On 7/8/2026 3:06 AM, Thomas Gleixner wrote:
> add_random_kstack_offset() uses get/put_cpu_var() which is pointless
> overhead when it is invoked from low level entry code with interrupts
> disabled.
>
> Provide a irqsoff() variant, which avoids that.
>
> Signed-off-by: Thomas Gleixner <[email protected]>
> Cc: Kees Cook <[email protected]>
> ---
> include/linux/randomize_kstack.h | 19 +++++++++++++++++++
> 1 file changed, 19 insertions(+)
>
> --- a/include/linux/randomize_kstack.h
> +++ b/include/linux/randomize_kstack.h
> @@ -77,8 +77,27 @@ static __always_inline u32 get_kstack_of
> } \
> } while (0)
>
> +/**
> + * add_random_kstack_offset_irqsoff - Increase stack utilization by a random
> offset.
> + *
> + * This should be used in the syscall entry path after user registers have
> been
> + * stored to the stack. Interrupts must be still disabled.
> + */
> +#define add_random_kstack_offset_irqsoff()
> \
> +do {
> \
> + lockdep_assert_irqs_disabled();
> \
> + if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
> \
> + &randomize_kstack_offset)) {
> \
> + u32 offset = prandom_u32_state(raw_cpu_ptr(&kstack_rnd_state));
> \
> + u8 *ptr = __kstack_alloca(KSTACK_OFFSET_MAX(offset));
> \
> + /* Keep allocation even after "ptr" loses scope. */
> \
> + asm volatile("" :: "r"(ptr) : "memory");
> \
> + }
> \
> +} while (0)
Or can we avoid these repetitions and reuse the same code?
> +
> #else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
> #define add_random_kstack_offset() do { } while (0)
> +#define add_random_kstack_offset_irqsoff() do { } while (0)
> #endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
>
> #endif
>
