linux-kernel - Re: [PATCH V4 6/6] Add API to query supported rseq cs flags

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Message-ID: <df3840a6-36ae-4429-80f2-b1f089917e2c@efficios.com>
Date: Fri, 23 May 2025 15:57:10 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Prakash Sangappa <prakash.sangappa@...cle.com>,
 linux-kernel@...r.kernel.org
Cc: peterz@...radead.org, rostedt@...dmis.org, tglx@...utronix.de,
 bigeasy@...utronix.de, kprateek.nayak@....com
Subject: Re: [PATCH V4 6/6] Add API to query supported rseq cs flags

On 2025-05-13 17:45, Prakash Sangappa wrote:
> For the API, add a new flag to sys_rseq 'flags' argument called
> RSEQ_FLAG_QUERY_CS_FLAGS.
> 
> When this flag is passed it returns a bit mask of all the supported
> rseq cs flags in the user provided rseq struct's 'flags' member.
> 
> Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Signed-off-by: Prakash Sangappa <prakash.sangappa@...cle.com>
> ---
>   include/uapi/linux/rseq.h |  1 +
>   kernel/rseq.c             | 17 +++++++++++++++++
>   2 files changed, 18 insertions(+)
> 
> diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
> index 015534f064af..44baea9dd10a 100644
> --- a/include/uapi/linux/rseq.h
> +++ b/include/uapi/linux/rseq.h
> @@ -20,6 +20,7 @@ enum rseq_cpu_id_state {
>   
>   enum rseq_flags {
>   	RSEQ_FLAG_UNREGISTER = (1 << 0),
> +	RSEQ_FLAG_QUERY_CS_FLAGS = (1 << 1),
>   };
>   
>   enum rseq_cs_flags_bit {
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index c4bc52f8ba9c..997f7ca722ca 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -576,6 +576,23 @@ SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
>   		return 0;
>   	}
>   
> +	/*
> +	 * return supported rseq_cs flags
> +	 * It is an or of all the rseq_cs_flags;
> +	 */
> +	if (flags & RSEQ_FLAG_QUERY_CS_FLAGS) {
> +		u32 rseq_csflags = RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT |
> +				   RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL  |
> +				   RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE |

Those 3 flags (above) are already or'd by RSEQ_CS_NO_RESTART_FLAGS,
and they are all deprecated. See rseq_warn_flags().

So I would not return them within the set of supported
rseq flags.

Thanks,

Mathieu



> +				   RSEQ_CS_FLAG_DELAY_RESCHED |
> +				   RSEQ_CS_FLAG_RESCHEDULED;
> +		if (!rseq)
> +			return -EINVAL;
> +		if (copy_to_user(&rseq->flags, &rseq_csflags, sizeof(u32)))
> +			return -EFAULT;
> +		return 0;
> +	}
> +
>   	if (unlikely(flags))
>   		return -EINVAL;
>   


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