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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>
Subject: [PATCH 4.9 106/264] random: simplify arithmetic function flow in account()
Date:   Thu, 23 Jun 2022 18:41:39 +0200
Message-Id: <20220623164347.069392524@linuxfoundation.org>
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From: "Jason A. Donenfeld" <Jason@zx2c4.com>

commit a254a0e4093fce8c832414a83940736067eed515 upstream.

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@dominikbrodowski.net>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/char/random.c |   17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1344,7 +1344,7 @@ EXPORT_SYMBOL_GPL(add_disk_randomness);
  */
 static 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);
@@ -1352,20 +1352,15 @@ static size_t account(size_t nbytes, int
 	/* 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;