Re: [RFC/RFT PATCH 3/6] random: Use u32 to keep track of batched entropy generation
From: david laight <hidden>
Date: 2025-11-27 10:11:45
Also in:
linux-hardening, lkml
On Thu, 27 Nov 2025 10:22:30 +0100 Ard Biesheuvel [off-list ref] wrote:
From: Ard Biesheuvel <ardb@kernel.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.
Probably best to use a u32. While it will always(?) be the same as 'unsigned int' it is more descriptive.
quoted hunk ↗ jump to hunk
Signed-off-by: Ard Biesheuvel <ardb@kernel.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); \
Isn't that cast pointless? David
if (batch->position >= ARRAY_SIZE(batch->entropy) || \
next_gen != batch->generation) { \
_get_random_bytes(batch->entropy, sizeof(batch->entropy)); \