From b86c65d31c61af42a3b634c6a3150917509e3e8b Mon Sep 17 00:00:00 2001
From: Damien George <damien.p.george@gmail.com>
Date: Mon, 3 Jul 2017 14:52:00 +1000
Subject: [PATCH] extmod/modubinascii: Add check for empty buffer passed to
 hexlify.

Previous to this patch hexlify(b'', b':') would lead to a bad crash due to
the computed length of the result being -1=0xffffffff.
---
 extmod/modubinascii.c                     | 6 ++++++
 tests/extmod/ubinascii_micropython.py     | 3 +++
 tests/extmod/ubinascii_micropython.py.exp | 1 +
 3 files changed, 10 insertions(+)

diff --git a/extmod/modubinascii.c b/extmod/modubinascii.c
index 07b8b15bf..cf250d27f 100644
--- a/extmod/modubinascii.c
+++ b/extmod/modubinascii.c
@@ -42,6 +42,12 @@ mp_obj_t mod_binascii_hexlify(size_t n_args, const mp_obj_t *args) {
     mp_buffer_info_t bufinfo;
     mp_get_buffer_raise(args[0], &bufinfo, MP_BUFFER_READ);
 
+    // Code below assumes non-zero buffer length when computing size with
+    // separator, so handle the zero-length case here.
+    if (bufinfo.len == 0) {
+        return mp_const_empty_bytes;
+    }
+
     vstr_t vstr;
     size_t out_len = bufinfo.len * 2;
     if (n_args > 1) {
diff --git a/tests/extmod/ubinascii_micropython.py b/tests/extmod/ubinascii_micropython.py
index a4c00a2cb..77084ec9e 100644
--- a/tests/extmod/ubinascii_micropython.py
+++ b/tests/extmod/ubinascii_micropython.py
@@ -10,3 +10,6 @@ except ImportError:
 # two arguments supported in uPy but not CPython
 a = binascii.hexlify(b'123', ':')
 print(a)
+
+# zero length buffer
+print(binascii.hexlify(b'', b':'))
diff --git a/tests/extmod/ubinascii_micropython.py.exp b/tests/extmod/ubinascii_micropython.py.exp
index 0ae3d768e..a195d2602 100644
--- a/tests/extmod/ubinascii_micropython.py.exp
+++ b/tests/extmod/ubinascii_micropython.py.exp
@@ -1 +1,2 @@
 b'31:32:33'
+b''