From 423acb9483f672df5cf220e386b7d85e35b95fb8 Mon Sep 17 00:00:00 2001
From: Mark Wooding <mdw@distorted.org.uk>
Date: Sun, 11 Sep 2016 14:29:49 +0100
Subject: [PATCH] math/mprand.[ch], rand/grand.c: Check range of arguments.

  * mprand: It doesn't make sense to ask for a zero-bit integer whose
    low bit is set; or, indeed, a four-bit integer whose fourth bit is
    set.  So check the mask against the bit length.

  * mprand: On the other hand, it /does/ make sense to ask for a
    zero-bit integer, and the answer is simply zero.  But the code used
    to segfault.  Fix this.

  * mprand_range, grand_defaultrange: It doesn't make sense to ask for
    an integer in [0, 0), because there aren't any.  Check before
    trying.
---
 math/mprand.c | 21 ++++++++++++++++++---
 math/mprand.h |  5 ++++-
 rand/grand.c  |  1 +
 3 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/math/mprand.c b/math/mprand.c
index eb779cb8..5805e6ed 100644
--- a/math/mprand.c
+++ b/math/mprand.c
@@ -49,17 +49,27 @@
  *		uniformly distributed in the interval %$[2^{b - 1}, 2^b)$%.
  *		The result is then ORred with the given @or@ value.  This
  *		will often be 1, to make the result odd.
+ *
+ *		The length @b@ may be zero; but %$\texttt{or} \ge 2^b$% is
+ *		not permitted.
  */
 
 mp *mprand(mp *d, unsigned b, grand *r, mpw or)
 {
   size_t sz = (b + 7) >> 3;
   arena *a = (d && (d->f & MP_BURN)) ? arena_secure : arena_global;
-  octet *v = x_alloc(a, sz);
+  octet *v;
   unsigned m;
 
+  assert(b >= MPW_BITS || !(or >> b));
+
+  /* --- Special case --- */
+
+  if (!b) return (MP_ZERO);
+
   /* --- Fill buffer with random data --- */
 
+  v = x_alloc(a, sz);
   r->ops->fill(r, v, sz);
 
   /* --- Force into the correct range --- *
@@ -74,7 +84,11 @@ mp *mprand(mp *d, unsigned b, grand *r, mpw or)
   /* --- Mask, load and return --- */
 
   d = mp_loadb(d, v, sz);
-  d->v[0] |= or;
+  if (or) {
+    assert(d->sz);
+    if (!MP_LEN(d)) d->vl = d->v + 1;
+    d->v[0] |= or;
+  }
   memset(v, 0, sz);
   x_free(a, v);
   return (d);
@@ -91,7 +105,7 @@ mp *mprand(mp *d, unsigned b, grand *r, mpw or)
  *		interval %$[0, l)$%.
  *
  * Use:		Generates a uniformly-distributed pseudorandom number in the
- *		appropriate range.
+ *		appropriate range.  We must have %$l > 0$%.
  */
 
 mp *mprand_range(mp *d, mp *l, grand *r, mpw or)
@@ -119,6 +133,7 @@ mp *mprand_range(mp *d, mp *l, grand *r, mpw or)
    * As usual, the number of iterations expected is two.
    */
 
+  assert(MP_POSP(l));
   b = ((b - 1) & 7) + 1;
   m = (1 << b) - 1;
   do {
diff --git a/math/mprand.h b/math/mprand.h
index 0c193c03..a088df8a 100644
--- a/math/mprand.h
+++ b/math/mprand.h
@@ -58,6 +58,9 @@
  *		uniformly distributed in the interval %$[2^{b - 1}, 2^b)$%.
  *		The result is then ORred with the given @or@ value.  This
  *		will often be 1, to make the result odd.
+ *
+ *		The length @b@ may be zero; but %$\texttt{or} \ge 2^b$% is
+ *		not permitted.
  */
 
 extern mp *mprand(mp */*d*/, unsigned /*b*/, grand */*r*/, mpw /*or*/);
@@ -73,7 +76,7 @@ extern mp *mprand(mp */*d*/, unsigned /*b*/, grand */*r*/, mpw /*or*/);
  *		interval %$[0, l)$%.
  *
  * Use:		Generates a uniformly-distributed pseudorandom number in the
- *		appropriate range.
+ *		appropriate range.  We must have %$l > 0$%.
  */
 
 extern mp *mprand_range(mp */*d*/, mp */*l*/, grand */*r*/, mpw /*or*/);
diff --git a/rand/grand.c b/rand/grand.c
index e4451917..a95c89f0 100644
--- a/rand/grand.c
+++ b/rand/grand.c
@@ -97,6 +97,7 @@ uint32 grand_defaultrange(grand *r, uint32 l)
    * @byte@, but by that point it must be able to satisfy us.
    */
 
+  assert(l);
   if (r->ops->max && r->ops->max >= l) {
     w = r->ops->raw;
     m = r->ops->max;
-- 
2.11.0