 
         while (1) {
                 arg = *++argv;
diff --git a/coreutils/printf.c b/coreutils/printf.c
index 72acbc751..76524f706 100644
--- a/coreutils/printf.c
+++ b/coreutils/printf.c
@@ -348,8 +348,11 @@ int printf_main(int argc UNUSED_PARAM, char **argv)
          * even if libc receives EBADF on write attempts, it feels determined
          * to output data no matter what. So it will try later,
          * and possibly will clobber future output. Not good. */
-        if (dup2(1, 1) != 1)
-                return -1;
+// TODO: check fcntl() & O_ACCMODE == O_WRONLY or O_RDWR?
+        if (fcntl(1, F_GETFL) == -1)
+                return 1; /* match coreutils 6.10 (sans error msg to stderr) */
+        //if (dup2(1, 1) != 1) - old way
+        //      return 1;
 
         /* bash builtin errors out on "printf '-%s-\n' foo",
          * coreutils-6.9 works. Both work with "printf -- '-%s-\n' foo".