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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     linux-kernel@vger.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Sultan Alsawaf <sultan@kerneltoast.com>
Subject: [PATCH] random: align entropy_timer_state to cache line
Date:   Wed, 30 Nov 2022 03:08:15 +0100
Message-Id: <20221130020815.283814-1-Jason@zx2c4.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

The theory behind the jitter dance is that multiple things are poking at
the same cache line. This only works, however, if those things are
actually all in the same cache line. Ensure this is the case by aligning
the struct on the stack to the cache line size.

On x86, this indeed aligns the stack struct:

 000000000000000c <try_to_generate_entropy>:
 {
    c:  55                      push   %rbp
-   d:  53                      push   %rbx
-   e:  48 83 ec 38             sub    $0x38,%rsp
+   d:  48 89 e5                mov    %rsp,%rbp
+  10:  41 54                   push   %r12
+  12:  53                      push   %rbx
+  13:  48 83 e4 c0             and    $0xffffffffffffffc0,%rsp
+  17:  48 83 ec 40             sub    $0x40,%rsp

Cc: Sultan Alsawaf <sultan@kerneltoast.com>
Fixes: 50ee7529ec45 ("random: try to actively add entropy rather than passively wait for it")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/char/random.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 67558b95d531..2494e08c76d8 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1262,7 +1262,7 @@ static void __cold entropy_timer(struct timer_list *timer)
 static void __cold try_to_generate_entropy(void)
 {
 	enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 15 };
-	struct entropy_timer_state stack;
+	struct entropy_timer_state stack ____cacheline_aligned;
 	unsigned int i, num_different = 0;
 	unsigned long last = random_get_entropy();
 	int cpu = -1;
-- 
2.38.1