Don't assume that if std{in,out,err} is a tty, it is the user's tty.

Previously, sudo only checked that the fd was a terminal, not that
it matched sudo's idea of the user's terminal.  This matters when
input or output is redirected to a different terminal.  In that
case we want to interpose the fd with a pipe even if it refers to
a terminal.  Bug #1056.

src/exec.c

@@ -376,6 +376,28 @@ sudo_needs_pty(const struct command_details *details)
     return false;
 }
 
+/*
+ * Check whether the specified fd matches the device file that
+ * corresponds to tty_sb.  If tty_sb is NULL, just check whether
+ * fd is a tty.  Always fills in fd_sb (zeroed on error).
+ * Returns true on match, else false.
+ */
+bool
+fd_matches_tty(int fd, struct stat *tty_sb, struct stat *fd_sb)
+{
+    bool ret;
+    debug_decl(fd_is_user_tty, SUDO_DEBUG_EXEC);
+
+    if (fstat(fd, fd_sb) == -1 || !S_ISCHR(fd_sb->st_mode)) {
+	 /* Always initialize fd_sb. */
+	memset(fd_sb, 0, sizeof(*fd_sb));
+	debug_return_bool(false);
+    }
+
+    /* Compare with tty_sb if available, else just check that fd is a tty. */
+    debug_return_bool(tty_sb ? tty_sb->st_rdev == fd_sb->st_rdev : isatty(fd));
+}
+
 /*
  * If we are not running the command in a pty, we were not invoked as
  * sudoedit, there is no command timeout and there is no close function,