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.
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); \
if (batch->position >= ARRAY_SIZE(batch->entropy) || \
next_gen != batch->generation) { \
_get_random_bytes(batch->entropy, sizeof(batch->entropy)); \
--
2.52.0.107.ga0afd4fd5b-googOn Thu, 27 Nov 2025 10:22:30 +0100
Ard Biesheuvel <ardb+git@google.com> 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.
>
> 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)); \
On Thu, 27 Nov 2025 at 11:11, david laight <david.laight@runbox.com> wrote:
>
> On Thu, 27 Nov 2025 10:22:30 +0100
> Ard Biesheuvel <ardb+git@google.com> 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.
>
> >
> > 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?
>
Depends on your definition of pointless :-)
We're deliberately truncating here, and the cast makes that explicit.
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