common: add AUTOBOOT_FLUSH_STDIN option

The key-sequence based unlock mechanisms are sensitive to junk symbols
that could have been sent to stdin and are still waiting to be retrieved.
Enabling this option will read all symbols off stdin before displaying the
autoboot prompt (and starting to read the password from stdin).

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

diff --git a/common/Kconfig.boot b/common/Kconfig.boot
index f9551b206f5d0db6abffaf5315691381e5c0e3e9..e0cca226da4fbb59012f14c76e03ae1ed447799b 100644 (file)
--- a/common/Kconfig.boot
+++ b/common/Kconfig.boot
@@ -791,6 +791,15 @@ config AUTOBOOT_KEYED
          U-Boot automatic booting process and bring the device
          to the U-Boot prompt for user input.
 
+config AUTOBOOT_FLUSH_STDIN
+       bool "Enable flushing stdin before starting to read the password"
+       depends on AUTOBOOT_KEYED && !SANDBOX
+       help
+         When this option is enabled stdin buffer will be flushed before
+         starting to read the password.
+         This can't be enabled for the sandbox as flushing stdin would
+         break the autoboot unit tests.
+
 config AUTOBOOT_PROMPT
        string "Autoboot stop prompt"
        depends on AUTOBOOT_KEYED

diff --git a/common/autoboot.c b/common/autoboot.c
index 2564ef8a561d7de65447c9579fd41fff8d29a400..35ef526c42f17e7d6f328a88be9621d33eb8aaa7 100644 (file)
--- a/common/autoboot.c
+++ b/common/autoboot.c
@@ -297,6 +297,15 @@ static int passwd_abort_key(uint64_t etime)
        return abort;
 }
 
+/**
+ * flush_stdin() - drops all pending characters from stdin
+ */
+static void flush_stdin(void)
+{
+       while (tstc())
+               (void)getchar();
+}
+
 /***************************************************************************
  * Watch for 'delay' seconds for autoboot stop or autoboot delay string.
  * returns: 0 -  no key string, allow autoboot 1 - got key string, abort
@@ -306,6 +315,8 @@ static int abortboot_key_sequence(int bootdelay)
        int abort;
        uint64_t etime = endtick(bootdelay);
 
+       if (IS_ENABLED(CONFIG_AUTOBOOT_FLUSH_STDIN))
+               flush_stdin();
 #  ifdef CONFIG_AUTOBOOT_PROMPT
        /*
         * CONFIG_AUTOBOOT_PROMPT includes the %d for all boards.