From 89bf0a79059699d137c964ac9dc59fb5255f152a Mon Sep 17 00:00:00 2001
From: Hanno Becker <hanno.becker@arm.com>
Date: Thu, 4 May 2017 13:39:22 +0100
Subject: [PATCH] Backup errno in net_would_block

Safe and restore the value of errno in net_would_block to be sure
it's not affected by the guarding call to fcntl. Fixes #845.
---
 ChangeLog     | 6 ++++++
 library/net.c | 7 ++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index fe5ce6535e..eb99ce2434 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,11 @@
 mbed TLS ChangeLog (Sorted per branch, date)
 
+= mbed TLS x.x.x branch released xxxx-xx-xx
+
+Bugfix
+   * Fix net_would_block to avoid modification by errno through fcntl call.
+     Found by nkolban. Fixes #845.
+
 = mbed TLS 2.1.7 branch released 2017-03-08
 
 Security
diff --git a/library/net.c b/library/net.c
index b6b08ed720..f08bbec51f 100644
--- a/library/net.c
+++ b/library/net.c
@@ -259,13 +259,18 @@ static int net_would_block( const mbedtls_net_context *ctx )
  */
 static int net_would_block( const mbedtls_net_context *ctx )
 {
+    int err = errno;
+    
     /*
      * Never return 'WOULD BLOCK' on a non-blocking socket
      */
     if( ( fcntl( ctx->fd, F_GETFL ) & O_NONBLOCK ) != O_NONBLOCK )
+    {
+        errno = err;
         return( 0 );
+    }
 
-    switch( errno )
+    switch( errno = err )
     {
 #if defined EAGAIN
         case EAGAIN: