linux-kernel - [PATCH] random: simplify arithmetic function flow in account()

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Date:   Mon, 17 Jan 2022 18:52:37 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: [PATCH] random: simplify arithmetic function flow in account()

Now that have_bytes is never modified, we can simplify this function.
First, we move the check for negative entropy_count to be first. That
ensures that subsequent reads of this will be non-negative. Then,
have_bytes and ibytes can be folded into their one use site in the
min_t() function.

Suggested-by: Dominik Brodowski <linux@...inikbrodowski.net>
Reviewed-by: Dominik Brodowski <linux@...inikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 drivers/char/random.c | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index a1720a48f583..ecb45be9535f 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1291,9 +1291,9 @@ EXPORT_SYMBOL_GPL(add_disk_randomness);
  * This function decides how many bytes to actually take from the
  * given pool, and also debits the entropy count accordingly.
  */
-static size_t account(size_t nbytes, int min)
+static noinline size_t account(size_t nbytes, int min)
 {
-	int entropy_count, orig, have_bytes;
+	int entropy_count, orig;
 	size_t ibytes, nfrac;
 
 	BUG_ON(input_pool.entropy_count > POOL_FRACBITS);
@@ -1301,20 +1301,15 @@ static size_t account(size_t nbytes, int min)
 	/* Can we pull enough? */
 retry:
 	entropy_count = orig = READ_ONCE(input_pool.entropy_count);
-	ibytes = nbytes;
-	/* never pull more than available */
-	have_bytes = entropy_count >> (POOL_ENTROPY_SHIFT + 3);
-
-	if (have_bytes < 0)
-		have_bytes = 0;
-	ibytes = min_t(size_t, ibytes, have_bytes);
-	if (ibytes < min)
-		ibytes = 0;
-
 	if (WARN_ON(entropy_count < 0)) {
 		pr_warn("negative entropy count: count %d\n", entropy_count);
 		entropy_count = 0;
 	}
+
+	/* never pull more than available */
+	ibytes = min_t(size_t, nbytes, entropy_count >> (POOL_ENTROPY_SHIFT + 3));
+	if (ibytes < min)
+		ibytes = 0;
 	nfrac = ibytes << (POOL_ENTROPY_SHIFT + 3);
 	if ((size_t)entropy_count > nfrac)
 		entropy_count -= nfrac;
-- 
2.34.1