linux-kernel - [RFC/RFT PATCH 3/6] random: Use u32 to keep track of batched entropy generation

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Message-ID: <20251127092226.1439196-11-ardb+git@google.com>
Date: Thu, 27 Nov 2025 10:22:30 +0100
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-hardening@...r.kernel.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Ard Biesheuvel <ardb@...nel.org>, Kees Cook <kees@...nel.org>, Ryan Roberts <ryan.roberts@....com>, 
	Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...db.de>, Jeremy Linton <jeremy.linton@....com>, 
	Catalin Marinas <Catalin.Marinas@....com>, Mark Rutland <mark.rutland@....com>, 
	"Jason A. Donenfeld" <Jason@...c4.com>
Subject: [RFC/RFT PATCH 3/6] random: Use u32 to keep track of batched entropy generation

From: Ard Biesheuvel <ardb@...nel.org>

The batched entropy containers each have a generation field, to keep
track of the base_crng generation from which it was last reseeded.

This use case does not require all bits of the unsigned long to be
stored: storing only 32 bits is sufficient to determine whether or not
we're at most 4 billion generations behind, which seems ample.

So use an unsigned int instead: this will allow a future patch to treat
the generation and position as a single 64-bit quantity, which can be
used locklessly in a compare-and-exchange() operation.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 drivers/char/random.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index b8b24b6ed3fe..0e04bc60d034 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -507,7 +507,7 @@ struct batch_ ##type {								\
 	 */									\
 	type entropy[CHACHA_BLOCK_SIZE * 3 / (2 * sizeof(type))];		\
 	local_lock_t lock;							\
-	unsigned long generation;						\
+	unsigned int generation;						\
 	unsigned int position;							\
 };										\
 										\
@@ -521,7 +521,7 @@ type get_random_ ##type(void)							\
 	type ret;								\
 	unsigned long flags;							\
 	struct batch_ ##type *batch;						\
-	unsigned long next_gen;							\
+	unsigned int next_gen;							\
 										\
 	warn_unseeded_randomness();						\
 										\
@@ -533,7 +533,7 @@ type get_random_ ##type(void)							\
 	local_lock_irqsave(&batched_entropy_ ##type.lock, flags);		\
 	batch = raw_cpu_ptr(&batched_entropy_##type);				\
 										\
-	next_gen = READ_ONCE(base_crng.generation);				\
+	next_gen = (unsigned int)READ_ONCE(base_crng.generation);		\
 	if (batch->position >= ARRAY_SIZE(batch->entropy) ||			\
 	    next_gen != batch->generation) {					\
 		_get_random_bytes(batch->entropy, sizeof(batch->entropy));	\
-- 
2.52.0.107.ga0afd4fd5b-goog