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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Mon, 1 Aug 2022 15:32:12 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H . Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api@vger.kernel.org, Peter Oskolkov <posk@posk.io>
Subject: Re: [PATCH 2/2] rseq: Kill process when unknown flags are
 encountered in ABI structures
Message-ID: <YufV3PmAOfo1Gt7g@gmail.com>
References: <20220622194617.1155957-1-mathieu.desnoyers@efficios.com>
 <20220622194617.1155957-2-mathieu.desnoyers@efficios.com>
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* Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> rseq_abi()->flags and rseq_abi()->rseq_cs->flags 29 upper bits are
> currently unused.
> 
> The current behavior when those bits are set is to ignore them. This is
> not an ideal behavior, because when future features will start using
> those flags, if user-space fails to correctly validate that the kernel
> indeed supports those flags (e.g. with a new sys_rseq flags bit) before
> using them, it may incorrectly assume that the kernel will handle those
> flags way when in fact those will be silently ignored on older kernels.
> 
> Validating that unused flags bits are cleared will allow a smoother
> transition when those flags will start to be used by allowing
> applications to fail early, and obviously, when they attempt to use the
> new flags on an older kernel that does not support them.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> ---
>  kernel/rseq.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index 81d7dc80787b..bda8175f8f99 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -176,7 +176,7 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
>  	u32 flags, event_mask;
>  	int ret;
>  
> -	if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS))
> +	if (WARN_ON_ONCE(cs_flags & RSEQ_CS_NO_RESTART_FLAGS) || cs_flags)
>  		return -EINVAL;
>  
>  	/* Get thread flags. */
> @@ -184,7 +184,7 @@ static int rseq_need_restart(struct task_struct *t, u32 cs_flags)
>  	if (ret)
>  		return ret;
>  
> -	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS))
> +	if (WARN_ON_ONCE(flags & RSEQ_CS_NO_RESTART_FLAGS) || flags)
>  		return -EINVAL;

Just to make it clear: no existing libraries/tooling out there have learned 
to rely on the old ABI that ignored unset flags, right? Only then is this 
patch ABI-safe.

Thanks,

	Ingo