From 6da2658fddded3d2f88c53f27f3707be7e910486 Mon Sep 17 00:00:00 2001
From: Rajan Gill <rajanjg89@gmail.com>
Date: Sun, 16 Sep 2018 12:22:36 -0600
Subject: [PATCH] libs/libc/math:  Add variable convergence in log() and logf()
 to avoid hangs caused by failure to converge for very specific input values.

---
 libs/libc/math/lib_log.c  | 37 +++++++++++++++++++++++++++++++++++--
 libs/libc/math/lib_logf.c | 27 ++++++++++++++++++++++++++-
 2 files changed, 61 insertions(+), 3 deletions(-)

diff --git a/libs/libc/math/lib_log.c b/libs/libc/math/lib_log.c
index 10c9ccc025..c2d414eb8e 100644
--- a/libs/libc/math/lib_log.c
+++ b/libs/libc/math/lib_log.c
@@ -3,7 +3,7 @@
  *
  * This file is a part of NuttX:
  *
- *   Copyright (C) 2012 Gregory Nutt. All rights reserved.
+ *   Copyright (C) 2012, 2018 Gregory Nutt. All rights reserved.
  *   Ported by: Darcy Gong
  *
  * It derives from the Rhombus OS math library by Nick Johnson which has
@@ -35,19 +35,44 @@
 #include <math.h>
 #include <float.h>
 
+/****************************************************************************
+ * Pre-processor Definitions
+ ****************************************************************************/
+
+/* To avoid looping forever in particular corner cases, every LOGF_MAX_ITER
+ * the error criteria is relaxed by a factor LOGF_RELAX_MULTIPLIER.
+ * todo: might need to adjust the double floating point version too.
+ */
+
+#define LOGF_MAX_ITER         10
+#define LOGF_RELAX_MULTIPLIER 2
+
 /****************************************************************************
  * Public Functions
  ****************************************************************************/
 
+/****************************************************************************
+ * Name: log
+ ****************************************************************************/
+
 #ifdef CONFIG_HAVE_DOUBLE
 double log(double x)
 {
-  double y, y_old, ey, epsilon;
+  double y;
+  double y_old;
+  double ey;
+  double epsilon;
+  int    relax_factor;
+  int    i;
 
   y = 0.0;
   y_old = 1.0;
   epsilon = DBL_EPSILON;
 
+  i            = 0;
+  relax_factor = 1;
+
+
   while (y > y_old + epsilon || y < y_old - epsilon)
     {
       y_old = y;
@@ -65,6 +90,14 @@ double log(double x)
         }
 
       epsilon = (fabs(y) > 1.0) ? fabs(y) * DBL_EPSILON : DBL_EPSILON;
+
+      if (++i >= LOGF_MAX_ITER)
+        {
+          relax_factor *= LOGF_RELAX_MULTIPLIER;
+          i = 0;
+        }
+
+      epsilon *= relax_factor;
     }
 
   if (y == 700.0)
diff --git a/libs/libc/math/lib_logf.c b/libs/libc/math/lib_logf.c
index dc6bcdb5b9..0faaf3b55e 100644
--- a/libs/libc/math/lib_logf.c
+++ b/libs/libc/math/lib_logf.c
@@ -3,7 +3,7 @@
  *
  * This file is a part of NuttX:
  *
- *   Copyright (C) 2012, 2017 Gregory Nutt. All rights reserved.
+ *   Copyright (C) 2012, 2017-2018 Gregory Nutt. All rights reserved.
  *   Ported by: Darcy Gong
  *
  * It derives from the Rhombus OS math library by Nick Johnson which has
@@ -32,8 +32,20 @@
 #include <math.h>
 #include <float.h>
 
+/****************************************************************************
+ * Pre-processor Definitions
+ ****************************************************************************/
+
 #define FLT_MAX_EXP_X   88.0F
 
+/* To avoid looping forever in particular corner cases, every LOGF_MAX_ITER
+ * the error criteria is relaxed by a factor LOGF_RELAX_MULTIPLIER.
+ * todo: might need to adjust the double floating point version too.
+ */
+
+#define LOGF_MAX_ITER         10
+#define LOGF_RELAX_MULTIPLIER 2
+
 /****************************************************************************
  * Public Functions
  ****************************************************************************/
@@ -48,11 +60,16 @@ float logf(float x)
   float y_old;
   float ey;
   float epsilon;
+  int   relax_factor;
+  int   i;
 
   y       = 0.0F;
   y_old   = 1.0F;
   epsilon = FLT_EPSILON;
 
+  i            = 0;
+  relax_factor = 1;
+
   while (y > y_old + epsilon || y < y_old - epsilon)
     {
       y_old = y;
@@ -70,6 +87,14 @@ float logf(float x)
         }
 
       epsilon = (fabsf(y) > 1.0F) ? fabsf(y) * FLT_EPSILON : FLT_EPSILON;
+
+      if (++i >= LOGF_MAX_ITER)
+        {
+          relax_factor *= LOGF_RELAX_MULTIPLIER;
+          i = 0;
+        }
+
+      epsilon *= relax_factor;
     }
 
   if (y == FLT_MAX_EXP_X)