From 936f4e3a97bea275d23cb7b2ad2cf50640675a7b Mon Sep 17 00:00:00 2001
From: Neil Shepperd <nshepperd@gmail.com>
Date: Tue, 1 Sep 2015 14:22:47 +1000
Subject: [PATCH] Implement BSD-style /dev/[u]random behaviour

---
 drivers/char/random.c | 32 +++++++++++---------------------
 1 file changed, 11 insertions(+), 21 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index d0da5d8..cebed43 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -281,6 +281,7 @@
 #define OUTPUT_POOL_WORDS	(1 << (OUTPUT_POOL_SHIFT-5))
 #define SEC_XFER_SIZE		512
 #define EXTRACT_SIZE		10
+#define MINIMUM_ENTROPY_BITS    256

 #define DEBUG_RANDOM_BOOT 0

@@ -468,6 +469,7 @@ static struct entropy_store blocking_pool = {
 static struct entropy_store nonblocking_pool = {
 	.poolinfo = &poolinfo_table[1],
 	.name = "nonblocking",
+        .limit = 1,
 	.pull = &input_pool,
 	.lock = __SPIN_LOCK_UNLOCKED(nonblocking_pool.lock),
 	.pool = nonblocking_pool_data,
@@ -1036,22 +1038,20 @@ static void push_to_pool(struct work_struct *work)
 static size_t account(struct entropy_store *r, size_t nbytes, int min,
 		      int reserved)
 {
-	int entropy_count, orig;
-	size_t ibytes, nfrac;
+	int entropy_count;
+	size_t ibytes;

 	BUG_ON(r->entropy_count > r->poolinfo->poolfracbits);

 	/* Can we pull enough? */
-retry:
-	entropy_count = orig = ACCESS_ONCE(r->entropy_count);
+	entropy_count = ACCESS_ONCE(r->entropy_count);
 	ibytes = nbytes;
-	/* If limited, never pull more than available */
+	/* If limited, block until we have MINIMUM_ENTROPY_BITS bits in the pool */
 	if (r->limit) {
-		int have_bytes = entropy_count >> (ENTROPY_SHIFT + 3);
-
-		if ((have_bytes -= reserved) < 0)
-			have_bytes = 0;
-		ibytes = min_t(size_t, ibytes, have_bytes);
+		int have_bits = entropy_count >> ENTROPY_SHIFT;
+		if (have_bits < MINIMUM_ENTROPY_BITS) {
+			ibytes = 0;
+		}
 	}
 	if (ibytes < min)
 		ibytes = 0;
@@ -1060,18 +1060,8 @@ retry:
 		pr_warn("random: negative entropy count: pool %s count %d\n",
 			r->name, entropy_count);
 		WARN_ON(1);
-		entropy_count = 0;
 	}
-	nfrac = ibytes << (ENTROPY_SHIFT + 3);
-	if ((size_t) entropy_count > nfrac)
-		entropy_count -= nfrac;
-	else
-		entropy_count = 0;
-
-	if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
-		goto retry;

-	trace_debit_entropy(r->name, 8 * ibytes);
 	if (ibytes &&
 	    (r->entropy_count >> ENTROPY_SHIFT) < random_write_wakeup_bits) {
 		wake_up_interruptible(&random_write_wait);
@@ -1343,7 +1333,7 @@ void get_random_bytes_arch(void *buf, int nbytes)

 		if (!arch_get_random_long(&v))
 			break;
-
+
 		memcpy(p, &v, chunk);
 		p += chunk;
 		nbytes -= chunk;
--
2.5.1