[RFC,05/41] random: don't reset entropy to zero on overflow

Commit Message

Nicolai Stange Sept. 21, 2020, 7:58 a.m. UTC

credit_entropy_bits() adds one or more positive values to the signed
entropy_count and checks if the result is negative afterwards. Note that
because the initial value of entropy_count is positive, a negative result
can happen only on overflow.

However, if the final entropy_count is found to have overflown, a WARN()
is emitted and the entropy_store's entropy count reset to zero. Even
though this case should never happen, it is better to retain previously
available entropy as this will facilitate a future change factoring out
that approximation of the exponential.

Make credit_entropy_bits() tp reset entropy_count to the original value
rather than zero on overflow.

Signed-off-by: Nicolai Stange <[e-mail]>
---
 drivers/char/random.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 35e381be20fe..6adac462aa0d 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -706,7 +706,7 @@  static void credit_entropy_bits(struct entropy_store *r, int nbits)
 	if (WARN_ON(entropy_count < 0)) {
 		pr_warn("negative entropy/overflow: pool %s count %d\n",
 			r->name, entropy_count);
-		entropy_count = 0;
+		entropy_count = orig;
 	} else if (entropy_count > pool_size)
 		entropy_count = pool_size;
 	if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)