Re: [PATCH v2 2/2] rseq: slice ext: Ensure rseq feature size differs from original rseq size

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2026-02-18 15:14, Mathieu Desnoyers wrote:
[...]
> diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
> index 863c4a00a66b..d5b3c489901e 100644
> --- a/include/uapi/linux/rseq.h
> +++ b/include/uapi/linux/rseq.h
> @@ -87,8 +87,10 @@ struct rseq_slice_ctrl {
>   };
>   
>   /*
> - * struct rseq is aligned on 4 * 8 bytes to ensure it is always
> - * contained within a single cache-line.
> + * The alignment of struct rseq used by the kernel needs to be 32 bytes
> + * so the kernel can interact with legacy userspace which allocate
> + * struct rseq with its original alignment.
> + * Userspace aligns struct rseq allocation on 32 bytes or more.

I plan to send a v3 of this series to clarify this uapi comment with:

/*
  * The original size and alignment of the allocation for struct rseq is
  * 32 bytes.
  *
  * The allocation size needs to be greater or equal to
  * max(getauxval(AT_RSEQ_FEATURE_SIZE), 32), and the allocation needs to
  * be aligned on max(getauxval(AT_RSEQ_ALIGN), 32).
  *
  * As an alternative, userspace is allowed to use both the original size
  * and alignment of 32 bytes for backward compatibility.
  *
  * A single active struct rseq registration per thread is allowed.
  */

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com